|
Sunday, January 03, 2010
If You Have To Ask The Price...You can't afford it. The latest print catalog from Orion Telescopes arrived, and offered 36", 40", and 50" reflectors. And no, the print catalog coyly avoided listing the prices. The price tags vary from $55,600 to $123,000--and curiously enough, shipping is promised directly from "the North American factory."Yes, a little out of my price tag. If you want evidence that America is a wealthier country than it was when I was young--telescopes this size used to be considered world class research tools that only the very largest and most well-funded universities would own. Labels: telescopes
posted by Clayton at 7:32 PM permalink
Thursday, November 19, 2009
Clear SkiesLast night was something of a rarity: a crisp clear night. And let me emphasize how crisp it was. I rolled Big Bertha 2.0 out a bit late--around 8:30 PM--and by then, the target that I had originally planned to go after, M101 had dipped too low in the sky for a decent view. After spending some time collimating the mirror, I decided, "It is just too cold out here!" and went back inside. I am pleased to report that the diagonal mirror holder and mirror cell that I made for Big Bertha 2.0 are very easy to adjust for perfect collimation. Labels: telescopes
posted by Clayton at 11:21 AM permalink
Thursday, October 08, 2009
Watch NASA Make Things Go BoomFrom October 8, 2009 CNN: NASA's Lunar Crater Observation and Sensing Satellite is scheduled to drop its Centaur upper-stage rocket on the lunar surface at 7:31 a.m. ET. NASA hopes the impact will kick up enough dust to help the LCROSS probe find the presence of water in the moon's soil. Four minutes later, the LCROSS will follow through the debris plume, collecting and relaying data back to Earth before crashing into the Cabeus crater near the moon's south pole. The LCROSS is carrying spectrometers, near-infrared cameras, a visible camera and a visible radiometer. These instruments will help NASA scientists analyze the plume of dust -- more than 250 metric tons' worth -- for water vapor.
... "We expect the debris plumes to be visible through midsized backyard telescopes -- 10 inches and larger," said Brian Day at NASA's Ames Research Center at Moffett Field, California. Day is an amateur astronomer who is leading education and public outreach for the LCROSS mission.
Assuming I wake up early enough, I'll roll Big Bertha 2.0 out for a look see. Labels: telescopes
posted by Clayton at 8:18 PM permalink
Wednesday, July 15, 2009
Suerrier Truss AgainI can't find a formula that exactly does the job, but I can use a worst case scenario to solve my problem, I think. Unlike the current design for Big Bertha 2.0, where the load is about 1/4 of the way up the channel that is the primary support, here the deflection on the tubes that position the upper optical cage is just its weight on the end of the round tubes. For a single round tube, the worst loading will be gravity pulling the upper optical cage when the telescope is horizontal. (This is a position that it isn't in very often.) There are a total of six tubes in a typical Suerrier truss, but I'll do the math here for a single tube holding the force. The reason is that this is by far the worst case--so whatever I come up with for an answer will be more than adequate. The deflection for a beam supported at one end with a force at the other end is computed by the formula: D=FL^3/3EI where D = deflection F = force L = length E = Young's modulus for the beam I = moment of inertia for the beam The moment of inertia (or second moment of area) for a tube is calculated by the formula: I = pi/4 * (RO ^4 - RI^ 4) where RO = outside radius of the tube and RI = inside radius of the tube For the 1" OD, .050" wall aluminum tubes that Moonlite Telescope Accessories sells, I = 1.12 x 10-7. For aluminum, E is approximately 70 gigapascals. For my application, I need 62 inch long tubes, so that I can use the existing bolt holes for the square tubes that I am currently using. (If I went a bit shorter, it would reduce deflection very slightly, but put a bunch of unsightly holes in the lower cage.) I'm assuming that the entire ten pound load of the upper cage (including possibly having a camera in the eyepiece focuser) is carried on one tube. Converting everything to metric, this gives us a length of 1.57 meters, a force of 4.55 kg, and a worst case deflection with the telescope horizontal of .00075 meters, or about .03" inches. In practice, with six tubes carrying the load, and the upper cage providing additional stiffness, I suspect that I will get down into the thousandths of an inch of deflection. Here's where it gets a little more tricky. While each tube by itself only weighs 0.41 kg (because they are hollow), six of them gets the weight up to 2.46 kg, or a bit under five and a half pounds. My goal was to knock at least ten pounds off the weight of the scope, by removing the square tubes, the turnbuckles, the guy wires, and most of the aluminum channel that provides both the mounting base for the telescope, and the bottom stiffness member. That aluminum channel right now weighs 10.22 pounds; cutting most of it away, so that it only connects the lower cage to the saddle for the mount, knocks off 6.25 pounds. Losing the turnbuckles and guy wires probably gets me another pound. Losing the two square aluminum tubes gets me another 0.44 pounds--so perhaps this would gain me about two pounds--hardly worth the effort. A couple of possibilities: 1. Skeletonize the aluminum channel to reduce weight, since even it flexes slightly, that doesn't matter much--the upper and lower cages will be rigid relative to each other, and that's what matters. 2. Find some way to reduce the number of tubes, since it looks like one is barely sufficiently. 3. Use carbon fiber composite tubes. These are about three times as stiff, and even at the same size, about 1/2 the weight. There are commercial sources for carbon fiber composite tubes of these dimensions, and instead of five pounds, we're talking 2.5 pounds. Or look for some way to use even smaller carbon fiber composite tubes instead to get the same stiffness as aluminum, and get the weight down to perhaps 1.5 pounds. (And the price of carbon fiber composite tubes is such that going smaller both saves money, and increases the number of suppliers.) Unfortunately, as you shrink the diameter of the tubes, the stiffness declines rapidly, so you can't really go below 0.75" outside diameter--and that takes away much of the weight savings. The temptation is to look at ways to replace that aluminum channel completely--and this might be a place where using carbon fiber composite could be most cost effective--if I could find an off the shelf carbon fiber composite channel of the right dimensions. UPDATE: Here's a supplier of carbon fiber composite tubes that are small enough that they would probably give me the right stiffness, and would only weigh .24 pounds for all six. My concern is that the deflection for one tube would .1". If the load was evenly distributed, that would be .016" deflection--just barely acceptable. And they have carbon fiber channel--at a typical hefty price. UPDATE 2: Yet another possibility is to remove the aluminum channel completely, and replace it with a a cradle that supports the lower cage where it needs to be supported--but is otherwise flat. The reasons for the channel were: 1. Stiffness. 2. To prevent the lower cage from rocking back and forth. If the Suerrier truss provides all the stiffness, I can start with a flat piece of aluminum, and add some supports where the lower cage will be. (If I had a big vertical mill, I would start with a 1/2" thick piece of aluminum, and mill away everything else!) I also notice that DragonPlate is quoting 138 gigapascals for the Young's modulus for their carbon fiber. I've had to adjust my calculations accordingly. UPDATE 3: I took a nap this evening--hence, I'm full of energy late into the evening. If I dispense with the aluminum channel completely, I could use a 4" wide, 1/4" thick, 24" long piece of aluminum to tie the lower cage to the mounting plate. This would give me a half pound where I currently have more than ten pounds, and a maximum deflection of about .02"--in a place where that's a completely acceptable deflection, since it is outside the optical axis of the scope. In combination with the other proposed changes above, using aluminum tubes, I could get it down by about six pounds. If I can find 1", .050" wall carbon fiber tubes at least 62" long, I can get the weight down by eleven pounds--so the whole telescope assembly would probably weigh about 45-47 pounds--light enough that the current mount should be sufficient. Of course, the price of the 1" carbon fiber tubes is horrifying. Finding them long enough is also a problem, but these splices let you epoxy shorter pieces together, and keeping the stiffness of the individual pieces. Labels: telescopes
posted by Clayton at 8:55 PM permalink
Saturday, July 04, 2009
Calculating Serrurier Truss DeformationIf you don't know what this is, see here. I'm thinking of seeing if I can reduce the weight of Big Bertha 2.0 by replacing the current combination of aluminum tubes, turnbuckles, and guy wires, with a single Serrurier Truss design. Moonlite Accessories sells components; I need to figure out how to compute the deformation of a Serrurirer Truss design, to see if I can replace the current scheme with something that would be just as stiff, but lighter. My intutive sense (which is often wrong), is that the deformation of any single tube will be the same as a tube parallel to the optical axis, but with cos and sin included as well! In addition, the top and bottom tube assemblies necessarily add some stiffness, so the total deformation of a six tube truss will be a maximum of 1/6th of the deformation of a single tube. (And in practice, of course, a bit less.) A book called The Design of Welded Structures has been recommended, but if I can find something online (or an expert in my readership--very likely!), all the better. The current scheme where I have a single aluminum channel that runs the length of the telescope can be replaced with a single piece of channel that runs from the lower section (where more than 3/4 of the weight is located) to the mounting plate. This, by itself, will knock at least six pounds off the total weight. If replacing the current square tubes, the guy wires, and the turnbuckles, managed to knock another two or three pounds off the total weight, it would be worth it. (That would get Big Bertha 2.0 down to about 48 pounds.) The other advantage is that I could disassemble Big Bertha 2.0 easily and quickly for vehicle transport, and it would simply putting on the mount, because I could put the lower assembly onto the mount as a somewhat lighter, and considerably less unwieldy structure, then put the tubes in place, and mount the upper cage on the tubes. Labels: telescopes
posted by Clayton at 7:47 PM permalink
Thursday, July 02, 2009
Am I Being Unreasonable?I received a ScopeRoller order on July 1. I emailed the customer back the same day, informing it that because we were waiting on wheels to ship from Missouri, it would be July 10 before we could ship. The response? A refund request because I couldn't ship immediately. I'll make a refund--a customer this easily upset isn't likely a customer that I want that badly. But I don't think responding same day to an order with an anticipated production and ship date 10 days in the future is particularly absurd. Has the Internet bred a generation of hopelessly impatient sorts? Labels: telescopes
posted by Clayton at 9:33 PM permalink
Saturday, June 13, 2009
New ScopeRoller CastersI've just added several new tripods to the supported line of products: ScopeRollerTM LXD55 for the Meade LXD55 tripod. ScopeRollerTM VHAL110 for the Vixen HAL-110 tripod. ScopeRollerTM VHAL130 for the Vixen HAL-130 tripod. ScopeRollerTM OptMini for the iOptron Minitower tripod. The sets for the Vixen HAL-110 and HAL-130 are actually more like a relaunch. I had grown too frustrated trying to machine these slightly complex parts before figuring out how to better grip the workpieces and how to select the right endmill for the job. Labels: machining, telescopes
posted by Clayton at 8:21 AM permalink
Sunday, April 19, 2009
No Longer FadedI mentioned that I had acquired a used but functional Louisville Ladder for climbing to the top of Big Bertha. The paint was faded--and having a high visibility ladder in the dark is somewhat important. Problem solved! Before 
Click to enlargeAfter 
Click to enlargeThe joys of fluorescent orange spray paint! Labels: telescopes
posted by Clayton at 7:48 PM permalink
Monday, April 13, 2009
Climbing To The Top of Big Bertha
Last July I mentioned that I was looking at Louisville Ladder's products for solving the problem of getting to the eyepiece of Big Bertha. (American made, by the way. We still make serious stuff in America, not just broken financial instruments.) In a number of positions, Big Bertha's eyepiece is either too high, or awkwardly placed (or both) to get to while standing on the ground--and the stepladder that I was using was a bit too scary to use in the dark. While trying to decide whether to splurge on the more expensive but taller version of this rolling warehouse ladder, the layoff at HP happened, and I decided that I had more important places to spend my money. I was looking through Craig's List recently, and I found a Louisville GSX2407 for sale, used, for $200. This is the really big version that I had not considered buying. New, they cost $920. Way too much money! But $200? A bargain. So I went over to the shop of the guy who was selling it. He has a machine shop that would make my Sherline lathe and vertical mill feel like they had landed in Land of the Giants. This shop has two vertical mills that can easily turn five foot long, 24 inch diameter objects. It has at least one of the really big Bridgeport mills. Machine tool lust! Anyway, the ladder was made in 1997, and the paint has faded, but everything works fine.
Click to enlargeThere's a brake at the bottom. You press a lever at the first step, and it lowers it down so that the wheels no longer turn. You press another lever to raise it. The wheels are squealing a little--probably in need of Break-Free--and I'll probably give it a fresh coat of safety orange paint to make it look better and protect it--but it works great. I can climb into positions with Big Bertha that would otherwise be difficult, or scary, and the railings mean that even in darkness, there's no real danger of falling off. Oh yes: at the top landing, I can look into the roof gutters, and it is far more stable and safe than a ladder. Labels: telescopes
posted by Clayton at 9:47 PM permalink
Saturday, January 31, 2009
Big Bertha Is Still Too Heavy
I mentioned back in June that I should clearly have used a Serrurier truss, instead of my own clever (?) design. While adding turnbuckles and wires gave Big Bertha sufficient stiffness to hold collimation, it is still too heavy for the Celestron CI-700 mount. The CI-700 had a nominal weight capacity of 60 pounds, and Big Bertha is right at 60 pounds--but that's part of why this is a nominal weight capacity. When this became apparent a few months ago, I thought of selling the CI-700 mount, and buying a Losmandy Titan, which has a nominal capacity of 100 pounds, and should be more than sufficient. But a Losmandy Titan new costs about $6000. That was a sufficiently breathtaking amount of money that I didn't just run right out and buy one--and then my job at HP evaporated. So I went back to asking, "How can I knock some more weight off of Big Bertha?" Something closer to 50 pounds. In the meantime, carbon fiber composite materials have become easier to find, and cheaper (at least for off the shelf components). Dragonplate, for example, sells square carbon fiber composite tubes from which I could construct a Serrurier truss. But the more that I looked at this, the more that I liked the idea of buying off the shelf parts for this. Moonlite Telescope Accessories sells connectors and poles for exactly this purpose. I did the math, and concluded that I could replace my current Frankenstein collection of parts with a total of six poles--keeping at least that part of the aluminum channel that bolts the telescope to the mounting plate (which also adds stiffness to that part of the telescope with the most deflection problem). I'm not quite sure how to calculate the stiffness of a Serrurier truss, but I am quite sure that because of the diagonals, it is stiffer (probably substantially stiffer) than the same tubes parallel to the optical axis, as I have now. Even parallel to the optical axis, the six 1" aluminum tubes would give maximum deflection of 0.00251" for the heavy (mirror) end of the telescope--and the total weight would be about 52 pounds (which was about my original goal for Big Bertha). Most attractive of using these off the shelf parts is that they are designed for quick assembly and disassembly. I could turn six bolts at the top of the scope, and six bolts at the bottom, and end up with two fairly short assemblies that could be put in the trunk of almost any passenger car. The six poles are five feet long, but can be put into almost any front seat without problem. It also simplifies putting Big Bertha onto a mount. The lower end will weigh about 35 pounds, and is small enough to pick up and handle by myself. Once located on the mount, I would bolt the poles in place, install the upper assembly (which should weigh less than ten pounds), and tighten down the bolts. I'll scratch my head about this for a while, then look to see if I can find carbon fiber composite tubes that would be lighter than the aluminum tubes--even knocking 2-3 pounds off the total weight would be a win. Labels: telescopes
posted by Clayton at 3:05 PM permalink
Thursday, January 22, 2009
Doing My Part For The Balance of PaymentsScopeRoller is shipping orders to Britain and Belgium tomorrow! That's my first customer in Belgium! Labels: telescopes
posted by Clayton at 7:28 PM permalink
Friday, January 02, 2009
ScopeRoller Orders Flying InOkay, not enough to quit the contract C# work, but suddenly I'm getting lots of orders, in the dead of winter. And lots of nice remarks from customers who have received their shipments: Hi Clayton, Got the plugs in Wed mail. Thanks for the super fast shipping--it's amazing. Hat's off to you for running a real customer oriented business. You rank right at the top! and from someone replacing a Scope Buggy with my product: The advantages are that the tripod now takes up less space in the garage, I frequently 'tripped' on or over the large wheels in the dark while using the scopebuggy, and I never quite felt completely comfortable with the 'fit' of the tripod legs in the rings provided. (Too much 'extra' room in those rings.) Labels: machining, telescopes
posted by Clayton at 5:42 PM permalink
Thursday, August 07, 2008
The World Wide Web: So Many Answers, Some RightI needed the formula for calculating the sagitta of a spherical mirror (which is the depth of the hole you have to gouge out to make a flat mirror into a spherical mirror). Go ahead: search for the formula. There many different formulas out there--some of them right, some of them wrong. This one is correct:Now you are almost ready to start grinding. Before you start you need to figure out how deep the hole is that we are going to need. The formula for this is pretty simple. You need to know the diameter of the mirror (D) and the Focal Length (F) that you want to make and you get the Sagitta (S) which is the depth of the hole that you need to carve into the glass. The formal formula is:
S = 2F - sqrt( (2F)2 - (D/2 )2) The second way (an approximation) to calculate the sagitta with this formula which is probably a lot easier to calculate:
S = (D/2)2 / 4*F  "Sqrt" is the square root of the number inside of the brackets. Sagitta is how deep the curve of the mirror is going to be in the center of the glass. The more accurately you calculate and measure this dimension, the closer you will be to the Focal Length you want when you get done. Either formula should provide the same basic answer unless you're doing a fast mirror. This one has the right answers in the example, but definitely the wrong formula:
SAGITTA This is the depth of the curvature of the primary mirror, measured at the center. The deeper the curve, the shorter the radius of curvature, and of course the focal length. The formula for sagitta is: sagitta = R - Square root(R2 - d2/4)  | R = Radius of Curvature | | d = diameter of mirror | The following table ties it all together with some examples: | Mirror diameter | F ratio | Radius of Curvature | Focal Length | Sagitta | | 8" | 7 | 112" | 56" | .071" | | 10" | 7 | 140" | 70" | .089" | | 12.5" | 6 | 150" | 75" | .130" | | 16" | 5.5 | 176" | 88" | .182" |
The World Wide Web is an amazing resource--but only slightly more authoritatively accurate than watching television news. Labels: telescopes
posted by Clayton at 8:00 PM permalink
Saturday, July 19, 2008
Moon Over Bogus Basin; JupiterUnfortunately, I was so ga-ga over how beautiful the framing was, I didn't notice that I wasn't very sharply focused--on either Moon or trees (although they should be effectively identical focus). This is a prime focus at ASA 100 with the 17.5" f/4.5 reflector, 1/90th of a second. 
Click to enlargeI also tried to do eyepiece projection on Jupiter--still having a heck of a time getting a decent focus. You can tell that the cloud bands are there. Being still low in the sky, and with the Moon washing everything out, the cloud bands weren't dramatically more crisp in the eyepiece. This was 1/20th of a second, ASA 400, with an 18mm orthoscopic eyepiece projection. 
