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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
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