Click to enlargeI have also decided that whatever the limitations of Big Bertha's mirror, it is not clear that it has a turned edge. I am beginning to think that the problem was the lack of support of Big Bertha 1.0. I took the mirror mask off last night, and I couldn't see that there was any decline in image quality. I also tried to do the star test on Antares. While I couldn't get the diffraction rings (Antares was low in the sky, and there was a bit of turbulence), I didn't see any of the outside focus symptoms of turned down edge. It may not be a great mirror, but I can feel comfortable using the full aperture of the mirror now. The big problem is that I need to get the telescope back down to Earth! I have the mount sitting on a 10.5" plus 12" column right now to get it high enough off the ground board to avoid collisions. What I need to do is is take out the 10.5" column, and build appropriate hardware to bolt the 12" column to the ground board. I still need a better stepladder than I am using, but even taking 10.5" out of the elevation would make a world of difference. Labels: astrophotography, telescopes
posted by Clayton at 8:43 AM permalink
Tuesday, July 15, 2008
Taking More Weight Off Big BerthaI mentioned a couple of days ago that the CI-700 mount just wasn't quite beefy enough to handle Big Bertha's weight. At the same time, the next step up--a Losmandy Titan or Mountain Instruments 250--would be $6000 to $7000--an enormous amount of money for what is, after all, a hobby. I occurred to me that my original goal in slimming down Big Bertha was because the more weight you shave off the telescope, the less expensive of a mount you need. I would guess that every dollar you spend lightening the telescope saves you four or five dollars on a beefier mount. And maybe it is time to start looking at the use of carbon fiber composite components to take some more weight off Big Bertha. Even a ten pound reduction would probably get Big Bertha light enough for the CI-700 to handle well. It is certainly worth investigating. Labels: telescopes
posted by Clayton at 4:39 PM permalink
Monday, July 14, 2008
Need A Suggestion For a Part I have a screw that will be lifting a plate through about a 40 degree turn--but because of the angle that it will be at, the plate has to be captive. I can't just put a screw under the plate--I have to make sure that the upper plate won't go flying freely, and when I turn the screw back that it pulls the plate back down.  I had thought of using a ball joint or a clevis joint to allow the pivot. Ball joints in this size (like, 1/4" threads) don't have enough angle of motion. Clevis joints do, but clevis joints I can find are usually threaded, so turning the screw to move the plate up and down will be turning the joint round and round. Any suggestions? Is there a part intended for this? Perhaps something that might be used in model aircraft that allows a screw motion to move a part up and down? UPDATE: One reader suggested a universal joint. A universal joint transfers rotation. In this case, I don't really want to do that. (Or need to--and since this might go into mass production, I'm looking for a cheap and simple way to do this.) Ideally, I want something that lets the screw coming up from the bottom move the upper plate up and down, while the joint allows the angle to change. This won't be moving quickly and very far. UPDATE 2: Thank you for all the useful suggestions. I think I have enough now to figure out a solution! Labels: telescopes
posted by Clayton at 12:35 PM permalink
Sunday, July 13, 2008
Balancing Big Bertha 2.0Getting a telescope on an equatorial mount balanced is a bit of an operation--and I have not been completely happy with my efforts so far. One side effect of being unbalanced is that the motors just don't have enough power to move the telescope in either axis if they are unbalanced. You can disguise the lack of balance by tightening down the locks on both axes of the mount, but the motors still won't have enough power--especially when the telescope is up at the upper limits of the mount's capacity, as is the case with Big Bertha 2.0 and this Celestron CI-700 mount. Balancing a telescope on a CI-700 mount involves loosening two clamps and moving the telescope back and forth--but that's harder than it sounds when the tube assembly weighs 55 pounds, and is 6 1/2 feet long. Even worse, when you replace a light eyepiece with a heavy eyepiece, or worse, a camera, the balance changes--again. A common solution to this problem is to put a balance adjustment weight on a bar that you can move back and forth. I found a somewhat simpler solution. I took two pieces of scrap black nylon, cut them so that they were a bit wider than the bottom rail of Big Bertha, then used a 3/8" end mill to make two slots in each so that they would slide up and down on the bottom rail. 
Click to enlargeSorry it is such a lousy picture--flash was too bright, and without the exposure dragged on too long. And here's a full picture of it. 
Click to enlargeI had originally thought of using aluminum or Delrin, but the nylon has the advantage of: 1. It is scrap that cost me almost nothing. 2. Easy to cut and machine. 3. Because of its flexibility (even relative to Delrin), I didn't need to add a fastener to hold it on the rail. It is a friction fit, and I can move it back and forth with a wave of my hand. I still think the CI-700 mount is too small for Big Bertha 2.0, but at least it tracks objects across the sky okay now. My big problem now is wind, which tends to whip it around a bit. There's less wind on the south side of the house--but perhaps a wind screen or a heavier mount makes sense. Unfortunately, from CI-700 size mounts to the next step up--the Losmandy Titan--is a leap of enormous size. (The Titan mount lists for $6000--far more than I can justify until I get the spare house in Boise sold, and even then, that's a lot of money.) Labels: telescopes
posted by Clayton at 9:41 PM permalink
Friday, June 20, 2008
Big Bertha 2.0: We're Getting There!I spent some time today checking collimation, and I am finally happy with the results. There is still a little bit of miscollimation as I move the telescope from horizontal to vertical, but it isn't ferocious. If I collimate with the telescope at about a 30 degree angle, moving it to vertical produces very little change, and horizontal isn't really one of the more useful directions for an astronomical telescope. If I could identify the source of the bending, it might be worth trying to get this a bit better--but with the inherent limitations of this turned edge mirror, it may be polishing a cinder. I also experimented with the light shroud. The purpose of a light shroud is to keep straight light from hitting either the main mirror or the diagonal mirror. Under night conditions, there is usually almost no stray light anyway; light that hits the main mirror from any direction from straight on will be reflected off to the side, anyway. The place where a light shroud is of greatest value is when you are using the telescope at twilight, and there is still a bit of skyglow. Well, I had the telescope in the garage at lunchtime, with the north facing door open. I was using the telescope to examine hillsides several miles away. I couldn't see that the light shroud made any difference--and this is daytime! I won't bother with a light shroud. Labels: telescopes
posted by Clayton at 11:29 PM permalink
Thursday, June 19, 2008
Rolling Safety Ladder I strikes me that I should get one of these for access to the eyepiece on the telescope. A conventional step ladder has no rails, and it is easy for someone (not myself, of course!) to become disoriented in the dark and fall sideway or forward into the telescope. You can see an example of what these ladders look like here. They are just expensive enough that I find myself wondering where I might be able to find one used. I suspect that after 15 years of use, these are still fully functional, but sufficiently beat and ugly that I might get a deal on one--if I knew where. Labels: telescopes
posted by Clayton at 1:36 PM permalink
Wednesday, June 18, 2008
Big Bertha 2.0: On High HeelsI think I mentioned that the bottom end of the scope was too low to reach the zenith. Since I had to struggle mightily to get this piece of 6" OD tube bored to fit the CI-700 mount head into it, I was not happy about the prospect of doing this again. But it turned out that I had a 12" Losmandy extension sitting around that fit in the middle just fine. 
Click to enlargeOkay, I was 2" too low before; now I'm 10" higher than I need to be. At least it clears the ground board! A couple of additional annoyances: 1. Before, I was only occasionally in awkward positions. Now, I am a little reluctant to stand on the stepstool required to get into some positions (such as the Ring Nebula this evening). This is a downside of an equatorially mounted Newtonian reflector this massively large. (And not the only downside.) The only real alternative is a rotating tube, either in full, or at the eyepiece end. This may be more work than it is worth. 2. We are coming up on the summer solstice in a few nights--and we are so far north that you still don't have much in the way of stars out until 10:30 PM. 3. There is still some sort of problem with not having quite enough movement in the mirror cell to get perfectly collimated. It's not bad, but it's not perfect, either. 4. Something is still flexing a bit as I move the scope across the sky. I wondered if the diagonal spider (the part in the upper assembly that holds the diagonal mirror in place) was too flexible, and this was causing my problems. So I machined an adapter that let me put my laser collimator tool into the diagonal assembly, and then watch the laser beam's position on the main mirror as I moved the scope around. Dead solid--no motion. So perhaps the problem is flex in the main mirror cell. Labels: telescopes
posted by Clayton at 11:35 PM permalink
Saturday, June 14, 2008
Turnbuckles And ThreadingI tried to add the last two stiffening components to Big Bertha this afternoon. Because I didn't measure the length of the cable quite right, I decided to solve the problem by cutting some of the excess metal out of the bolts that turn into the turnbuckle. The first try worked great. On the second turnbuckle, I chewed up the threads on the bolt. So the simple solution was to re-thread the bolt. But I forgot that the threads in the turnbuckle are the opposite direction of the standard thread direction--so if I turn the turnbuckle, it doesn't tighten the cable at all. Oh well, at least the turnbuckles are cheap. I'll buy another one tomorrow and replace the chewed up part, and the cable that is attached to it. Labels: telescopes
posted by Clayton at 10:17 PM permalink
Friday, June 13, 2008
Progress For Big Bertha!One of my readers made a very good suggestion on how to stiffen up Big Bertha: use steel wire to brace the various parts. The first try wasn't very successful; the guy at Lowe's told me to clamp the ferrules onto the wire with pliers. I used a vise. That didn't work. So I asked a friend of mine, J. Norman Heath, perhaps the only person who is both a circus rigger a published scholar on the Second Amendment. (Weirder things have happened, but I don't know where.) He gave me some tips--including why using a vise to close a ferrule doesn't work. His blog is here. The right solution was to go back to Lowe's, and hunt around until I found this swaging tool for crimp these correctly. 
Click to enlargeIt does rather look like something with which Jack Bauer would torture midgets to get information out of them, doesn't it? Here you can see the way that the turnbuckles tighten the steel wires, tensioning the system diagonally. 
Click to enlarge
Click to enlarge
Click to enlargeAnd no, the mask on the mirror is black--it's just overexposed. 
Click to enlargeIt still isn't right. I ran out of turnbuckles, and so this is asymmetric. Once these four are tensioned, it moves the optical components enough of alignment that there isn't enough play left in the mirror cell screws to collimate correctly. So I fumbled around a bit, and settled for something that wasn't quite right--but still did okay. (I'll finish this tomorrow.) Here's a picture of the Moon at prime focus, 1/125th of a second, ASA 100: 
Click to enlargeHere's a shot done with eyepiece projection, using an 18mm orthoscopic, 1/45th second at ASA 800: 
Click to enlargeIt should collimate better tomorrow. By the way: through the eyepiece looks far better than these pictures. Labels: astrophotography, telescopes
posted by Clayton at 11:06 PM permalink
Friday, June 06, 2008
It's Been Busy Around Here...ScopeRoller seems to be averaging almost an order per business day now--which doesn't sound like much, but they are a diverse range of casters assemblies, so there's no great advantage to doing a production run all alike--and it is hard to keep enough stock on hand, because it is always different sizes. The double secret project is also gobbling up lots of time--all the desire to make the working model beautiful (as opposed to merely functional) is making it take longer than it should. It is astonishing how beautiful a piece of boring aluminum is once you have used Mother's Mag Wheel polish on it! Labels: telescopes
posted by Clayton at 9:57 PM permalink
Tuesday, June 03, 2008
Not Stiff Enough Big Bertha 2.0 isn't stiff enough. It won't hold collimation as I move it across the sky. A little experimentation with the laser collimator in place, watching where the laser spot moves on the diagonal mirror, suggests that the 4" wide aluminum channel is stiff enough--but the 1" square aluminum tubes are not. There's a reason that Serrurier truss designs are so popular--and I guess that I just didn't want to accept that there was a reason for this. A Serrurier truss (like the example shown at Moonlite Telescope Accessories) is a series of triangles that hold the two ends of the telescope in the correct position. Because they are triangles, they hold the parts in tension. I'm still learning, but the most obvious solution for me is to buy two double ball and socket blocks to put on the focuser cage, and four single ball and socket blocks to put on the mirror cage. (Since I already have to have the 4" wide aluminum channel to mount the telescope to the equatorial mount, and this is very, very stiff, I'll leave it place.) The aluminum tubes fit in between these ball and socket blocks. The total weight of these parts is quite small--comparable the square aluminum tubes that I have on there now. It will mean putting a few more holes in the two cages, but perhaps I will be able to patch over the existing holes when I am done. Labels: telescopes
posted by Clayton at 6:39 PM permalink
Monday, June 02, 2008
Big Bertha 2.0: Finally Clear WeatherWe finally had clear skies last night. After fiddling a bit with the collimation, I aimed at Saturn and I was reasonably happy. There was a bit of turbulence, so I was only getting fraction of a second periods when the image was really sharp. I didn't get any advantage going above 160x, but then again, with a mirror this large, 160x provides as much resolution as the 8" reflector does at 235x. (A 17.5" mirror should do better, actually, but this is only a so-so mirror.) I'm still working on getting the balance correct, but at least the mount is able to track Saturn across the sky. I was still seeing some slight discrepancies--perhaps because I wasn't aligned on Polaris (which I couldn't see--at this latitude, darkness comes very late in June), and perhaps because I still need to move the scope a fraction of an inch to get the balance correct. Tonight I may try and getting the digital setting circles working well enough to go hunting galaxies. Labels: telescopes
posted by Clayton at 8:18 AM permalink
Thursday, May 29, 2008
Oregon-Like WeatherThe last couple of weeks (ever since I finished balancing Big Bertha 2.0) has been like Oregon: cloudy when it isn't raining. Tonight is the first night that I have been tempted to roll out the telescope. If this keeps up, everything will be green and beautiful--and then the liberals will move here and make the place uninhabitable. Labels: telescopes, welcome to Idaho
posted by Clayton at 10:39 PM permalink
Friday, May 16, 2008
Big Bertha 2.0: Yes, It WorksA few tweaks required: 1. I couldn't buy any blackout cloth at the Joann Fabrics on Fairview last night--they were all out. And in the early evening, there was still enough ambient lighting that it was a problem. As darkness fell, the image quality improved quite a bit. 2. Because the mirror is better supported, it does seem that Big Bertha 2.0 does have a somewhat better image than before. 3. There is still a turned edge on the main mirror--but it is much easier to get in and add a mask to cover that over now. 4. I probably need to slot the holes where the upper cage is held to the rails, so that I can turn the focuser--otherwise you get some rather awkward positions. 5. I still need to attach the finder--there's little hope of finding anything except the Moon otherwise. 6. It is still a little creaky--lots of discomforting noises as I move it around, but it seems pretty stiff. Labels: telescopes
posted by Clayton at 6:59 AM permalink
Wednesday, May 14, 2008
Mounting Big Bertha 2.0Okay, finally have it up on the equatorial mount.  You will notice that the CI-700 equatorial mount head is on a 10.5" tall aluminum tube. It wasn't pretty, but I managed to turn the interior to the required 5.54" inside diameter by putting an end mill in the drill press, and slowly turning the pipe so that the end mill got all parts. Yes, this isn't exactly how you are supposed to use a drill press. But it worked. The lower elevation relative to the standard CI-700 (or even G-11) tripod means that it was a lot easier for my wife and I to pick the scope up and get it into the dovetail--and I won't need to be standing tippy-toe on a stepladder to get to the eyepiece, either. At the zenith, the eyepiece is 75 inches from the ground. I still need to add the finderscope to the telescope. I'm a little torn as to whether to add it at the balance point, near the eyepiece, or closer to the mirror. Traditionally, finderscopes are near the eyepiece so that you can quickly move from finderscope to eyepiece. Adding it there would add a pound or two to the light end of the scope, requiring me to move the scope down slightly in the saddle--and I'm already on the edge of scraping the board to which the tube is mounted. I knew that I was going to need to put a light shroud on the scope--but in spite of that, it works surprisingly well in daylight. Mirror collimation was a bit of a struggle--it is still possible that the tube structures that I am using don't provide enough stiffness. I don't like some of the creaking noises as I move the scope from position to position. We'll find out the next time the clouds clear out at night. This is still a work in process; I'm learning a lot along the way. Labels: telescopes
posted by Clayton at 11:23 AM permalink
Sunday, May 04, 2008
Flying Buttresses, Solid Buttresses, & TripodsOkay, the square tube is plenty stiff; the 1/4" thick L-brackets that link the square tubes to the vertical tube that holds the tripod isn't. It occurs to me that nearly all tripods use some variant of a triangle. Either they have the legs going out at a angle, or they use a triangular bracket to tie each leg to the vertical tube. The reason is that the diagonal distributes the load. I am thinking that perhaps what might be the right way to do this would be to either add a diagonal from the top of the tube (which is 10.5" tall) to the end of the leg, so that most of the load would be distributed directly to where the casters are located, or replace the L-bracket with a right triangle 3/4" thick, and perhaps 10" high. The right triangle is much easier to make, although the material cost if made of aluminum is substantial. I could drill out lightening holes to get the weight down. Adding a diagonal member might not be so difficult. I could use some square tube (I still have plenty), cut it at the correct angles to make the L-brackets be the right angle. Somewhere there must be something that shows how the load works for cathedrals, flying buttresses, and solid buttresses. The good news is that it won't be written in Latin; medieval cathedral builders did everything like this by experiment. The cathedrals that stayed up inspire wonder in us today; a number didn't stay up. You could say that they buried their mistakes. UPDATE: I went back out to the garage, took another look at everything, and had a sudden inspiration. Dobsonian telescopes (as Big Bertha 1.0 was) consist of: 1. A flat base. 2. A cradle that rotates in azimuth on the flat base. 3. The telescope tube that rotates in altitude on bearings on the cradle. I thought about it for a couple of minutes, and realized that I had already put the casters onto the flat base--which is two inches thick of solid oak. Hmmm. I think that would hold the 5.5" ID tube into which the CI-700 mount should go. All I had to do was: 1. Remove the casters that I had removed from the flat base, and put them back in the flat base. 2. Remove the square aluminum tubes from the L-brackets. 3. Drill and tap (yes, you can tap oak) the flat base to match the holes in the L-brackets that were attached to the square aluminum tubes. 4. Screw 3" long 3/8"-16 bolts through the L-brackets into the flat base. 5. Put nuts on the bottom of the flat base to hold the bolts--in case, for any reason, the threads in the oak give way. It is an elegant reuse of materials! It might look better to use a 1/4" thick stainless steel square instead, and it would certainly be resistant to the weather, but I already have the oak, and I know that it will handle the load--it has been handling the far greater load of Big Bertha 1.0. Tomorrow: I need to drill and tap 3/8"-16 holes at the top of the 5.5" ID tube for the bolts that hold the CI-700 equatorial mount head into the tube. At that point, it's just a matter of moving the mount, bolting everything together, and then moving Big Bertha 2.0 into the saddle. Labels: telescopes
posted by Clayton at 8:08 PM permalink
Sunday, April 06, 2008
Thanks For All The Machining SuggestionsI finished the part, got it installed--and then decided it wasn't worth taking the mount apart to take a picture. It mostly looks terrible (at least the parts that I machined first), but it works, and the segment that was broken on the original is now twice as thick on the replacement. This may limit my ability to use this mount in the tropics, but I can't picture taking it there. There are two next steps before I can mount Big Bertha 2.0 on the mount: 1. Get another Losmandy counterweight. (I didn't have enough.) 2. Build a lower tripod. Because of the size of Big Bertha 2.0, the eyepiece would be at ladder height if I used the current tripod. Also, the center of gravity is so high that it makes it less stable than I would prefer. Finally, lifting Big Bertha 2.0, all 55 pounds of her, to the height of the saddle is an enormously difficult and slightly risky maneuver, even with two of us doing so. So, to build a lower tripod, I need to find some pipe 5 1/2" inside diameter (or perhaps slightly larger) and about 12 inches long. Then I will bolt legs made from 1/4" x 1" steel, about 20" long. This will give me a very wide, very low base to lower the center of gravity and make it less likely to tip--even with 55 pounds of scope, 30 pounds of equatorial mount head, and 75 pounds of counterweights. UPDATE: I found a vendor with short sections of 5 1/2" ID, 1/4" aluminum tubing--exactly what the current tripod uses. I've done the math, and I can cut some of the 6' sections of aluminum square tube left over from the first attempt at Big Bertha 2.0 into 2' legs. These provide enough stiffness to handle the weight of this behemoth with only fraction of an inch of deformation. I'll use 1/4" thick, 3/4" wide L-brackets to bolt the legs to the 5 1/2" ID tube, with the L-brackets held in both places using 3/8"-16 bolts, and the L-bracket inside the square tube. Labels: machining, telescopes
posted by Clayton at 6:29 PM permalink
Sunday, March 30, 2008
The Part I Need To MakeThis is the part that I need to make to replace the broken part.  UPDATE: The more I look at this, the more it just looks like a pain to make--lots of careful thought before I go on to each step. The biggest struggle is going to be making that .606" diameter hole. A 19/32" drill bit is .5938"--just a little small. There are .600" reamers available, and for only $27.38 from MSC Direct. I suspect that you start out drilling the hole with a 19/32" drill bit, then use the reamer to enlarge the hole. Or it is possible that I can just use the 19/32" drill bit, and then put a great big piece of sandpaper on a smaller drill bit, and run it in and out of the hole until the sandpaper takes off .0061" of aluminum--which might happen pretty quickly. Labels: machining, telescopes
posted by Clayton at 7:36 PM permalink
What Ever Happened To The Big Bertha Rebuild?This state senate campaign has just gobbled up way too much time (as you might expect), so something had to give--and what gave was Big Bertha 2.0. But I did receive the 4" wide aluminum channel, and after a bit of examination, I concluded that there really wasn't a need to epoxy the 1/8" thick piece of aluminum into the channel. Here you can see the channel bolted to the tube rings: 
Click to enlargeOf course, none of the existing 1/4"-20 bolts were the right length (some too long, some too short). Here you can see the top side of the channel, where the bolts holding the dovetail plate go: 
Click to enlargeAnd here's the saddle plate side: 
Click to enlargeThere are a total of four 1/4"-20 bolts attaching the saddle plate to the channel, and since two should have been theoretically more than enough, four is way more than enough. However: my wife is anxious to get the enormously huge Big Bertha 1.0 tube assembly out of the garage, so we went ahead and tried to put Big Bertha 2.0 on the Celestron CI-700 mount this afternoon. It turns out that: 1. I don't have enough counterweights to balance Big Bertha 2.0. I have two 23 pound counterweights that came with the CI-700, as well as an 11 pound, and an 8 pound weight that came with the Losmandy GM-8. It's close, with all 65 pounds at the end of the counterweight shaft, but not quite. So I need to buy some more counterweights. Probably one of the Losmandy 23 pounders should do the job, allowing the 11 and 8 pound weights to go back to the GM-8. 2. Remember that I knocked the CI-700 over a few weeks back, breaking one of the parts, which I then had to get welded? It turns out that the welds didn't survive the load of the counterweights on one end of the shaft, and Big Bertha 2.0 on the other end--and the parts that broke off the end, broke off again. I had noticed when I got the part out last time that it was almost something that I could machine myself, if I needed to do so. It might not be as elegant as the original, but it would be close. I guess that I need to do so. I'm sure that if I machined this part from a solid piece of aluminum, it would be strong enough to handle the load. I confess that I am tempted to machine it out of a piece of steel, however, just to make sure. If I could find the part for sale, I would buy it. Today being the Sabbath, I think I'm going to concentrate on relaxing instead. Labels: machining, telescopes
posted by Clayton at 3:14 PM permalink
Monday, March 03, 2008
Aluminum ChannelI'm having a heck of a time finding aluminum channel that is 4" or so wide, and 1/4" thick. Generally, aluminum channel doesn't come quite that thick. Sometimes, the vertical legs of the channel are more than 1/4" thick--but not the horizontal leg. You see, the way that channels with low walls (as a channel that will cradle a large diameter round tube will be) get their stiffness, is primarily dependent on the thickness of the horizontal leg. The stiffness of a channel is roughly in direct proportion to the increase in width--but if you double the thickness of the base of the channel, it gets about five times stiffer with only an approximate doubling of the channel's weight. If you double the thickness of the vertical legs, you only get about a 30% improvement in stiffness. What this means is that a 4" wide channel that is .25" thick (or perhaps even .30" thick) provides very nearly the perfect tradeoff between weight and thickness for this application--and I'm having trouble finding such a channel that is available off the shelf. I had thought about cutting off one side of a 4" square, 1/4" wall tube, or perhaps a 4" x 1" rectangular tube with a 1/4" wall--but these also seem to be unavailable. Square or rectangular tubing that size is typically 1/8" wall or thinner--just not stiff enough for this purpose. I looked at perhaps using steel channel instead, which is cheaper than aluminum--but steel turns out to be, for the same weight, no great advantage. Mild steel has Young's modulus typically of about 190 to 210 GPa; aluminum is 70 Gpa. The steel is therefore 2.7x to 3x the stiffness of aluminum--and 2.44x as dense. Because of the non-linear advantage of a thicker channel for enhancing stiffness, it turns out that steel doesn't buy me anything, except harder to machine, and a little cheaper on the raw material. UPDATE: I've got a vendor offering me 3" wide, .25" thick, or 5" wide, .375" thick. The first choice gives me a .019" deflection (which, because there are two other members also providing support, means I will probably get closer to .010" of deflection), and a weight of 5.7 pounds. The second choice gives me a deflection of .0026" (which far stiffer than I need), but a weight of 14.8 pounds--which is just too much. If I don't find something better by tomorrow, I may go with the 3" wide channel. (The prices are really, really good, too.) If I had a really big mill, I suppose that I could take the 5" piece and take an 1/8" off the inside of the vertical legs.... UPDATE 2: A reader suggested using the 5" piece, then drilling lightening holes in it to make it lighter--especially since it is far stiffer than I need. This is an intriguing thought. There's really no room in the vertical legs to drill anything but tiny, tiny holes, but putting a series of 2" holes through the horizontal leg every few inches might make this feasible. Of course, I've got to put some holes in the base for mounting the dovetail plate. This could get ugly, although not difficult. One other thought just occurred to me: I have two 1" square aluminum tubes that I won't be using. I might be able to have the welding shop I used the other day weld these to the bottom of the 3" channel to increase stiffness, or have a 2 1/2" wide, 1/8" strip of aluminum plate welded into the bottom of the 3" channel. UPDATE 3: It turns out that they can only really weld where the channel and the plate meet--so it sounds like using JB Weld as an adhesive between the channel and the plate would be the way to go--and that opens up a lot more possibilities. I could use a 4" wide by 1/8" channel with a 1/8" plate glued to it. This gives me nearly optimal tradeoff of stiffness and lightness. Labels: telescopes
posted by Clayton at 10:53 AM permalink
Friday, February 29, 2008
Channels The Right Size Are ScarceIt turns out that the aluminum channels turn out to be either too thin or too narrow. I am thinking that buying a 5" wide 1/4" wall rectangular aluminum tube, then running it through the bandsaw to get a channel might be the simplest solution. Of course, I'm a little unsure whether my bandsaw is going to be happy cutting 1/4" wall aluminum. Hmmmm. Labels: telescopes
posted by Clayton at 1:09 PM permalink
Thursday, February 28, 2008
Miracles of Channel Stiffness
I mentioned yesterday that I was hoping someone would tell me if my calculations were correct. I spent a bit of time checking the spreadsheet (and updating it), and I am now confident that the calculations are correct. Of course, that requires the underlying equations be right.... It is astonishing how non-linear the relationship between thickness and stiffness for a channel is. As an example, with a 4" wide channel, .125" thick, and the verticals are .5" high gives a maximum deflection of .0326" inches. At .25" thick, the deflection drops to .0107". Doubling the thickness cut the deflection by about 2/3. Making the channel wider, however, is also non-linear in its effects. Doubling the width of the channel only gets about 1/3 more stiffness. When making the trade-off, if stiffness were the only goal, you would want thick but not very wide. Now, the trick is to call Metal Supermarkets, and find out what sizes of channel they have in stock, plug the dimensions in, and see what makes the best fit. Because the goal is to reduce roll, wider is better. At the same time, the price goes up, and thick contributes more to the stiffness. Since I have to trim the verticals so that the tube fits into the bottom of the channel, I don't want two thick of a channel--because I have to run this through my table saw to make it fit. Labels: telescopes
posted by Clayton at 9:40 PM permalink
Wednesday, February 27, 2008
Solving the Roll ProblemI mentioned a few days ago that I was thinking of using a piece of aluminum channel to solve the problem of the round tube (especially the part holding the main mirror) from rotating off the square tube that mounts to the dovetail plate. I've been doing the math, and I think I've got a reasonable solution. A channel definitely is stiff than a plate, but as the height of the verticals approaches zero, the more closely the channel approaches the stiffness (or lack of stiffness) of a plate. A channel that was 20.4" wide that captured both sides of the tube (as one person suggested) would be immensely stiff--and far too heavy. It would also be hard to find off the shelf! So, I have resigned myself to a channel of a more reasonable width, probably just a bit wider than the dovetail plate, which is four inches wide. A 5" wide channel that is .5" thick needs to have verticals that are .5" high so that the tube fits into the bottom of the channel, where the bolts lock the tube to the channel. (The verticals prevent rotation.) My first reaction was that a channel with such low shoulders isn't going to be a lot stiffer than a plate of the same width and thickness. Yet when I run the numbers, I get results that tell me that even with these low shoulders, it is really, really stiff! So much so that I don't trust my calculations. When computing deflection of a beam with a central support, the formula appears to be: deflection = FL^3/48*E*I where F is the force, L is the length of the beam, E is Young's modulus for the beam, and I is the moment of inertia. E is approximately 70 GPa for aluminum. The moment of inertia for a plate is really simple: I=bh^3/12, where b is the width of the beam, and h is the height. Because the plate is different width and height, the moment of inertia in the Y-axis is 0.00000002 and 0.00000217 in the X-axis. (No surprise: the plate is more resistant to bending on the 5" than the 1/2" dimension.) Using this formula, a 5" wide by 1/2" thick aluminum plate with a 35 pound weight (156.07 Newtons) 18 inches (or .46 meters) from the end produces two results: a deflection in the Y-axis of 0.0002048 meters (.0081") and 0.0000020 meters (.0001") in the X-axis. The weight of the plate comes to 4.39 pounds. At this point, only the real engineers, or masochists (which are somewhat the same thing) are still reading. Now we do the more complex set of equations for computing deflection for a channel. Over at eFunda.com you can see this lovely set of equations for computing the moment of inertia for a square channel. I'm going to put my spreadsheet up at the end of this post for those who really want to check the math. (And if you do so--I'm grateful.) In this case, I'm using a channel that is 5" wide, 1/4" thick, with 1/2" high verticals (the right size to cradle the 20.4" tube). The moment of inertia in the Y-axis is 0.00000022, and in the X-axis, 0.00004966. Using the same force of 156.07 Newtons, the same length .46 meters (18"), gives an X-axis deflection of 0.00000009 meters (0.00000352") and a Y-axis deflection of 0.00002037 meters (0.00080216"). As you can see, this is substantially stiffer than a much thicker plate--and a weight of only 2.85 pounds. Anywhere, here's the spreadsheet. If you see any errors, let me know. UPDATE: I found a couple of errors in the spreadsheet--which actually understated the stiffness of the channel. I have updated it as of 2/28/08 9:28 PM Mountain time. Labels: telescopes
posted by Clayton at 3:58 PM permalink
Broken Telescope Mount PartI mentioned a couple of days ago that my attempt at using 5 minute epoxy to repair the broken part on this CI-700 mount was unsuccessful. This morning I took it in to Riverside Welding and Fab in Eagle for repair. Because this is cast aluminum, apparently you need to heat the part up before you can start welding--and the two pieces that broke off were so small that they were starting to melt before the welder could get the big piece hot enough. So the solution was that they welded a large piece of aluminum on in place of the broken parts, and cut off the excess. It doesn't look very good (especially because the original part was black anodized), but it works just fine. You have to look inside the mount to see the bare aluminum, so I consider this sufficient. (I might use some glossy black spray paint here in there to make it look a bit better.) I was also able to straight out the bent bolt. It still isn't perfect, but it is close enough that you have to look carefully to see the bend, and it works just fine. Best of all, it only cost $35 (their minimum shop charge) to have this done. Labels: telescopes
posted by Clayton at 12:48 PM permalink
Monday, February 25, 2008
Epoxy As A Repair Agent For Aluminum
I mentioned several days ago a stupid accident that snapped a couple of pieces of aluminum out of the CI-700 mount. I have not found any solution that I am completely happy with for fixing this. Welding the two broken parts back in place would require complete disassembly of the mount. There is a soldering product called Alumaloy--which appears to be largely zinc. You heat the aluminum parts up with a blowtorch, and that's hot enough to melt the zinc. Zinc is not as strong (or as flexible) as aluminum, but it isn't orders of magnitude worse than aluminum. A number of comments that I found indicate that Alumaloy works, but it isn't as easy as the demonstrations of it suggest. And I would still have to the take the mount entirely apart to do this. However, epoxy might make sense. Epoxy seems to have a yield strength in the 12,000-15,000 psi range, while aluminum is in the 15,000-20,000 psi strength range. This isn't going to be anywhere as strong, but it's worth a try. I've got some five minute epoxy that I used to repair the counterweight lock knob which broke, and while I will never want to treat it roughly, it doesn't seem to be preparing to fall apart. Once I have everything in position, I'll use the blow dryer to accelerate the curing process. It's a confined space, so this should work well. UPDATE: For some reason, the 5 minute epoxy would not set--and being as the broken parts were upside down, getting them to stay in position was hopeless. I'm told that J.B. Weld epoxy, which is made specifically for metals, works better. Fortunately I was able to disassemble the mount head, and ended up with a single small part with nothing electrical on it that I could feel comfortable taking to a welding shop. 
Click to enlargeLabels: machining, telescopes
posted by Clayton at 11:16 PM permalink
Channel vs. Tube DeformationAs I mentioned a couple of days ago, it appears that the solution to the Big Bertha 2.0 problem is replace the 1" square aluminum tube with an channel. This will support the tube on both sides, instead of leaving it only supported at the point where the screws go through both parts.  I'm having a little trouble finding the formula for computing the deflection on a channel. I was thinking of using a 4" wide aluminum channel. Obviously, the wider the channel is, the more support the sides of the channel create on the tube, and the less strain it puts on the screws that hold the tube and channel together. Of course, the wider the channel is, the heavier it is, too. I have an intuitive sense that a channel is less stiff than square aluminum tube of the same dimensions because the channel is missing the top of the square. I also intuit that stiffness declines as the verticals of the channel get shorter. The ideal channel, from a longitudinal stiffness perspective, has the vertical as tall as the width of the channel. Obviously, the only way that I could use a channel like that for this application would be if the channel was 20.4" wide, and the verticals were 10.2" tall. Of course, that would be a very heavy channel. The thicker the channel, the more stiff it is longitudinally--but the heavier it is, too. My guess is that to get a channel of approximately the same stiffness as the 1" square tube (which was 1/8" wall) I will need a 4" wide channel that is 1/4" thick. I haven't done the trigonometry yet to figure out how tall the verticals will have to be to get the telescope tube assembly to fit to the bottom of the channel, but a 4" wide channel is going to have relatively short verticals. Perhaps I would be better off with a wider channel of a thinner material? The wider channel means taller verticals, and thus more stiffness with a thinner material. There's no magic solution on this--whatever I build is going to be heavier than the 1" square aluminum tube, I know that. Anyway, the net result is that I need a formula for computing how much deformation is created by applying force N on material with Young's modulus E, and length L. I believe that I could even use the current formulas that I have if I knew how to compute the moment of inertia for a channel with dimensions for the thickness, width of the base, and height of the verticals. Or perhaps there is some way to use the moment of inertia calculation for an I-beam, and modify it for a channel, which is essentially a I-beam that has been turned on its side and chopped? From that example, it looks like a channel's moment of inertia (where T=thickness, W=width of the base, and V=height of the verticals) would be computed by adding the moment of inertia for each of the verticals (TV^3/12) and for the section between the verticals (W-(2*T)*((T^3)/12) I=2*(TV^3/12) + (W-(2*T))*((T^3)/12) Did I miss something here? UPDATE: Here's the formula for moment of inertia of a channel. It doesn't seem to be that simple! Labels: telescopes
posted by Clayton at 1:07 PM permalink
Friday, February 22, 2008
Big Bertha 2.0: Something That I Didn't Think AboutIt finally came time to mount Big Bertha 2.0 on the equatorial mount today--and I ran into a problem that I now realize is pretty significant. Deformation along the length of the telescope isn't a problem (especially now that I have switched the rail that the dovetail plate attaches to from 1/16" wall to 1/8" wall). But I hadn't considered the problem of deformation across the telescope. It was immediately apparent that when over at an angle, the primary mirror assembly would twist the rail quite severely. I think the problem here is that all the stress of the primary mirror assembly is being transferred to the one rail to which the dovetail plate attaches. Within the existing design, I can see the following possible solution: 1. Add three more rails, so that I get a hexagon. This only adds about five more pounds to the telescope. 2. Add supports that transfer the load that is currently entirely on one rail (and at the bottom part of the rail alone, where most of the weight is) to distribute the load to the other five rails. This might be something as simple as using 1" square aluminum tube sections to connect all six rails together. This involves making a series of 60 degrees cuts (easy with the chop saw), then drilling and tapping holes that will allow them to lock to the rails going lengthwise. At least at the moment, I am having a little trouble figuring out exactly how this will work. The 1" square tubes are stiff enough, however, that I suspect that it would not take a lot of these segments to do the job. Remember that they don't have to be terribly strong themselves; they just need to stiff enough to transfer the load that is currently on one rail to the other five rails. UPDATE: It occurs to me that the big problem isn't even twisting of the telescope itself--but that the lower assembly (which is very heavy, because of the mirror) is twisting on the rail because there is a single point of contact (round surface on a flat surface). Perhaps using an aluminum extrusion that consists of two right angles might work better. In that case, the round tubes drop into the extrusion. The tube then rests not only on a single point of contact, but also against the two uprights. In this case, a 4" wide base with two uprights would prevent the rotation across the flat surface. Labels: telescopes
posted by Clayton at 3:55 PM permalink
Monday, February 18, 2008
The Telescope Is Together It is all assembled--and weighs fifty pounds--which is darn impressive for a 17.5" reflector. I still believe that it will be stiff enough that I won't need to add the other three rails--but if I need to do so, it would still only be fifty-five pounds. The balance point is 14.5" from the mirror end of the scope. This means that the dovetail plate that will attach it to the mount will be centered at that point on the bottom rail. I have some concern that over time, the rail might bend under load. If I see any sign of this, I might swap that rail for one that is still 1" square, but with a .125" or even .25" wall. This would add 1.68 pounds for the .125" wall, or about 3.7 pounds for the .25" wall--quite acceptable increases in weight if it lets me keep the rest of it light. Some of you have asked why I didn't go with a more conventional truss design. My primary reason is this: I wanted something that I could mount on a conventional equatorial mount, and I haven't seen any truss designs that would do that. I wish that I had the energy to finish this tonight--we had a wonderfully clear (although cold) evening. Labels: telescopes
posted by Clayton at 10:04 PM permalink
The Telescope Is Coming Together (Part 2)Here you can see that even though I didn't have the right length and finish of bolts, I had enough of a collection of 1/4"-20 fasteners to get everything together. I don't have the mirror cell in it, so I don't know for sure whether three rails will be enough, but at least right now, I can't detect any bending when I hold it by one rail. 
Click to enlargeAnd as I mentioned, it is going to need some repainting before final assembly. More by dumb luck than planning, there is no problem adding three more rails of the same size if needed to reduce deflection. This would only be another five pounds, and should reduce deflection by half. Labels: telescopes
posted by Clayton at 11:27 AM permalink
Saturday, February 16, 2008
Assembling Big Bertha 2.0I have the aluminum rails, and I have started putting them together. The only difficult part of the process is getting them exactly parallel to the optical axis of the telescope. Here you can see me using a square to make sure that the rail is exactly perpendicular to the tube. I suspect that if geometry class had required me to prove some of these problems, I might have been a bit more interested! (I feel so sorry for my geometry teacher!) 
Click to enlargeI hold the square tube in position with a couple of hand clamps, and then drill through the holes in the square tube to mark where the holes go in the cylindrical tubes. It turns out that the 1 1/2" long black oxide bolts I bought at Industrial Hardware on Thursday aren't tap bolts (the kind where the threads go all the way to the head). Therefore they are too short, so I'm using some shiny 1 1/2" hex head tap bolts as a placeholder. 
Click to enlargeIndustrial Hardware, unfortunately, is like a computer: they sell me what I ask for, instead of what I need. They seem to be deficient in their ability to read my mind in the future, but that's okay, they are very nice people, and they seem to have everything I can imagine. Here we have the first rail attached. 
Click to enlargeThe holes in the cylindrical tubes are tapped 1/4"-20. I had originally drilled the holes in the square aluminum tubes at 1/4" on the drill press. While these holes were pretty accurately marked, they weren't quite accurate enough to get the bolts to slide in without a struggle, so I went up two drill sizes. Tomorrow after church I'll try and get the other two square tubes attached. I had wondered how I was going to get these square tubes attached exactly at 120 degree angles, but it turns out that the holes in the cylindrical tubes were measured pretty carefully, so if I get one of these holes flat on the floor, I just have put the square tube exactly at the top of the tube to be in position. I'm afraid that I am going to have to do some more paint work on the exterior of the tubes once I am done--the paint has gotten a bit scratched as I been drilling holes. Labels: telescopes
posted by Clayton at 7:28 PM permalink
Thursday, February 14, 2008
Painting Telescope PartsI couldn't get an adequately polished finish on the outside of the tubes, so I decided to go for a gloss white finish on the outside. Obviously, all interior components have to be flat black. 
Click to enlargeThe exterior finish looks better in the picture than it does in real life--I'm afraid that spray paint doesn't come out as professional looking as I would like. I'm buying the square aluminum tubes today that will hold the lower and upper assemblies in position. UPDATE: I thought that I was buying the square aluminum tube today, but when I arrived at work, I decided to see if I could find a better deal online. I didn't find a better deal (the price s were actually somewhat higher online--and that's before figuring shipping), but I found a lot more sizes available to me, including 1" square with 1/8" wall, and 1 1/4" square with 1/8" wall. But I didn't have my handy-dandy spreadsheet for calculating deflection with me, so I waited until I got home. The gain from going to a 1/8" wall from 1/16" wall was about 3/10,000ths of an inch less deflection at the mirror end--and it increased the weight for all three tubes from 5.03 pounds to 9.39 pounds. Why bother? I guess I'll buy the 1/16" wall tubes tomorrow. We've been having glorious weather--not consistently clear at night, but often enough that I want to get Big Bertha 2.0 operational. The tubes cost $2.07 per foot, so it is less than $40 for all three. Labels: telescopes
posted by Clayton at 8:06 AM permalink
Saturday, February 09, 2008
The World's Clumsiest Optical BenchBefore I spend the money on the square aluminum tubes on which to mount the upper and lower tube assemblies, I wanted to make sure that I had the dimensions right. I set them up at about the right distance in the garage. 
Click to enlargeYou may be wondering, "Can you really get everything aligned adequately to test whether the focal point is correct?" By eye, no. I was beginning to get rather frustrated with all this, then I pulled out the Orion Deluxe Laser Collimator that I bought several years ago, turned it on, and everything became much easier. Then it was just a matter of making some adjustments to the diagonal, and adjusting the collimation screws on the main mirror. Then I opened up the garage, and tried to bring the 35mm eyepiece to focus. That should be about 56 power. What did I see? The snowbank in front of the house, which is far too close to focus--but I could see what seems to be a Ford pickup truck at the edge of the field--and quite large. So I stand up, look that direction--and where's the pickup truck? 
Click to enlargeI think it might be over at the Good's house (the people that were neighborly a few days ago and brought the road grader up our driveway). But I sure couldn't find it with the zoom lens on my camera. I have increased confidence that I haven't wasted a pile of money! Labels: telescopes
posted by Clayton at 1:25 PM permalink
Tuesday, February 05, 2008
A Peculiar NeedI'm full of peculiar needs. I need some dust covers to go over the ends of the telescope tube assemblies. I'm thinking either elastic bowl covers large enough to go around a 20" diameter bowl (so like something a restaurant would buy), or shower caps that you might find aboard an alien spaceship--especially the bigheaded aliens that the original Star Trek series so often used. Any hints? I've tried Google, but looking for 20" bowl covers isn't doing the job. UPDATE: Large (22" diameter) stuff sacks, such as are used for large sleeping bags, have been suggested. Since I am going to have to create a light shroud for the telescope anyway (attached to the tube ends with Velcro), I may just buy a little extra, and some elastic, and make the dust covers myself. Hey, at least I am not going to make the cloth myself! Labels: telescopes
posted by Clayton at 4:40 PM permalink
Monday, February 04, 2008
The Telescope Is Coming TogetherActually, rather literally. As you can see from this picture, the top tube assembly now has the eyepiece focuser and the elliptical mirror installed. 
Click to enlargeAt this point, I need to drop the mirror into the mirror cell, put the parts in rough alignment so that I can verify the exact spacing required between them, and the buy the aluminum square tubes that will hold it all together. I mean, alternatively, I could just visualize spacedness, and have a very Marin County kind of telescope! Labels: telescopes
posted by Clayton at 4:37 PM permalink
Saturday, February 02, 2008
The Upper Tube Is Squeezed To RoundI mentioned that using a bar spreader, I managed to finally stretch the short diameter of the upper tube assembly to within 1/8" of round--which is good enough. I can always adjust small discrepancies with washers. (If only I had--or could afford--a lathe large to turn a 20" ID piece of aluminum!) Anywhere, here it is. 
Click to enlargeI still have to drill the hole in the side of the tube for the eyepiece focuser. That's a 2.25" diameter hole. Fortunately, I have a 2.25" hole saw. (The saw is actually more like 2.23", but I suspect that it will actually make it 2.25" by the time it gets done--and I can sand or file to make it a few thousandths of an inch larger.) I have been holding off on this operation until I had the new elliptical mirror that I am going to use--one that has a 3.1" minor axis. It has arrived. I bought it used for $100, including shipping, but I can't tell that it isn't new. Purportedly it is 1/10th wave flat. If you don't know what that means: it means that the across the surface of the mirror, there is no variation from flat that is more than 1/10th of a wavelength of blue-green light. If this seems like startling accuracy--opticians do this all the time. Or at least they advertise that they do it all the time, and I know the methods used for verification. Newton's Rings is an inteference phenomenon produced by the air gap between two surfaces. The example that Wikipedia gives involves a flat surface and a spherical surface, but the same technique can be used with a mirror of known flatness, and another mirror of unknown flatness. The air gap between them produces the rings--and the number of rings that you can see tells you how many wavelengths of light are involved. The pretty rings that you sometimes see on when oil is floating on water are produced by the same mechanism, apparently, and it was this that led to Newton's work on this. Labels: telescopes
posted by Clayton at 12:37 PM permalink
Saturday, January 26, 2008
Making the Tube RounderI've mentioned that the two aluminum tubes that I had fabricated came out far from round. I've been trying to stretch the upper tube assembly to get it closer to round--and I'm getting somewhere! When I started, this tube was about 20 5/8" ID on one diameter, and about 19 7/8" on the diameter perpendicular. Now, by using a clamp/stretcher device I bought at Harbor Freight, it is now 20 1/4" on the smaller diameter, and 20 1/2" on the larger diameter after removing the stretcher. I'm hoping that if I keep this up, I will get it to 20 3/8" both dimensions--and if not, this is getting close enough to round to do what I need to do. Labels: telescopes
posted by Clayton at 10:36 AM permalink
Monday, January 21, 2008
The Mirror Cell Is In The TubeAfter repeatedly running the base plate through the sander, the mirror cell now slides into the tube. It is a tight fit--but not so tight that I can't loosen the bolts in the flanges and slide the entire mirror cell back and forth. 
Click to enlargeI took off only as much of the base plate as I needed to barely fit into the tube. For that reason, the tube, which was delivered somewhat out of round, is now close enough to round for my purposes. Unfortunately, I can't use the same trick for the upper tube assembly. The spider legs were designed to be light, and to hold the diagonal mirror holder in position under tension. But because the upper tube assembly is so far out of round (even more so than the lower tube assembly), I don't have any easy way to get the diagonal mirror holder exactly centered using the tensioned legs approach. I am thinking of replacing the .050" carbon steel legs with something a bit thicker and stiffer--perhaps .100" carbon steel. These might be stiff enough to actually force the tube walls closer to circular. In this case, they aren't held in tension, but add rigidity to the upper tube assembly. I wish that there was some way to force the upper tube to be round, but the aluminum is just stiff enough that while I can bend it to round, it won't stay there once I remove the clamp. I can't imagine a technique for applying that force that won't obstruct the diameter. I thought about applying an epoxy coat to the outside of the tube, while I have it clamped to round--but once the clamp comes off, I expect the tube's desire to return to its natural state will crack the epoxy. Labels: telescopes
posted by Clayton at 9:01 PM permalink
Thursday, January 17, 2008
The Tube Assembly ProblemIt turns out that while I can use a clamp to get the tube assemblies properly round, once I remove the clamps, it returns to out of round. The only real solution is to reduce the diameter of the base plate of the mirror cell. I don't have a lathe capable of turning a 20" diameter round, but the solution is to sand the edge of the base plate. I have a small belt sander that includes a rotating round disc. I have found that holding the base plate up to the disc, maintaining a steady pressure against the disc, and carefully turning the base plate, I can remove a very consistent, even amount from the edge. So far, have taken about 1/4" off the diameter without too much effort. I need to take a little bit more to get the base plate to slip easily into the tube--then I can put the mirror cell back together, and move forward. Once I bolt the mirror cell flanges to the inside of the tube, it will pull the tube back into round (or at least close enough to round). Labels: telescopes
posted by Clayton at 8:08 PM permalink
Wednesday, January 16, 2008
The Tube Assemblies ArrivedI'm a bit disappointed with National Metal Fabricators. In spite of specifying 20.4" ID, +- .05", what I received for the lower tube assembly was 20" ID on one diameter, and 20.5" ID on another diameter. For this level of imprecision, I could have paid half the money and received the same or better results. I went with National Metal Fabricators at roughly twice the price of what Parallax quoted me, because Parallax warned that the tubes would be as much as 1/4" out of round--or considerably closer to round than what I have received. I was able to, with a bit of squeezing and tapping, get the mirror cell into the tube. Part of why I built the flanges that hold the base plate in position with 2" of travel, was to be able to adjust the mirror cell fore and aft for the additional travel required for astrophotography. Something that has to be tapped into the tube isn't going to do the job. I think the solution is going to be to find a woodworking clamp that lets me stretch the tube a little in the 20" ID dimension, and trim the base plate of the mirror cell by perhaps 1/8". Since I don't have a lathe big enough to turn something this large, I'm going to have to use a grinder instead--which isn't going to win any elegance awards. Labels: telescopes
posted by Clayton at 10:46 AM permalink
Saturday, January 05, 2008
The Spider Has LegsI finally got around to slicing this .050" steel into three spider legs, and attaching them to the diagonal holder. 
Click to enlargeObviously, I still have to paint everything flat black. Those are 8-32 screws holding the legs to the diagonal holder. It was very satisfying to drill the holes in the legs by measuring .500" from the bottom edge, drilling a hole, then measuring another 1.000" over, and drilled another hole. Because I had used precision tools to drill the holes in the diagonal holder, everything just screwed right in, with no obvious discrepancies in location. Since all three legs are identical, I taped them together, and drilled all of them at once. Here you can see the one asymmetry. 
Click to enlargeThere are two set screws used for making fore/aft adjustments to the mirror position, at the top of the Delrin piece that holds the 1/4"-20 screw in position. The set screws are on opposite sides of the cylinder. I didn't plan quite far enough ahead, so I had to drill another hole in one of the spider legs to make sure that I could get access to both set screws. If it looks like the legs in the picture immediately above aren't in the same plane, that's a perspective problem. They are actually within a few thousandths of an inch of the same position relative to that Delrin cylinder. The .050" steel isn't stiff in the sense of extraordinarily rigid; that would mean something quite a bit thicker. Instead, better spider designs (as I fancy mine is) rely on tensioning the legs. Once I have the upper tube assembly (scheduled for January 18th delivery), I will bend the ends of the legs to a right angle, and bolt them to the inside of the tube. By having all three legs under similar tension, the diagonal holder will be in a very non-flexible position. Because the legs are 2" high, there's considerable resistance to rotation across the optical path, which is the most important direction. Labels: machining, telescopes
posted by Clayton at 4:20 PM permalink
Friday, January 04, 2008
Breaking 6-32 TapsI decided to use a total of 9 6-32 screws to hold the spider legs to the diagonal holder. I used the vertical mill to very accurately drill the holes--and then I broke two 6-32 taps trying to tap the holes. Maybe I didn't use enough thread cutting oil when doing this, or maybe tapping 6-32 holes in aluminum is just asking for trouble. Anyway, since I now had two pieces of aluminum with broken taps in them, I gave up, and made the next piece of Delrin. It machines easier, and is still adequately stiff and strong. I also switched to 8-32 screws on this one because I had run out of 6-32 taps. Since there will be three legs on the spider, how do you get the holes exactly 120 degrees apart on a cylinder without a rotary table on your mill? Since I have a 1/4"-20 hex head bolt going through the length of the cylinder, this was pretty easy. I used a right angle against the hex head bolt's edge. Each rotation of the face moved the cylinder 60 degrees. Labels: machining, telescopes
posted by Clayton at 8:21 AM permalink
Monday, December 31, 2007
The Fiberglass FolliesIt's a very small world. Someone that I used to work with--and who, several years before that, sold a small telescope to a friend of mine--has done extensive work with carbon fiber composites for aircraft. Anyway, he tells me that the polyester resin style of fiberglass that I am using is much less stiff than the epoxy kind--and for the intended purpose (repairing car fenders and such) that is probably a good thing. Also, as my wife suggested, and I surmised, fiberglass stiffness is highly dependent on cross-section, and scaling up the diameter of the tube requires scaling up the wall thickness as well. As a result, four layers of fiberglass cloth for a 1.75" ID tube is far less stiff than four layers of fiberglass cloth for a 20.4" ID tube. I had thought about contacting Sky Valley Scopes about a tube because they used to make honeycomb composite fiberglass tubes that were very strong, and very light. See this example of a remarkably strong and light tube. The honeycomb composite use a very strong honeycomb layer in between two thin skins of fiberglass. You get all the benefits of the strength of the honeycomb and its lightness. (The XB-70, to my knowledge, was the first to use this approach, with stainless steel honeycomb sandwiched between titanium skins.) Unfortunately, Ken Ward, who ran Sky Valley Scopes, reached a point where his arthritis prevented him from continuing to make the tubes. His wife Judi forwarded his instructions on how he made these tubes to me. It sounds like something that I could ruin a lot of materials before I got good at it! Anyway, I have abandoned the idea of making a fiberglass tube for the Big Bertha rebuild. As soon as National Metal Fabricators opens for business Wednesday I am going to order up aluminum tube sections for this. I will also return the unused container of polyester resin--but not the fiberglass cloth. I may continue to experiment with making fiberglass tubes, using epoxy instead of polyester, and the honeycomb material that you can buy from operations like Aircraft Spruce and Specialty. I have been tempted for some time to rebuild the 3" f/4.5 reflector that I built some years ago. Because of the scarcity of parts in this size, there are a number of compromises to it. I used a larger tube than I needed (4" ID) because the only mirror cell that I could find had that as an OD for the base plate. The diagonal was bigger than it needed to be, because of what I could buy at the time, and the limitations of a high profile focuser. Now that I can machine the parts that I need (especially in this inky-dinky size), I may do a complete rebuild, with an optimally sized diagonal, tube, and focuser. And maybe I will replace that heavy piece of PVC that is the current tube with something made of fiberglass. Even the polyester resin should be capable of making an adequately stiff and light tube, based on the 1.75" ID experiment. Labels: telescopes
posted by Clayton at 4:14 PM permalink
Saturday, December 29, 2007
The Diagonal HolderI've decided to give up on fiberglass, and have National Metal Fabricators make the two tube sections I need. Just to be sure that the mirror cell tube can handle it, I'm having them make it out of .125" aluminum sheet, and the focuser/diagonal section out of .080" aluminum sheet. The focuser and diagonal components only weigh about eight pounds, so most of the increased weight on the mirror cell section is gained by going thinner at the other end. (Unfortunately, most of the weight that causes deflection problems is at the mirror cell end--if only there was a way to go lighter on that section.) Here you can see the diagonal holder, almost complete. I haven't attached the spider legs yet, which will be .050" carbon steel, or the three clips that will hold the diagonal mirror to the 45 degree face. Why carbon steel for the legs? Because the thinner the legs, the less they interfere with the optical path. If thickness didn't matter, I would use aluminum instead of steel; aluminum is slightly stiffer for the same weight. But when it has to be really thin--.050" steel will be stiffer than .100" aluminum. 
Click to enlarge
Click to enlargeThe main body is constructed of a 3" diameter piece of Delrin, hollowed out to reduce weight. I used Delrin also because it is, relative to aluminum, very light, and yet still adequately strong and stiff for this application. To make sure that I can reach in and easily turn the collimation wing nuts, the cylinder to which the legs will attach has to be small, so I used aluminum, because it is very stiff for its weight (unlike Delrin). There's a 0.25" hole bored through the center of that piece of aluminum, and a 1/4"-20 hex head bolt allows me to move the entire diagonal assembly up and down. There are two 8-32 x 0.5" socket head screws in the side of that piece of aluminum to let me lock the screw in position. I had thought of using thumb screws for that, but once I get the diagonal positioned relative to the main mirror, I can't imagine having to move it again. The hex head bolts that go into the 45 degree cutoff section are going into threaded holes. There are also nuts on the top of the 45 degree cutoff section to prevent any motion of those hex head bolts. The plate that the hex head bolts go through (with wing nuts on the top) is also Delrin. This is attached to the aluminum cylinder with the 1/4"-20 bolt which is screwed into a threaded hold in the Delrin plate. There is a nut and a lock washer on the bottom of the plate to prevent motion, and a nut on the top of the plate to prevent rotation there as well. (Maybe I am being overly cautious, but the prospect of the diagonal assembly dropping down on to the main mirror makes me nervous.) Would I have ever attempting anything this ambitious without a lathe and a drill press? No way! Labels: machining, telescopes
posted by Clayton at 6:43 PM permalink
Abandoning FiberglassThis stuff is just too inconsistent on results. That first small tube worked beautifully; nothing that I have tried since as come out anywhere near as well. One try gave me a hard resin, but very little stiffness. This morning's try, even after 25 minutes of baking in the oven, is still tacky. I suspect that perhaps the hardener I bought at Home Depot yesterday might be old (although I can smell both the peroxide and the ketones very readily). The advantage of fiberglass over having an aluminum tube rolled is cost. The materials cost to make these tubes of fiberglass is about $40 each, as opposed to $100 each to have them fabricated of aluminum and shipped to me. The problem is that if you have to make four or five attempts to get an adequate fiberglass tube, it turns out to be cheaper to have them made of aluminum. At this rate, I don't have any confidence that even four or five attempts will be successful. Labels: telescopes
posted by Clayton at 10:14 AM permalink
Friday, December 28, 2007
Second Try With Fiberglass Worked Better
I mentioned the lessons that I learned from the first attempt at making a big tube. While I did a better job of laying up the fiberglass cloth, there were still some wrinkles in the plastic, and the resulting tube wasn't as round as I had hoped. (But it does fit over the mirror cell!) The bad news is that even with five complete turns, the tube just isn't stiff enough. It's too big to bake in the oven, so I used a blow dryer--which made an obvious difference--then I put in the spare bathroom with a heater set to bring the room up to 85 degrees for several hours. Now it is back out in the garage, with an electric heater warming the room. Perhaps I need more turns of fiberglass? Even though I haven't trimmed it to the correct dimensions, it only weighs a bit more than two pounds. Perhaps five more turns? It would still only weigh four pounds. The good news is that the form came out very easily, without damage. Labels: telescopes
posted by Clayton at 8:37 PM permalink
Thursday, December 27, 2007
Fun With Fiberglass On A Bigger Scale
I mentioned the success of making a small fiberglass tube. I decided to go bigger today--and I seem to have learned what not to do. 1. I needed to enlarge the external diameter of the Sonotube that I am using as a mold. It is very slightly larger than 20.25" O.D., and I need something closer to 20.375" O.D. So I started wrapping masking tape around it. It took ten turns. Then I ran out of masking tape--so I switched to traditional duct tape--that only took three turns. Then I switched to a new style of duct tape that is thinner--it took about ten turns. 2. Okay, I wanted to make sure that the mold wouldn't get stuck in there, and that I would be able to get it out again (since I had to destroy the mold that I used for the small fiberglass tube). So I ran it through the table saw, to allow me to bend one side of the circle. To make sure that it held a circle, I then duct taped the cut. 3. Then I grabbed a piece of 3 mil plastic sheeting, which came folded in half, and I thought, "Great! I'll just put this double fold over, and tape it in place!" 4. Then I tried to apply the 8" wide strips of fiberglass to the resin on the plastic sheeting. What didn't work: 1. The cut in the mold just made it too flexible. 2. The 3 mil plastic sheeting needs to be a single layer, wrapped ten times around. That lets me skip the laborious application of masking tape. 3. I neglected to tape one end of the plastic sheeting to the Sonotube--and with the double layer irregularities, I ended up with a plastic sheet that kept moving around as I tried to apply the fiberglass cloth. 4. The longest piece of the fiberglass cloth was 51" long. The circumference is 64". Adding the next piece where the previous piece ended worked, but it isn't very elegant, compared to working with a single piece of cloth (hard to stretch to a tight fit). 5. It still hasn't finished hardening--and it is too big to go in the oven. It is hardening over night in the garage, with electric heaters warming up the air in there. 6. I ended up applying only three 51" lengths, making for a bit more than two complete turns of fiberglass on the mold. Last night I was concerned that it was going to be too heavy if I made four complete turns, because I was mistaken in thinking that the weight would increase with the square of the increase in diameter. I was tired, and not analyzing the problem correctly. The weight increases directly with the increase in diameter, and with the increase in length. Four turns would give me a 1-2 pound tube. It is apparent that two turns, even once it is completely hard, isn't going to be all that stiff, and four turns is so light that I can afford to go to six turns or eight if need be. What I have learned: 1. Don't cut the mold to make it easier to remove. If I use 10 turns of the 3 mil plastic, there should be no problem removing the mold from the tube. 2. Use a single sheet of plastic wrapped repeatedly so that you can get a reasonably tight fit to the mold. 3. Tape the starting end of the plastic securely to the mold--perhaps with packing tape. 4. Once you have the fiberglass cloth cut into strips of the correct length, sew them together so that you have one continuous length that you can wind in one continuous action. There's a gross enough weave in the cloth that it should be possible to hand sew the ends together with just a little overlap--maybe go in and out every third hole. This should be strong enough to tolerate the tugging to get a tight fit. 5. Plan on making it long enough to do at least four complete turns on the mold. That's part of why the small fiberglass tube I made worked out so well. Fortunately, I still have a second, unused Sonotube that I can use. Since these 12 inch sections cost all of about $6.50, if this one gets destroyed removing it, it isn't that big of a deal to go buy another. UPDATE: Sewing fiberglass cloth is right up there with nailing Jell-O to the wall. I guess I will just have to apply in 51" lengths, using the resin to hold it in place. Unfortunately, I have run short of the hardener that came with this 5 gallon container of resin. This must be a pretty common problem, because I notice that Home Depot sells the hardener both with the resin, and separately as well. UPDATE 2: Here's a picture of this first attempt with the split tube mold: 
Click to enlargeLabels: telescopes
posted by Clayton at 10:39 PM permalink
Wednesday, December 26, 2007
Fun With FiberglassOkay, I'm trying again, this time to make a fully fiberglass tube--not fiberglassing a piece of Sonotube. But since the last experience was so unpleasant, I am starting small (using this set of instructions). I started out with a paper towel roll cardboard tube, wrapped it in Saran wrap, which is taped to itself. (The theory is that you should be able to pull the cardboard tube out of the plastic on which the fiberglass has set.) The cardboard tube will be used strictly as a mandrel around which to wrap the fiberglass--and only the middle of the tube gets used for this, providing a non-sticky, non-messy section to grab and turn. Then I coated the outside of the tube with resin, and did a four times wrap of fiberglass cloth, applying resin liberally as I turned it. While the cardboard tube wasn't perfectly round (my wife fished it out of the trash compactor), the surface of the fiberglass came out surprisingly smooth and glassy--and will probably look even better after it hardens and I sand it. 
Click to enlargeEven with the difference in scale factored in, this was much easier than trying to fiberglass a piece of Sonotube--so much easier that I am now prepared to seriously consider using Sonotube as the mandrel around which to form the tubes that I need. Now it just needs to harden. It's unfortunate that I can't put it in the oven--I understand that most of these resins cure remarkably quickly at 185 degrees--but the wife can't stand the smell, and putting this sample in the oven just isn't going to make her happy! A rotisserie--that would be perfect--rotating and heating at the same time. UPDATE: After a few hours sitting next to a forced air electric heater in the garage (and after my wife figured out a way to set up reflectors to force more of the heat back onto the tube), it was sufficiently non-pungent that my wife relented, and let me bring it inside, and cure it in the oven at 170 degrees for ten minutes. And wow, it went from pretty hard where the heated air had been hitting it to absolutely solid over the entire length. Removing the cardboard tube wasn't quite as neat as the pictures linked to above suggest. I had to crush the cardboard tube, but once I did, it popped right out. The Saran wrap mostly came out, but some ended up on the inside. I couldn't very easily get inside to sand it out. I was able to smooth it on the outside with a belt sander. 
Click to enlargeAs I mentioned, the cardboard tube wasn't all that round, having been through the trash compactor, so don't let the shape make you think this was a failure. It wasn't! The tube averages about about .090" thick, is a bit over six inches long, 1.75" ID, and weighs just slightly more than one ounce. It is incredibly stiff, and strong enough that I don't think it would break unless I intentionally crushed it between both hands. I'm not sure how this is going to scale up on weight. The weight will increase directly as the length increases. I think (in my exhausted, ready to go to bed state) that the weight will increase with the square of the increase in diameter, which would mean the 8" long section I need would weigh 11 pounds--which is way too heavy. However, this was four turns, and I've read that three turns of fiberglass cloth is as much as you need to make a telescope tube. It is certainly clear to me that this little tube here is far stiffer and stronger than I needed. Two turns would be about a 5 1/2 pound tube section. I could live with that. Labels: telescopes
posted by Clayton at 4:17 PM permalink
Tuesday, December 25, 2007
Telescope TubesI am still struggling with the tube issue. It does seem as though I do need tubes both to position all the parts, but also to help provide some positioning for the square tubes. The difficulty is getting sufficiently round tubes. I experimented with rolling .065" 5052 aluminum sheet--and developed an increased appreciation for the difficulties in rolling .090" aluminum sheet into a reasonably round tube. It strikes me that it might be possible to start with a thinner sheet--say, .025" aluminum sheet--and roll more layers, something like doing a spiralbound cardboard tube. Four turns would produce a .100" wall tube, which I could then bolt the layers together, at least long enough to get it to a welder to weld the ends and the edge. I'm told that model airplane builders sometimes use epoxy and aluminum to get a result somewhat similar to making fiberglass tubes--and perhaps epoxy between each roll would give the same result. The big problem is that I can't seem to find a local supplier of .025" aluminum sheet that can provide longer than a 48" strip. Four turns on a 20.4" diameter means slightly more than 256 inches. Trying to get someone local to roll an aluminum tube hasn't been so successful. They either want a pretty frightful amount, or they aren't able to weld the resulting rolled sheet into a tube. Perhaps I gave up on making a fiberglass tube too quickly. I did so because I thought that I had a supplier of aluminum tubing the right size. Instead of struggling to get fiberglass cloth to wrap around the end of Sonotube, I should plan on using the Sonotube as a mandrel around which to form a pure fiberglass tube. The technique for this involves applying a layer of polypropylene as a release layer, then painting epoxy on the polypropylene, then applying the fiberglass cloth, then another layer of epoxy, repeating until you have something that is stiff enough for you to be happy with the result. Since the 20" ID Sonotube is about 1/4" wall, I would end up with a 20.5" ID fiberglass tube. The web pages showing how to do this make it sound relatively easy to do--maybe I just need to make another try. Certainly, the materials cost isn't bad, and I still have a good bit of the resin from the last attempt. Labels: telescopes
posted by Clayton at 1:57 PM permalink
Monday, December 17, 2007
Tubes? Tubes? We Don't Need No Stinkin' Tubes!With apologies to The Treasure of Sierra Madre. The more I have been looking at the problem of how to reduce the weight of the tubes that carry the mirror and mirror cell on one end, and the diagonal mirror holder and eyepiece focuser on the other end, I find myself wandering back to my original concept--totally tubeless. The original brilliant (or deranged) scheme was three square tubes to which the mirror cell will attach at one end. This has never been the big problem; there's three bolts to hold the mirror cell to the square tubes that provide the necessary stiffness. At the other end, I have to attach a eyepiece focuser, a diagonal holder to position the secondary mirror in place, and a finder scope. But without a tube, how do I attach those items?  The green squares are the 1" (or perhaps less) tubes that provide the rigidity. With a three vane spider, the spider "legs" attach directly to the tubes. These have to be very rigid; probably steel, not aluminum, since you want them as thin as possible to reduce diffraction. But how do you attach the eyepiece focuser? With a piece of aluminum, probably 1/8" thick or more that forms half a hexagon, bolted to the square tubes at the 120 and 240 degree angles (where 0 degrees is the tube at the bottom). The finderscope can bolt directly to one of the square tubes. This approach knocks eight to twelve pounds off the total weight, and even better, knocks that weight off the ends of the scope--where they produce the maximum load on the square tubes. Even if I have to increase the thickness of the tubes somewhat, and even if the increase in the weight of the square tubes were to cancel out the weight savings from the round tubes, this means no money having the round tubes made, and no struggles finding a vendor to make them. The downside is that the diagonal spider legs have to be stronger than before, since they are providing some structural reinforcement at the far end. Alternatively, since I am already putting a shelf in place for the eyepiece focuser to mount, I could just complete this all the way around at both ends. I also have to come up with some way to cover the optics when not in use. I also have to completely wrap the telescope in black cloth when in use, but I was already expecting to have to do that. Labels: machining, telescopes
posted by Clayton at 3:35 PM permalink
Sunday, December 16, 2007
Big Bertha Rebuild: Mirror Cell Almost CompleteHere you can see it from the side, showing the spring suspension that allows you to collimate the mirror quite precisely to the optical axis of the telescope. Remember that because the collimation screws are 20 threads per inch, one complete rotation of the wing nut moves that screw 50 thousandths of an inch. Applying a little trigonometry to the angle involved shows that you can actually get motions of the mirror's face below a thousandths of an inch, if you make very, very slight movements of the wing nut. 
Click to enlargeHere's the back, where the wing nuts do the adjusting. There's only an inch or so of travel, but that's quite sufficient, as long as you aren't grossly mispositioning the mirror cell in the tube. 
Click to enlargeWhat's left? It turns out that the 3/8" long screws for the mirror clips are just a little long. I need to buy some aluminum washers. If I could buy some 1/4" long aluminum screws this size, that would be even better, but there's no local supplier, and buying a box of 100 of them mail order seems a bit excessive. It isn't like the shorter length is going to shave even an ounce off the total weight. I haven't put the furniture glides onto the mirror plate yet. The furniture glides provide both a metal/glass separation, and provide air flow to speed up mirror cooling. Until I paint or have it anodized, there's no point in putting them in place, since they stick to the surface with adhesive. I am ready to verify the dimensions before I order the aluminum tubes in which this, and the diagonal assembly, will fit. Labels: machining, telescopes
posted by Clayton at 3:40 PM permalink
Big Bertha Rebuild: The Joys of Aluminum Fasteners
I mentioned yesterday that I was going to buy some shorter bolts for the mirror cell. When I reached Home Depot, I was overjoyed to discover that they had a number of aluminum machine screws in the sizes that I needed. Reasons for switching from steel to aluminum for the mirror cell fasteners: 1. About 1/3 of the weight. I've knocked three ounces off the mirror cell this way. Final weight is almost certainly going to come in at or below four pounds. 2. If I get the cell anodized, I can get all the fasteners anodized as well. 3. Aluminum to steel interfaces means a bit more risk of corrosion (although Big Bertha is not going to be standing outside in the rain, and this isn't a moist climate). Aluminum to aluminum knocks this out completely. I'm told that using galvanized steel with aluminum helps; no surprise, because aluminum's electronegativity is 1.61, zinc's is 1.65, and iron (primary component of steel) is 1.83. Aluminum and zinc are so similar that their would very, very little electron flow, while aluminum and iron would have a bit more. 4. For certain applications, aluminum's lower tensile strength relative to steel means that I would have to go to much larger bolts. As I mentioned some weeks ago, two 1/4"-20 bolts should be many times more than sufficient to hold Big Bertha to the dovetail plate--but I will probably use four, just to be overcautious. But that's a load of more than 50 pounds. Here, I have a roughly 30 pound load distributed across six 1/4"-20 bolts in the mirror cell, and across three 1/4"-20 bolts holding the mirror cell to the tube. I may end up using galvanized steel bolts to hold the mirror cell to the tube--just to get the advantage of a dramatically higher tensile strength. Labels: machining, telescopes
posted by Clayton at 1:12 PM permalink
Big Bertha Rebuild: Mirror Cell ContinuedI made a bit more progress today. The base plate now has the flanges attached: 
Click to enlargeThe mirror plate now has the "corral" pieces in place. These I made by rather the same method as the flange brackets, except that unlike the flange brackets which bolt to the top of the base plate, these are bolted to the underside of the mirror plate. 
Click to enlargeHere you can see how the mirror clips attach to the corral brackets. 
Click to enlargeBecause this is only a 1.5" thick mirror, I was originally planning to make these mirror clips quite short--but then it occurred to me that if at some later time I wanted to use a higher quality 17.5" mirror--one that was a bit thicker--better to have enough room. The mirror clips are held in place by 1/4"-20 screws into the threaded hole in the corral brackets. I made a tight enough fit that once the mirror is in the corral brackets, they are effectively a spring fit. Of course, if the telescope ever gets upside down, it would be a very good thing to have something more than a spring fit holding that mirror in place! So I will go ahead and complete the mirror clips. I still need some more parts, which I expect to pick up tomorrow to put this all together. I need some 1/4" long 1/4"-20 bolts for the mirror clips (since they are holding two 1/8" thick pieces of aluminum together, and the extra length would run into the mirror), and some 1/2" long bolts to replace the existing 3/4" 1/4"-20 bolts that hold the flanges and corral brackets in place. It looks a little ugly to have the extra 1/4" of bolt sticking out, and it adds weight that I don't really want. I need some 2" long 1/4"-20 bolts for the collimation adjustments. I already have the springs (which you can see in the picture above) that I salvaged from the existing monstrosity. Because of some compromises that I had made to keep using the existing tube and back door assembly, the current adjustment screws are 4" long--simply too big. I also need some of the furniture pads that I use to lift the mirror off of a direct glass to metal contact (especially of concern where they will be touching the bolts that hold the corral brackets in place). The good news is that the total weight of the current mirror cell components is still only four pounds! I still have to add the 2" long collimation bolts, and the furniture pads, but I also get to knock perhaps an ounce or two off by replacing 12 of the 3/4" long bolts with 1/2" long bolts. I am quite confident that this will still be a 4 1/2 pound mirror cell--or about 1/4th the weight of the only commercial one this size that I could find. And I still have complete confidence in the stiffness of this cell! Materials cost so far is about $150--or about 40% of the cost of the commercial mirror cell for this size. Labels: machining, telescopes
posted by Clayton at 12:09 AM permalink
Friday, December 14, 2007
If You Live in the Eugene, Oregon Areaand you are looking for a telescope-- this appears to be a spectacular deal. The seller thinks that it is a Parks 10" f/5 reflector (no mount, just the OTA). He's trying to get $200 for it. This is about 1/8th of what I would expect it to sell for new. Parks is a well respected maker of traditional Newtonian reflectors. They may not be quite as stupendous as their ads make them sound, but they do have a good reputation for being a real step up from the average Meade or Celestron reflector. If I lived within two hours of Eugene, I would run over there, take a look, and probably buy it--just because it is so cheap that with a little effort, and assuming that it is in the condition described, you can probably find a buyer willing to pay several times that much. If you invest about $600 in a used equatorial mount for it, you could have a very nice setup for astrophotography. (Assuming that the sky ever clears in Eugene.) Labels: telescopes
posted by Clayton at 10:51 PM permalink
Big Bertha's Rebuild: Mirror CellNational Metal Fabricators promised the two aluminum rings from which I am planning to make the lightweight mirror cell for December 13. Sure enough, that's when they arrived. I was just slightly nervous about whether they would be stiff enough for the job, but the math said that 1/8" thick 6061 aluminum would be up to the task. Having put out about $135 for them, I was still concerned that I might end up with two of the more unusual paperweights. This is the 17.5" diameter ring that the mirror will rest on. (Actually, on some felt that will be on top of this.) 
Click to enlargeThis is the 20.4" diameter ring that the smaller ring will connect with springs and bolts. And yes, the protractor is because I was measuring 120 degree angles to get the flanges as close to even as possible. 
Click to enlargeThey are certainly stiff enough for the task. I don't think I would have wanted to go any thinner, but I don't think that they in any way marginal for the intended purpose. Best of all, these two rings weigh about four pounds! Even with the springs, bolts, flanges, and side supports for the mirror, it is going to be under six pounds. Since the mirror end of the scope is the heaviest part--and therefore the one most prone to deform the tubes--the more weight that comes off here the better. Here you can see the flanges that will bolt the mirror cell into the tube, in roughly their final position. 
Click to enlargeI made the flanges by taking a 3" x 2" rectangular aluminum tube (1/8" wall), cutting sections, then cutting these L-brackets out of the sections. There are two 1/4"-20 threaded holes in each of the horizontal sections that will be used to hold the base plate to the flanges. There is a 2.25" long, 1/4" wide slot in the vertical sections that will be used to hang the mirror cell to the tube wall. The reason for the slot is to let me move the mirror between the visual and photographic positions. (The focal point needs to move about 2.5" farther up for the camera; this lets me optimize the secondary mirror for visual use, and yet still be able to do prime focus astrophotography.) The L-brackets were an interesting experience. I had originally planned to rough cut them with the chop saw, then machine them more precisely to length and width with the vertical mill. But it turned out that the vertical mill doesn't have quite enough motion in one direction to do what I wanted. To my pleasure, I was able to get the length and width of these L-brackets with .010" using the bandsaw. Then I used the vertical mill to very precisely (within .005") position the slot. I used a 1/4" end mill to create the slot. The more precisely you position the mirror in the cell, and the cell in the tube, the less play you need in the collimation screws to get everything exactly aligned. I've seen some telescopes where sloppiness in placing the mirror cell in the tube meant a lot more movement was required in the collimation screws. The horizontal part of the L-bracket will be attached to the base plate of the cell with two 1/4"-20 hex head bolts. The L-bracket is tapped; the base plate will be through hole; and there will be nuts on the bottom of the base plate. I may use a lock washer on the bottom to make sure that once I have these screwed down, they don't move. Six 1/4" bolts should be more than sufficient to hold an assembly that weighs total (with the mirror) about 32 pounds. The slots will take 1/4"-20 bolts again, probably hex head. I will use a wing nut and a lock washer on the outside of the tube to hold the mirror cell in position to the tube. This way I only have to loosen the lock washers and push or pull the mirror cell to reposition it at either the visual or photographic position. Of course, that still means recollimating the mirror, but that's not all that hard to do. I may look for the higher grade of bolts on this, since that's a total of three bolts holding about 32 pounds. That still seems more than sufficient. The finish isn't much, but I am going to either have it black anodized or (if that turns out to be too expensive) flat black paint it. Labels: machining, telescopes
posted by Clayton at 7:08 PM permalink
Monday, December 03, 2007
Shear Stress of BoltsBig Bertha's rebuild will be mounted on what is called a dovetail plate, which slides into the saddle of the Losmandy mount. There are then two large screws (on the CI-700 mount) that tighten down against the dovetail plate, preventing motion. How does the dovetail plate attach to the telescope? Usually, this is done with some screws that go up through the bottom of the dovetail plate, and screw into the telescope tube, or a ring that holds the telescope tube. For Big Bertha 2.0, this is going to be screwed into a tapped hole in the square aluminum tubing that holds the components in place. But these are typically 1/4"-20 screws that hold telescopes to the dovetail. Will that be strong enough? I have become intoxicated with the power of computing this stuff, like a mechanical engineer would do, and so I decided to try and compute it. The definition of shear stress is the force per unit area that will cause a material to fail and that "act over an area which is in line with the forces." To compute the force that will cause a bolt or screw to fail, you compute its area, and then compare the force applied across that screw to the shear strength. For a 1/4" bolt, that's roughly (.00635^2) square meters. The shear strength of stainless steel is approximately 186 MPa (megapascals). This means that to shear a stainless steel bolt of that size would require about 7500 Newtons. (A Newton is one kilogram accelerating at 1 m/sec^2. Use 9.8 m/sec^2 as the force of gravity to compute the force on this planet.) For weight alone to shear the bolt would therefore require a bit more than 765 kilograms (or about 1684 pounds). There are two bolts that will hold Big Bertha 2.0 to the dovetail plate, and from what I have read, the stress is divided by the number of bolts (although I can see how in some positions, one bolt might get all the load). Big Bertha 2.0 is only going to weigh about 60 pounds. I think I'm safe using two 1/4"-20 bolts! UPDATE: A reader whose signature line indicates that he is a mechanical engineer working on commercial jetliner landing struts points out that: 1. A fully thread bolt's shear cross section is a smaller than the nominal diameter. I presume that this is because the minor diameter of the thread (the bottom of the threads) is a good bit smaller than the major diameter of the thread (which is about the nominal diameter). 2. He thought the numbers above for the shear strength of stainless bolts was a bit low--although he also pointed out that most of the bolts that you actually buy at a hardware store are less than perfect chunks of steel. 3. Apparently it is considered good rule of thumb to assume that only half the bolts holding an assembly in place actually are carrying the load. This is not surprising; as I pointed out above, depending on position and angle, the load may be disproportionately on one bolt, instead of evenly distributed. He still thinks that two 1/4"-20 bolts will more than do the job. UPDATE 2: Another reader points to these two pages for finding strength of various bolts, one of which shows strengths for various grades of bolts, and the other shows minor diameter of various threads, and area. The weakest grade of bolt (ASTM A320 Grade 8) in 1/4"-20 shows a yield strength of 954 pounds. It might be tempting when the time comes to buy two of the higher grades of bolts, which will take the yield strength in that size up into the 2000 pound range. UPDATE 3: The shear strength of bolts is typically about 55% of the yield strength. Labels: telescopes
posted by Clayton at 3:21 PM permalink
Saturday, December 01, 2007
The Advantages of Having a LatheI think I mentioned previously that the Chromacor is a very cool gadget that turns a so-so achromatic refractor into a well-corrected apochromat--but that because the Chromacor screws into the threaded barrel of diagonal assembly on the refractor, there's no way to use the Chromacor without the diagonal in place. This is a problem, because the diagonal adds several inches to the optical path, making it impossible for me to attach my camera to the telescope without removing the diagonal--and thus, the Chromacor. I decided to fix this, so I turned a piece of Delrin that replaces the diagonal. Since the 48mm threads are not a common tap (and I don't have a thread cutting attachment for the lathe), I bored out the interior end of the Delrin so that I can press fit a 48mm threaded filter in place. (I'll epoxy it in place for permanence.) Now I can screw the Chromacor onto the filter, and put it into the telescope without the diagonal. I now have the advantages of the Chromacor with the option of astrophotography! I didn't have a big enough piece of aluminum lying about; perhaps I'm look into making them out of aluminum for the ultimate niche market--Chromacor owners! Labels: machining, telescopes
posted by Clayton at 11:10 PM permalink
Continual Revision of the Big Bertha RedesignIt turns out that Big Bertha's current diagonal mirror is larger than it should be (reducing image quality and light)--4.25". It turns out that if I reduce the mirror size to 3.1"--and increase tube diameter to 20.4" from 20"--I can get a nearly optimum combination of full illumination of the eyepiece and minimal obstruction. Of course, that means slightly enlarging the base plate the mirror cell (which I have not yet ordered), which increases the weight by a few ounces, and the weight of the two tube assemblies by an ounce or two. On the other hand, a 3.1" diagonal mirror is much lighter than the current diagonal. However: my choices on the square tubes that hold the tube ends apart are becoming frustrating. I had plugged in the wrong value for Young's modulus--using the stiffness of carbon fiber composite instead of aluminum. Now my choices are to go to a larger, considerably heavier set of aluminum square tubes, or spend the money on carbon fiber composite. The net effect is that using 1", .125" wall aluminum square tube gives me a total telescope weight of 67 pounds to get the total deflection down into the thousandths of an inch range--just too heavy. Going to .995", .060" wall carbon fiber composite gets me a 48 pound telescope with a comparable worst case deflection down below .004". The price of this stuff, however, is breathtaking--like $400 for the four tubes. Unless, of course, you know of a surplus carbon fiber composite dealer... Of course, I may be going too far on this. The deflection calculations are for the worst case--the telescope is pointing at the horizon. In practice, there is probably enough flex in a solid telescope tube that perhaps I can accept a few hundredths of an inch of deflection. I suppose that the worst that happens is I use somewhat smaller aluminum square tube (or two instead of four). For two square tubes, this gives me a total telescope weight of 57 pounds, and a maximum deflection of .014". If I need to, I suppose that I can add two more tubes. Labels: telescopes
posted by Clayton at 1:49 PM permalink
Friday, November 30, 2007
More About Deformation of PlatesThat question I asked yesterday about calculating deformation caused by load generated a lot of useful leads! A reader pointed me to this calculation website that appears to be about as closely matched to my needs as I can imagine. It calculates deformation for a circle that is supported by a ring at various radii of the circle. All you have to do is enter the force (which should include that generated by the plate itself), the radius of the circle, the radius of the support, the Young's modulus in gigapascals, and thickness of the plate, and it calculates the displacement in millimeters. Now, three screws supporting this isn't quite the same as a uniform ring, but I suspect that it will be close enough for my purposes. I'm going to go a little thicker than absolutely required, just for that reason, but even for .09" thick aluminum, the displacement is in the hundredths of a millimeter area--or about .002". That's fine. Labels: machining, telescopes
posted by Clayton at 10:11 AM permalink
Thursday, November 29, 2007
Today's Mechanical Engineering QuestionI found this page that tells me that the deformation of a member is equal to the force times the length divided by the Young's modulus, and again divided by the cross section of the member. The application is I am trying to figure out how much deformation a sheet of metal (probably aluminum) of a particular thickness will suffer when a particular weight is placed on it, said weight being evenly distributed across the entire surface. When the surface if parallel to gravity would seem like the most severe strain, so I can use this as my worst case. I suspect that the equation looks something like: F = pressure in newtons (kg * 9.8) L = width of the sheet in meters X = thickness of the sheet in meters Y = Young's modulus for the material in newtons/square meter D = FL/Y/X If the force were exerted over only a small part of the sheet, the equation would be more complicated (and I suspect the deformation would be more severe). Any hints would be appreciated. I'm trying to get the minimum thickness of aluminum sheet for the mirror cell to reduce cost, weight, and the difficulty of cutting the parts. UPDATE: A reader points me to this explanation of determining deflection intended for those building model railways. It's still a linear situation where I am looking at a circular situation, but I suspect that treating the diameter of the circle the same as a rectangular beam is probably pretty close. Labels: machining, telescopes
posted by Clayton at 8:59 AM permalink
Wednesday, November 28, 2007
Making Big CirclesI need to make some big circles--20" and 17.5" in diameter, out of 1/4" aluminum plate. But that's too big for a lathe that I have. It turns out that there are several ways to do this, both using a router, and using a table saw. Here's one of the better examples of how do this with a table saw (probably the best choice for me in the size that I am going to need). I've been experimenting with what was laying around the garage, and I have some confidence that I can do this, perhaps even pretty well, and most importantly, while leaving all my fingers firmly attached to my hands. Three tricks to this: 1. The board that holds the workpiece needs to be big enough that you can clamp it very securely to the table. You won't be able to hold the board tightly enough when cutting anything as tough as Delrin or aluminum. 2. The thickness of the board that holds the workpiece, plus the thickness of the workpiece, needs to be less than the height of the blade on your table saw. In this case, I have a somewhat smaller than normal carbide blade in the table saw, so I may need to get the standard diameter blade to get the height up a bit. 3. The example above uses 1/4" holes in the board into which the bolt that acts as an axis of rotation. I've decided that for my purposes, I'm going to use a tapped 3/8"-16 hole in the board (which will be aluminum or steel, depending on what I can find most available). The axis hole in the workpiece is a through hole so that it can turn freely, but using a threaded bolt to hold the workpiece in position to the board prevents the torque of the blade from ripping the workpiece free and sending it flying. You have to make sure that you don't use too long a bolt, of course, because the board has to clamp to the table. I would not recommend using this approach to cut very small circles, because then your fingers are dangerously close to the blade. Since I'm planning to cut 17.5" and 20" circles (for the mirror cell), my fingers never have to get closer to the blade than a bit less than half the radius. I'm looking at making a mirror cell rather than buying it because: 1. There aren't many vendors of 17.5" mirror cells intended to put into a tube. Many of the commercial sources are Dobsonian-targeted (which means a big square frame). Discovery Telescopes apparently makes one, but review comments like this one aren't confidence inspiring, especially since Discovery Telescopes webpage seems to have disappeared. 2. AstroSystems makes a 17.5" mirror cell which is probably quite good, since Company 7 sells it, but the weight of 16.6 pounds seems excessive. I believe that I can fabricate my own for a fraction of the cost, and about half the weight. (And weight reduction is the whole reason that I am rebuilding Big Bertha.) A reflector mirror cell consists of the following parts: 1. A base plate that attaches to the tube. Typically, there are three screws (sometimes more) that pass through the tube into a flange on the base plate. Both for ventilation, and to reduce the weight, I'll will put in some lightening holes. 2. A mirror plate with a ring in which the mirror sits. At the top of the ring are usually three clips that prevent the mirror from falling forward out of the ring. The mirror plate usually has a series of 9 or 18 points that support the mirror arranged in a pattern that provide air flow around the base of the mirror, to speed up cooling. My experience has been, however, that a heavy ventilated solid mirror plate works well, too, and saves a bit of weight. 3. Three big screws that attach the mirror plate to the base plate, with springs in between the mirror plate and the base plate. Some designs have the screws coming off the mirror plate actually part of the stamping, but I have discovered that tapped holes let you use standard bolts. There are through holes in the base plate, and wing nuts on the screws, which provide a secure method of adjusting the collimation. I have used 1/4"-20 bolts in the past for this for big Big Bertha's current primitive mirror cell, but because of the weight involved, it would be tempting to switch to 3/8"-16 bolts. I think the springs that I am currently using for this will still fit over 3/8" diameter bolts. UPDATE: It turns out that I can get the circles cut out of 1/4" aluminum for $220--which suddenly makes the cost of buying a mirror cell not seem so extravagant. Hmmm. Maybe using 1/8" steel starts to make more sense. My guess is that even 1/8" aluminum would still be more sufficiently stiff, easier for me to cut into circles, and it would take some more weight off the complete telescope--especially at the tail end, where there's the most deflection caused by weight. UPDATE 2: A reader suggests that trying to use the technique above should be restricted to small cuts--that it would be safest to trim the square to a rough circle first. I may in fact go to hexagons instead. Labels: machining, telescopes
posted by Clayton at 1:57 PM permalink
Tuesday, November 27, 2007
Product ReviewI submitted this a couple of days ago to Astromart.com. It's a review of the Orion Dynamo power pack for telescope mounts. Labels: telescopes
posted by Clayton at 2:39 PM permalink
Sunday, November 25, 2007
More Fun With FiberglassI made another try, wrapping the fiberglass glass radially around the cardboard tube, instead of applying it to the exterior face. I can't say that it made any better of a surface, but it means that the interior of the tube is now too small for the 20" OD mirror cell that I was planning to use. The good news is that the fiberglass does definitely stiffen these tubes substantially with only a small increase in weight. The bare tube weighed 2.5 pounds; after applying the fiberglass cloth radially, it is now 3.5 pounds--and is obviously much stiffer. I'm not sure that it is quite stiff enough for a telescope tube, but perhaps one more layer (on the outside) would do the job, and only get the weight up to 4.5 pounds per section. I do think it might be worthwhile to try and buy the thicker Sonotube from the operation in Bozeman, Montana, even with the shipping charges, just to get the extra stiffness. Perhaps I won't need to use fiberglass cloth with the thicker Sonotube--maybe just applying the resin inside and outside will be enough. Since the thicker Sonotube weights 3.3 pounds/foot for the 20" ID form, the tube will be 2.2 pounds per section. With the resin applied, even in two layers on the outside, this should still be less than 4 pounds section. Labels: telescopes
posted by Clayton at 1:25 PM permalink
Sunday, November 18, 2007
Fun With Fiberglass
I mentioned Saturday's non-fun with fiberglass. My attempts to sand it smooth were less than successfull--the surface was just too uneven, especially with wads of fiber mat sticking up and out at all angles. Fortunately, I'm only out about $30 for materials--a small price to learn the importance of keeping the temperature warm enough for the resin to flow well. It turns out that the makers of Sonotube have introduced something called Sonotube Commercial that uses a plastic coating on the paper tube to make it more water resistant, and stronger. I have emailed the manufacturer to find out how much stiffer it is--and if there is a Boise distributor. My thought is that the standard Sonotube is so flexible that even fiberglassing it may not be sufficient. Perhaps starting with Sonotube Commercial (even if it is a bit heavier) might well be worth it. Perhaps it will be rigid enough that it doesn't need anything but a single layer of resin and some paint to meet my needs. UPDATE: I spoke to the technical sorts at the maker of Sonotube this morning. It turns out that the Sonotube that I grew up with--and that was widely used for making telescopes--is just about gone. What they now make is much thinner, and much flexible. It uses a coating called Rainguard to make it adequate for concrete pouring---but not so much for telescopes. I am not the first call that they have received. It turns out that the old style Sonotube (which only weighs 3.11 lbs./foot in the 20" diameter) is still made at the Lewiston, Idaho plant, largely because they don't have the paper and adhesives to make the new form. But no one closer than Bozeman, Montana, actually has it. It seems that it may make more sense to use the standard, not terribly stiff form of Sonotube, and fiberglass it--and do it correctly this time, at the right temperature! Labels: telescopes
posted by Clayton at 8:56 PM permalink
Saturday, November 17, 2007
If There Is Anything Messier and Less Elegant Than Learning to Lay Up Fiberglass TubesIt must be appendectomies done with a hatchet. I'm back to working on Big Bertha's weight reduction program. (She's not fat; she just has big bones--too much wood, not enough aluminum.) I decided to use two pieces of Sonotube, each about 12 inches long and a bit more 20" inside diameter. One will hold the mirror cell, and the other will hold the focuser, diagonal, and finder. This stuff isn't as stiff as I would like (but it was $5.75 per foot), so I decided to try and improvise fiberglassing. I bought a fiberglass repair kit and a big bag of latex gloves ( absolutely necessary for this job), and tried to fiberglass these pieces. I suspended both of them on a piece of wood between two chairs, then painted the exterior surface with the epoxy mix as quick as I could, then put fiberglass matting on the outside, and tried to paint another layer of epoxy mix on top. It is really ugly. I am beginning to wonder if I might be better off spending the money to buy these sections from someone who doesn't mind (for some big bucks) getting his hands messy. Yuck. UPDATE: A reader pointed me to this account of fiberglassing a cardboard tube which I remember reading. I remember the part about using a stick to hold the tube. I didn't remember the part about the temperature needing to be 60-80 degrees. That may be why the resin was so thick that it kept grabbing the fiberglass mat and pulling it loose--making a really ugly mess. This is a more ambitious effort that doesn't involve starting with a cardboard tube at all! Labels: telescopes
posted by Clayton at 4:35 PM permalink
Wednesday, November 14, 2007
48mm Camera Filter Tap & DieThere is a very common camera filter thread that I believe is M48x0.75. I am having trouble finding a source for tap and die for cutting threads for this size. I've tried the obvious places, like McMaster-Carr and MSC Direct, and I've searched the web. Any suggestions of who might have such a tap and die set? UPDATE: I received a number of good suggestions, and amazingly enough, it does not appear that anyone has such a tap available off the shelf at less than $150. The reason that I need this ability is that my 5" refractor uses an astonishing piece of optics called an Aries Chromacor, which is made by one of those Ukrainian optical wizards who I gather used to work for the Soviet military--and after the collapse of the Soviet Union, put his expertise to work on peace dividend stuff. In this case, the Aries Chromacor is a piece of rather specialized glass that does two things, simultaneously: 1. It corrects the chromatic aberration that is common to all but apochromatic refractors. 2. It also corrects for whatever spherical aberration your refractor has. If you have a 1/6th wave undercorrected achromatic refractor, you buy an Aries Chromacor that has a 1/6th wave overcorrection built in. Anyway, it works very well--turning what would otherwise be a so-so cheap refractor into something that is perhaps 85-90% of an apochromat for about 30% of the price. A detailed review that I wrote several years ago is here. As with all clever compromises, there are couple of irritations about the Chromacor. Because it goes inside the telescope tube, at the eyepiece focuser end, something has to hold it place. The Chromacor has a male, 48mm filter thread on it. Refractor diagonals (at least some refractor diagonals) have the end that goes into the focuser tube threaded to accept 48mm filters, so the Chromacor just screws onto the end. This works fine, except that the diagonal adds enough length to the optical path that you can't attach a camera at prime focus--the focal point won't be at the focal plane of the camera. To take prime focus photographs requires removing the diagonal--and thus the Chromacor. What is needed to solve this problem is something that goes inside the focuser tube, and has 48mm filter threads on one end. That's why I was looking for a way to tap M48x0.75 threads. Well, it turns out that others have had this problem, and one solution is this Televue 2" eyepiece barrel extension. I'm told that it has 48mm threads (probably at both ends) so that you can attach it to a 2" diameter eyepiece barrel, and lengthen the eyepiece. I think what I will do (once I have verified the thread details) is buy one (or perhaps two), and then machine a piece of aluminum that will slide into the focuser tube. On one end, it will be wide enough that it doesn't slide all the way into the tube. On the other end, it will be small enough that the threads on the barrel extension will slide in, and I can use some epoxy to hold everything together. Then I can screw the Chromacor into the other end of the barrel extension. Labels: telescopes
posted by Clayton at 10:11 AM permalink
Monday, November 05, 2007
Too Much Play for Astrophotography?I have had a couple of ScopeRoller customers who have complained that the Deluxe wheel set has too much play for astrophotography. One of my customers sent a picture of of his scope mounted on ScopeRoller Deluxe casters--and some long exposure astrophotos he took--more than two hours. You tell me: are these good enough?
click to enlarge
click to enlarge
click to enlarge
Labels: astrophotography, telescopes
posted by Clayton at 8:43 AM permalink
Sunday, September 02, 2007
ScopeRoller Ad in AstronomyThe October issue of Astronomy had a 1/4 page ad for ScopeRoller. I wasn't quite sure what to expect. I was hoping that have ten orders a day coming in--enough to justify running another ad, and make this my full-time business. I've received three orders in the last week that were because of the ad. If this continues for another four weeks, then it will at least pay for the ad--but it doesn't justify running it again. Labels: machining, telescopes
posted by Clayton at 9:55 PM permalink
Saturday, July 21, 2007
Zero Motion BearingsI've had another customer decide that there was too much play in the ScopeRoller caster assembly for his astrophotography needs, and I'm not one to let an issue like this stop me. He confirms that my solution to the problem of motion in the locking mechanism works--the play is now in the bearings of the caster itself. This is a few thousandths of an inch to perhaps a few hundredths of an inch. This may not sound like much, but .05" of motion here can turn into several trillion miles at the far end of the beam of light, so I am interested in finding a solution. Obviously, a caster with zero motion when locked doesn't exist--unless it's made by the Frictionless Surface and Dimensionless Point Corporation. (You remember Frictionless Surfaces and Dimensionless Points from physics class, right?) But does anyone know of a maker of locking caster assemblies that might be so well made that applying a force of several pounds to a locked caster will result in a thousandth of an inch or less of motion? This is a tall order, and I would probably have to pay a lot more for such casters--and charge customers that need such perfection a tall price. Labels: machining, telescopes
posted by Clayton at 4:22 PM permalink
Friday, July 20, 2007
ScopeRoller's First Shipment to HungaryYup! We're expanding our customer base once again! Labels: telescopes
posted by Clayton at 11:24 AM permalink
Thursday, July 12, 2007
Optimizing Machining TechniquesI spent way too much of yesterday evening making a caster set for the Meade LX200 tripod--and end up with something that wasn't good enough to ship. It would have worked, sure, but the bore was eccentric and off-center. That's fine for friends (I mean, it's fine to have friends that are eccentric and off-center--most of mine are), but not okay for products! So this evening, I scrapped about $15 worth of Delrin and three hours of labor, and started over. This time, it took less than hour, and I ended up with a much better result. The core problem here was that the version that I make that is an insert that slides into the legs of some tripods has to be pretty precisely machined on the outside diameter--2.35" +- .005", for example, for the Losmandy G-11. For the sleeve version, like I build for the Meade LX200, it is the inside diameter of the sleeve that matters--not the outside. But because I started out making the insert version, I have gotten into the unnecessary habit of turning the outside diameter of the sleeve to a particular size. Partly, it was just a habit, and partly, I rather like the very machined look it produces. Worse, because I was trying to turn the entire length of the cylinder in one operation, I had to drill and tap a hole in one end, put it on a special holder, rather than just use the 3-jaw chuck to hold the cylinder in place. I have never produced a perfectly centered hole, so if I start with a 2.265" outside diameter piece of plastic, I typically have to turn it down to 2.20" to get all the outside at the same radius around the center of rotation. This is slow (hence the long operation yesterday evening)--and even then, when it comes time to bore in the center of the sleeve, it is surprisingly easy to end up not being centered. Anyway, I figured out that I should just put the cylinder in the 3-jaw chuck, and sand off the external surface (which often has brand markings on it) with #80, #180, #800, and #1500 sandpaper. It is fast, and produces an acceptable looking finish that is very smooth. This speeds up the process a lot! I am also thinking that for the more precisely machined cylinder that are inserts, instead of going through the hassle of trying to get an exactly centered tapped hole in one end (something that often does not happen), it makes more sense to add 3/8" of an inch to cylinder so that I can turn it down for the entire length of the cylinder with the cylinder in the 3-jaw chuck. When it comes time to cut it at the 30 degree angle, I just move the up 3/8" an inch, and have a bit more scrap--but a lot less time spent turning the cylinder to size. Most of the Delrin rods that I start with cost a bit more than $2 an inch, so this turns out to be a materials cost of about $2.50 per set--but a saving of easily six minutes a set, and a more consistent result. I still have to put a tapped hole in one end, because that's how I attach the cylinder to a fixture when running it through the chop saw to get the 30 degree cut. (This way, I get to keep my fingers attached.) But it no longer has to be a precisely centered, perfectly square hole. It can be even a bit sloppy, because it is just to have an easy way to lock down the cylinder when I put it on the chop saw. Labels: machining, telescopes
posted by Clayton at 12:24 AM permalink
Wednesday, July 04, 2007
Crosshair RepairI had a little surprise the other night trying to find Saturn with my 5" refractor. I looked through the finderscope--and there were no crosshairs. There was a single line across the field of view--and after a bit more investigation, I found a squiggly line near the top of the field of view that looked like what was left of the vertical crosshair. This surprised me. I've read of World War II crosshair manufacturing where they literally stretched a strand of spider web. But many years ago, my father and I modified a World War II era tank range finder telescope. It had a series of horizontal lines across the field of view. When we disassembled the optics, these lines were scratched onto a glass reticle. All we had to do was scratch a vertical line across the horizontal lines, and voila! Crosshairs! The next step was to drill a small hole in the tube next to the glass reticle; I could hold a small flashlight (the kind that used "grain of wheat" bulbs) against the hole, and get an illuminated reticle--very useful for finding objects against a very dark sky. If even a World War II era scope had a glass reticle, it seemed implausible that even the cheapest Chinese-made finderscopes used fine threads for crosshairs, instead of reticles. So I took the eyepiece apart, and indeed, there were two very fine pieces of thread or whatever (for all I knew, spider web strands) that formed the crosshairs. They were sufficiently fragile that sometime after disassembly, the remaining crosshair also snapped. I wasn't thrilled with my used finderscope options, and I also didn't want to wait several days for a replacement to arrive, so I drilled four holes in the eyepiece assembly .495" from the end, where the crosshairs had been located. (The advantages of having a vertical mill and edge finder!) Then I ran the finest thread in my wife's sewing kit through the holes, and used a single drop of epoxy to prevent the threads from working loose. I put it all back together again, and it works! Because I didn't have a good way to clamp this eyepiece assembly (it was too fragile to clamp hard, and being round, tried to rotate), two of the holes are across a chord, not the diameter of the tube. Still, all that matters is if the crosshairs align with the aim point of the telescope--not if they are perfectly centered. The thread I used is obviously much coarser than the original crosshairs--one might even say grotesquely so. On the other hand, because this finderscope didn't have illuminated crosshairs, against a dark sky the fine crosshairs just disappeared. These coarser crosshairs won't have that problem. Labels: telescopes
posted by Clayton at 3:35 PM permalink
Friday, June 29, 2007
Getting More Clever on the ScopeRoller Web PagesI decided to redo the webpages so that the ordering buttons were more consistent in appearance, and easier to maintain, so I changed the pages to use server side includes. (A "server side include" is a method of making a particular collection of stuff appear throughout a web page without having to make multiple copies of it.) Along the way, I made the directory buttons at the top of each page use a server side include to make it easier to update these. I can change one file, and get all the directories to update at once. I could make this even more clever, rewriting it all in Perl, but that's too much like my day job! Anyway, it is now a lot easier to add more ordering buttons for different countries, so I will probably add Britain, Australia, and New Zealand next. Labels: telescopes
posted by Clayton at 10:29 AM permalink
Saturday, June 23, 2007
Fun With Band Saws; Happy CustomersI just finished making a set of casters for the Vixen HAL-110 tripod--by far the most challenging machining operation yet. This slides inside the tripod leg assembly, replacing the "foot" that ordinarily sits on the ground.  What makes it difficult is that I wanted something that would slide into a fairly complex hole (far more complex than the insert's end, and far more complex than really needed for Vixen's purposes) without being loose. And that's what I achieved--precisely enough machined that it doesn't fall out, but also doesn't have to be pressed in hard. Of course, I use the M4-0.7 screws that hold the foot in place to hold this insert in position as well.   It doesn't look like it should be all that hard to make, but with the amount of time it took, I think I may charge a pretty premium price to make any more. Even using the band saw to hack the big chunks out, while it saved a lot of time, still doesn't make these fast to make. The problem is that even with a 1.5" long cutting surface on this end mill, the power limitations of the Sherline means that you are removing at most .020" of Delrin at a time. If you try to remove more, you get some nasty vibrations and accuracy suffers. Do you notice the "waffle" pattern? This particular end mill was a roughing mill--meaning that it concentrates on speed, not finish quality. If you hold the final result in exactly the right position, it kinda looks like a Manhattan skyscraper. The Sears 10 inch band saw has turned out to be the best $170 I've spent in a long time. It is reasonably consistent. I set the fence at 1 1/8", and the piece that came out was 1.03" wide, so about .09" short. I set the fence at 2 1/8", and it cut a piece 2.04" wide, so about .08" short. I tried moving the scale on the edge of the table to correct for this, but there's only a very small amount of adjustment potential. I'll just remember that the cut is going to be a little under 1/8" narrower than the fence position, and that's good enough. The other thing about it that is nice is that I can see where the blade is while cutting. The chop saw that I have, as powerful and quick as it is, is just too dangerous to use for cutting anything small. The blade that came with the band saw works well for Delrin and for 6061 aluminum. I tried to trim a small piece of steel, and it was clearly not going to do anything but dull the blade. This is a woodworking blade, however, so I am not surprised. One aspect of the band saw that has me a little confused is that it produces a bit of a wavy cut in Delrin. It doesn't seem to do this in wood or aluminum--very odd. It isn't a problem, since the only band saw work I do in Delrin is just a first step towards machining, but I suspect that there's something that I don't know that might produce a smoother finish. I made a caster set for the Celestron 93493 tripod (first time for this model). The customer is singing my praises in this thread. Now, if all the other users of that tripod would just go ahead and order. Labels: machining, telescopes
posted by Clayton at 10:38 PM permalink
Thursday, June 21, 2007
Tuesday, June 19, 2007
Band SawsI am thinking that perhaps what I need to do the rough cuts on the next Delrin project is a band saw. Not a big band saw--Sears has a 9" band saw for $119--but a band saw seems a bit more controllable than a router. Since everything that I need to cut is a rectangular solid, this has potential. I need to see if my neighbor who does woodworking has one. I think I want to experiment a bit before I commit myself to one. And yes, I know that band saws have another common use--by butchers. Even more than a drill press, one must exercise great care in using a band saw. UPDATE: One of my neighbors has a band saw--a 12" Delta that looks ancient enough to have been made somewhere that at least they use a Roman alphabet, even if they don't speak English. This is definitely the way to do this. He didn't have a fence appropriate to my needs, but even free hand, I managed to do an adequate job of making a couple of rough cuts. Not wanting to wear out my welcome, I stopped with two rough cuts, and didn't make the other two, much larger rough cuts--and regretted that decision for the next several hours, as I removed a .35" x 1.57" x 2.75" chunk of Delrin with an end mill. To my surprise, I found a monstrously big end mill--0.75" diameter and about 1.75" tall cutting surface--and so I was able to make make all my motions in the X and Y planes. But because of the height of the cutting face of the end mill, I was only taking off 20/1000ths of an inch on the forward pass, and 5/1000ths of an inch on the backward pass. (Trying to take off more than that caused it to chatter.) In spite of, I thought, extraordinarily careful measurement, the final result would not quite fit into the leg, so I had to make a few cuts here and there. The good news is that the final version slides right in, and is actually more amenable to band saw work, with just a few minutes of precision work with the end mill. My plan to use the Vixen leg as a template for positioning the holes in my piece worked perfectly, and it actually looks like something that might have come out of the Vixen factory! (Provided, of course, you don't look at the ugly surfaces that are now hidden inside the tripod leg. Production will be better.) I also made a set of casters for a customer who sent me his leg (tripod leg, of course) because the Celestron 93493 tripod isn't exactly the same dimensions as other Celestron tripods that I have made for. This went very well; I received the leg today, measured it, picked out the stock to work from, and machined the parts in time that I will be shipping the leg and the caster set back tomorrow. Had I been thinking a bit farther ahead, I could have dropped them at the post office this afternoon. Labels: machining, telescopes
posted by Clayton at 10:46 PM permalink
Wednesday, May 23, 2007
Fasteners vs. AdhesivesI'm looking at how to fasten six pieces of aluminum together to form a hexagon for the mirror cell. The rectangular sections have a 60 degree cut on each end. Ideally, these will be made from 1/8" thick aluminum (which is stiff enough to support the weight of the mirror without bending). Here's the problem: how do you fasten them together? I had thought of drilling and tapping two small holes through adjoining sections, and then using two small screws to lock them together. The problem is that you need very small screws to do this. A 1/8" thick section cut at a 60 degree angle gives you approximately a .1975" face. A number 6 screw has a major diameter of .138"--not leaving much material on either side of the screw. (And yes, I would be locating and drilling these holes with a vertical mill.) A number 4 screw (about as small I want to go) has a .1150" major diameter--still not a lot of material on either side. Two alternative strategies: 1. Use 1/4" Delrin instead. It will be about the same weight as 1/8" aluminum, actually slightly stiffer (because of the extra thickness). This way I have a .395" face, so a number six screw has lots of material on either side. I also don't have to blacken the Delrin (it is already black)--just sand it a bit to make it non-reflective. 2. Use an adhesive to bond the sections together. A friend tells me about a new miracle adhesive from Loctite that Ford is now using to hold its cars together not only because it ways less than bolts, but adds vibration absorption as well. Can anyone suggest some other approaches that give great stiffness without adding weight or complexity? UPDATE: A reader suggests making 120 degree angle brackets that would go on either the outside or inside of the hexagon. I could make these out of 1/8" aluminum, and because I am not using the inside corners of the hexagon (a round mirror goes in there), I could put them on the inside. This has two advantages: 1. I would have a total of 1/4" of aluminum that I could drill and tap. Since this isn't carrying much of a load, a number 6 screw would work fine. 2. I would not need to have thousandths of an inch accuracy on the holes. This speeds up the process, since I could use the drill press instead of the vertical mill. On the other hand, putting the brackets on the outside puts clamping force in a direction that would help hold everything together more tightly, I think. Labels: machining, telescopes
posted by Clayton at 9:36 AM permalink
Monday, May 21, 2007
There Are Some Advantages to Being as Obscenely Rich as a DemocratLike this 10" f/9 apochromat, available for only $40,000. Drool, drool. Labels: telescopes
posted by Clayton at 2:46 PM permalink
Friday, May 18, 2007
Carbon Fiber CompositeI've been looking at aluminum square tubing for Big Bertha's rebuild--but I'm running into some interesting issues. I would prefer to use two square tubes on opposite sides of the optical components, primarily because it makes it easier to mount it in a Dobsonian mounting as a short term strategy. There's also a cost issue. One 4" square tube (which is as light as I can go if I only use one tube) gives a deflection of .00053", which I consider sufficient for my purposes. But that one 4" tube is substantially more expensive than two 3" tubes would be--and the two 3" tubes gives a deflection of .00068", assuming that the stiffness is additive. I'm told by a PhD in Mechanical Engineering that using two tubes on opposite sides of the optical components, as long as everything is firmly attached at both ends, will give a stiffness that is quite a bit more than the sum of each tube, because you are effectively creating an I-beam. But how much stiffer than the sum of two tubes is that? I'm not sure. Anyway, I'm looking at carbon fiber composite. I see figures for its modulus of elasticity quoted of 33 million pounds per square inch, or about 220 gigapascals--more than three times stiffer than aluminum or steel. At the same time, it is far lighter than aluminum. Unfortunately, while there are a lot of vendors of carbon fiber composite tubes, all that I am finding seem to be aimed at the bicycle enthusiast, so no square tubes, and finding one that is 72" is also difficult. (Perhaps I should check with whoever makes racing bicycles for the Jolly Green Giant.) Any suggestions on where I might find 2" square tubing made of this miraculous material? UPDATE: It turns out that the formula for computing the stiffness of an I-beam is described here. You compute the moment of inertia based on the cross-section of the flanges (the top and bottom horizontal strokes of the "I"), a factor that includes the height of the vertical member (and that gets squared), and a third factor that multiples the width of the bottom flange by the height of the vertical member--and then raises it to the third power. If I regard the two aluminum hexagons that will sit between the tubes as effectively a very tall, very wide vertical member, then the combination will be very stiff indeed. Unlike a conventional I-beam, the two hexagonal members are many times wider and taller than the flanges (although not full length). Best of all, because they are effectively round, unlike an I-beam, which is much stiffer vertically than horizontally (because of that cubed factor on the height of the vertical member), there should not be an enormous difference in stiffness of the telescope depending on whether the tubes are vertical or horizontal. I haven't tried to calculate the deflection of the combination, but I suspect that having these hexagonal structures between will enhance stiffness quite impressively. It also argues for going a little stiffer on the hexagons, so that I can go a bit thinner on the tubes. Labels: telescopes
posted by Clayton at 2:34 PM permalink
Thursday, May 17, 2007
The Big Bertha Rebuild Project
I mentioned this yesterday, and I know that some of you are very interested in it (or are really desperate for something to read). For those who are wondering why I care about a deflection measured in hundredths of inch, when I almost certainly can't make all the parts that accurately--it's very simple. With the telescope sticking straight up in the air, there will be no deflection. With the telescope pointing at the horizon, a few hundredths of an inch of deflection will screw up the collimation of the optical train. If I collimate for one position, that much deflection will screw up collimation in the other position. As far as I am concerned, deflection needs to be down in the thousandths of an inch area before I am happy. I went to Metals Supermarket today to look at what they had in stock, and see if comparing the stiffness of the square aluminum tubing with what the formulas tell me passed the giggle test. Yup! I tried to bend a 3", .125" wall piece of 6063T6 aluminum, and a 3", .25" wall piece of 6061T6 aluminum. Yes, extremely stiff! There's no difference in stiffness between 6061 and 6063--although 6061 is a bit harder. Both have a 68.9 gigapascal modulus of elasticity. The good news is that when I went out to measure the dimensions of Big Bertha, I discovered that some of my assumptions about the dimensions were wrong. The mirror weighs 26 pounds, and it is only 23 inches from the balance point for the telescope. This lower weight and shorter length substantially reduces the point load length and somewhat reduces the beam load length. This lets me use either a somewhat smaller tube, or get less deflection. Using real data, a 3" square tube with .125" wall would give me .001" total deflection from beam load and point weight load--and at least at this point, it appears that my total telescope weight will be somewhere around 46 pounds. I can get the total deflection below .001" by going to a 4.25" tube, which brings the telescope weight up to 48 pounds--still acceptable. I am still trying to find out if using two smaller tubes is additive--if it distributes the load across both tubes, and thus cuts the deflection in half. I have an email into a friend with a PhD in Mechanical Engineering--I'm hoping that he is educated enough to answer the question! As I mentioned, I have to build my own mirror cell to fit the rather odd geometry of not having a tube, but I think have come up with a design involving a hexagon that will work. I can't turn a piece of aluminum 17.5" inside diameter (as tempting as it is), but I think the solution is to make a hexagon from pieces of aluminum bar stock, cutting 60 degree corners, then drill, tap, and screw them together at the corners. I can use a similar, although slightly larger hexagon to suspend the diagonal mirror from, and on which to mount the eyepiece focuser and finder. I may build a small version of this first to house the 3" f/4.5 reflector I built some years ago--a chance to verify the design in Delrin. If it works in Delrin, aluminum should be no problem. Yes, the weight of something like this goes up with the cube of the increase in linear dimension, but aluminum has a somewhat higher modulus of elasticity than Delrin, so I suspect that if it works for the 3", I won't have to do much to make it work for Big Bertha. Labels: machining, telescopes
posted by Clayton at 8:49 PM permalink
Tuesday, May 15, 2007
I Am Always Impressed How Smart My Readers AreI mentioned here the engineering problem I was confronting on rebuilding Big Bertha, and the formulae provided by a reader for calculating deflection under load. I pulled together the appropriate data, and this is what I found. First of all: I discovered that Young's modulus is expressed in Pascals (the metric unit of pressure). Because the Pascal is defined as one newton of force over a one square meter area, I needed to convert all my numbers from centimeter/gram/second to meter/kilogram/second. Next, I used the equations to figure out the moment of inertia for a solid rectangle, rectangular tube, and a square tube. (I'm not considering the use of a round tube, simply because it is harder to get a solid connection between two pieces of metal if one of them is trying to roll.) I plugged in dimensions for several commonly available 6061 aluminum shapes, and the moment of inertia came out like this: | section | moment of inertia | kilograms per meter |
| rectangular solid (2" x 0.5") | 0.000000000009 | 1.74 |
| rectangular tube (3" x 1" with .125" wall) | 0.000000007 | 1.63 |
| square tube (2" x 2" with .125 wall) | 0.000000126 | 1.63 |
You can see what a difference a square tube makes relative to a rectangular tube of the same weight, or a rectangular solid that weighs slightly more! There are two loads to consider for computing deflection: the point load (which assumes a weight that is concentrated at one point), and the load that the weight of the tube itself inflicts. When I plugged in those formulae, using a weight of 40 pounds for the mirror end of the telescope (which is by far the heaver load) and a distance of 40 inches (it will actually be a bit less, depending on the balance point), the square tube gave a deflection of .00069 meters for the point load, and .00056 for the beam load. I'm told that adding these together is probably sufficiently accurate for these purposes, so that comes to .00072 meters, or about .028". That's not quite sufficient (especially because it will vary depending on whether the telescope is pointing horizontally or vertically), so I could either go to a larger tube, or plan on using two of them. Going to a 3" x 3" x .125" tube more than triples the moment of inertia, and knocks the deflection down to .008". I suspect that it may make more sense to use two or even four of the 2" square tubes instead. I don't know exactly how they would reinforce each other, but I suspect that two tubes would halve the point load per tube. UPDATE: One advantage of using a single large tube to mount everything--it makes it easy to adjust distances. When you are doing astrophotography, you either need a lot of adjustment range in the focuser, or you have to move the mirror closer to the camera than would be needed for visual use (typically 2" to 2.5" closer). It would be fairly use to drill two sets of holes in the base tube: one set for mounting the diagonal/eyepiece/finder cage at astrophotography distance, and another set for visual distance. It might even be possible to do this on a sliding mechanism with set screws to lock everything into position. Labels: machining, telescopes
posted by Clayton at 2:22 PM permalink
Monday, May 14, 2007
Mechanical Engineering QuestionYoung's Modulus tells you have much a piece of metal will bend under strain; it's a measure of stiffness. This website (which sounds like they know what they are doing) claims that titanium and aluminum are about 5% stiffer than steel for the same weight. Steel is much stiffer for the same thickness than aluminum, but steel is about 7.8 g/cc, while aluminum is about 2.7 g/cc. For the same stiffness as steel, you use a much thicker piece of aluminum--but you can still end up ahead on weight--and weight matters for this application. I'm looking to rebuild Big Bertha in a form that is substantially lighter. Right now, it is unnecessarily heavy--perhaps 250 pounds, which is absurd, considering that the optics, eyepiece focuser, and mounting hardware for the optics weigh perhaps 45 pounds. If I just replaced the current wooden superstructure with Sonotube, I can get the telescope down to 110 pounds--perhaps less. But an equatorial mount that would handle it would cost $6000--and even then, it would be a bit heavy for that mount. There's a saying in backpacking that every pound you take off your feet is equivalent to five to six pounds out of your backpack. The same is true with telescopes: for every dollar you spend taking weight off the telescope, you can save many dollars on the mount itself. As an example, the Losmandy GM-8 mount that I have is nominally capable of carrying a 30 pound instrument, and costs about $1500. The next step up is the G-11, nominal capacity 60 pounds, and costs about $2200. The next step from there is the HGM Titan, nominal capacity 100 pounds, and costs about $6000. I don't even look at the step up from that; it makes my wallet scream in agony to even think about it (and even the HGM Titan causes me to cringe at the price). I don't know that I can get Big Bertha light enough to fit on a G11, but if I can, it is worth spending a bit of money lightening it up enough to do so. If it is just too heavy for a G-11, then it will be at least light enough to cause no strain for the HGM Titan. So, one strategy is to build an octagonal skeleton tube, using eight aluminum tubes 81" long, cross braced with perhaps 1/4" thick aluminum flats, all bolted together with stainless steel bolts. How thin can the 81" long tubes be, and still provide sufficient stiffness once cross braced? I need it to be stiff enough that even with roughly 38 pounds at one end (where the mirror is), and about 7 pounds at the other end (diagonal mirror, its holder, and the eyepiece focuser), the deformation of those tubes by gravity will be barely measurable, or not measurable at all. (The telescope will be mounted at approximately the center of gravity, with a plate that will bolt to two of the long tubes.) There must be a way to calculate this--I know that mechanical engineers don't do everything by experiment. The telescope pictured here might be another strategy. It uses a single, extremely stiff member to hold the two ends in the correct positions. Perhaps this is another approach, since this very stiff member could also be the dovetail plate that slides into the telescope mount saddle. One other aspect of this design that is attractive--it is possible to disassemble it quickly into two "rings": the top one carries the eyepiece focuser, diagonal, and finder, and the bottom ring carries the primary mirror. These can attach to the base with several bolts, making it quickly come apart into two relatively light and compact parts, and one very long rail. UPDATE: One of my readers responded with: There's a measure of flexural rigidity for a beam which you can use to compare some alternatives (it doesn't account for twisting though). It's simply the elastic modulus of the material multiplied by the cross-sectional moment of inertia (E*I).
The moment of inertia for a solid circular rod is I = pi * (d^4) / 64. For a tube, you just subtract the inner moment from the outer one.
Circular Tube: I = pi * (d1^4-d2^4) / 64 or I = pi * (d^4-(d-t)^4) / 64
Solid Rectangle: I = b*h^3 / 12
Rectangular Tube: I = (b1*h1^3 - b2*h2^3) / 12
Square Tube: I = (w^4 - (w-t)^4)/12
For a point load (W) at the end of a weightless beam supported firmly at the other end, the vertical deflection at the loaded end is:
dy = W*L^3 / (3*E*I)
The angle of rotation at the end of the beam is:
theta = W*L^2 / (2*E*I)
You can consider the weight of the beam a distributed load (q, in lbs/in for example), so without a point load you have:
dy = q*L^4 / (8*E*I)
theta = q*L^3 / (6*E*I)
It's probably accurate enough (for small deflections anyway) to just add the results of both sets of calculations together for the net effect. Where the ^ indicates exponentiation. Labels: telescopes
posted by Clayton at 1:19 PM permalink
Monday, March 12, 2007
How Can You Tell That You Are a Serious Telescope Geek?When you mark on your calendar that the Travel Channel series Made in America has an episode where they visit the Astro-Physics manufacturing plant in Illinois.Labels: telescopes
posted by Clayton at 2:57 PM permalink
Sunday, March 11, 2007
Brass Compression RingsWhat, you ask, is a brass compression ring? My first telescope eyepiece focuser was a cheap Edmunds rack-and-pinion unit that was a press fit. To put an eyepiece into the focuser, you pressed it in, and the split in the upper part of the focuser tube spread just enough to hold the eyepiece. This wasn't ideal, because it tended to scratch the eyepiece barrel, and the force required often moved the telescope. My first focuser upgrade was to a University Optics focuser that used a set screw in the side of the focuser tube to hold the eyepiece. Typically these were a 4-40 set screw that grabbed the side of the eyepiece barrel. They were zero effort to insert and remove (once you loosened the screw), but the set screw approach had two annoying side effects: the set screw, being steel, often scratched the finish of the eyepiece barrel; because the friction holding the eyepiece in place is exerted over a very small contact patch, very, very heavy eyepieces, or camera assemblies, might not be held in as tightly as they should. And so, a few years ago, I started to see focusers that offered a "brass compression ring" instead. What was it? I thought that you turned something on the outside of the focuser tube, and it squeezed down against the eyepiece barrel. Nope! Instead, you still have a set screw (or perhaps two of them) on the outside of the focuser tube, but instead of directly squeezing on the eyepiece barrel in just one spot, the set screw (or screws) press down on a piece of brass that sets in a channel inside the focuser tube. The brass started out straight, and was bent to fit into the channel. It is not a complete ring; perhaps it is an 1/8" or 1/4" short of making a complete circle inside the channel. The brass ring's desire to straighten out again prevents it from easily slipping out of the channel. When you put pressure on the brass ring with a set screw, the set screw presses the brass ring against the eyepiece barrel--and because of the confining channel, the ring is pressing against the eyepiece barrel across a big chunk of the circumference of the eyepiece barrel. Because the pressure is spread out across perhaps 1/4" by 2" of area, you don't have a single point that cuts through the chrome surface of the eyepiece barrel. Because the total contact area is substantially larger than a single set screw (even though the pressure per square inch is less), a brass compression ring can exert more force to hold an eyepiece in place than a single 4-40 set screw. Anyway, why am I yammering on about brass compression rings? One of my customers has a very high end Takahashi mount, and he doesn't want the 1/4"-20 bolts that hold the ScopeRoller caster assembly onto the legs to scratch up the surface. My solution? I'm going to bore a channel perhaps 1/8" or 1/4" deep inside of the sleeve that goes over the leg, and put a brass compression ring in that channel. The 1/4"-20 screws will not be directly impinging on the legs. Instead, the brass compression ring (which is softer than steel) will be doing the pressing. Of course, along with ordering some 3" Delrin stock tomorrow (why aren't these ever a size that I already have around?), I have to go brass hunting. Labels: machining, telescopes
posted by Clayton at 9:10 PM permalink
Saturday, March 10, 2007
Clear Skies!The 8" f/7 reflector is out cooling right now--I'm going to try and take some astrophotographs of the Orion Nebula (M42) in another 20 or 30 minutes. UPDATE: And by the time the mirror had cooled...the clouds were coming in. Labels: telescopes
posted by Clayton at 8:36 PM permalink
Wednesday, March 07, 2007
Clear Skies Didn't LastA bit after sunset, I was able to get Big Bertha collimated and aimed at Saturn--and the new focuser really makes a difference. The fine focus knob let me bring Saturn in to a very sharp focus at 222x. It still isn't quite as sharp as the 5" refractor, because fundamentally, Big Bertha's mirror is only so-so, but still, I could see at least one cloud band on the planet itself. Cassini's Division was only visible at the ansae of the rings, partly because Saturn was still not terribly high in the sky. Then the clouds came in. Maybe I'll get lucky tonight. Labels: telescopes
posted by Clayton at 9:17 AM permalink
Sunday, February 04, 2007
Installed the Moonlite CF2 On Big BerthaIt was only a minor chore: drill new screw holes in the tube; enlarge the hole for the drawtube; cross-thread a nut on one of the stainless steel screws, and then break the screw trying to remove it; discover that Lowe's doesn't have 10-32 stainless steel screws, but they do have 10-24.  The "coarse" focus is quite a bit finer than the old focuser, and the "fine" focus barely feels like it is moving: 0.0725" per revolution of the knob. High clouds came in after I finished recollimating it, so this will have to wait for better weather. Labels: telescopes
posted by Clayton at 10:19 PM permalink
Thursday, February 01, 2007
Books Trickling In; My Relative Absence From BloggingYesterday, one of the three expected boxes of my new book arrived. Today, another box arrived. Since all three were mailed the same day, I'm utterly mystified why they are arriving day by day. But they are arriving! I have not been blogging much because I am busily trying to make ScopeRoller products. There has been an unexpected rush of orders the last few days. In the interests of efficiency, instead of making one at a time, I'm trying to make several sets alike in one operation, both to save time moving tooling around, and because it produces a more consistent set of results. But the net result is that I get very little free time in the evenings to blog. Also, the Moonlite Telescope dual rate focuser arrived--and considering that it ended up costing $330, I would say that I more than got my money's worth. The finish, the smoothness of the focuser, and the general appearance--wow! It makes the rest of Big Bertha look like it was assembled by cavemen--and gives me reason to replace Big Bertha's tube with something a bit less savage. The good news is that 20" inside diameter Sonotube (an excellent lightweight choice for this) will cost me about $35! Once I get done using some epoxy to make it into pseudo-fiberglass, it will look dramatically sharper, and weigh perhaps 60 pounds less than the current Sonotube and wooden construction. Labels: telescopes
posted by Clayton at 11:15 PM permalink
|
