Measuring Quality in Small Refractors

How do you measure the optical quality of a small refractor?Like quite a number of amateurs, I’ve wondered why serious amateurs spend so much money for the top of the line refractors: Televue, Astro-Physics, Takahashi.Besides the name, what else are you getting when you spend big money for these high reputation refractors?I have read raves (sometimes unbelievable raves) from other amateurs on the Internet—though seldom any detailed, quantifiable measurements of image quality.Occasionally, I run into one of these high priced refractors at a star party, but I have never had the opportunity to do a direct comparison between the high-end glass and the more reasonably priced refractors.

Some people insist that a telescope like the Ranger is overpriced, that no refractor so small can be worth so much money.I have seen these expensive refractors compared to fine jewelry, or a Ferrari—lots of style, wonderful craftsmanship, but little actual performance advantage over the much less expensive Corvette.Since I have long regarded prestige items with suspicion, and I am fundamentally a cheapskate, this status symbol theory sounded very satisfying.But while satisfying, I still had a nagging suspicion that the Televue fanatics were right, that you could buy spectacular optics in small packages.

I have seen an enormous range of seeing conditions over the last twenty-five years, with various combinations of turbulence, transparency, and darkness.As a result, I know that user reports from different locations, places, and times, are simply not a meaningful way to compare telescopes.Variations in eyepieces are also significant.Some of the cheap refractors have short focal length Ramsden eyepieces, completely inappropriate for the telescope and the mount.These excuses for eyepieces would make even a good telescope perform as if it had an objective made from the bottom of a Coke bottle.

Most depressing of all, at least for those of us with 40 year old eyes, is that even with corrective lens, some people simply see better than others.I sometimes apologize for poor viewing conditions with, “I can’t see much on Jupiter.Under better viewing conditions, you can see belts,” only to have other people excitedly report that they can see four dark belts across the planet.

The only way to get a meaningful answer to my questions was to do a side-by-side comparison: the same viewing conditions; the same eyepiece types; the same observer.While this methodology does not give any absolute measures of what a scope can do, it does give a relative measurement of image quality.

For my testing, I bought a Televue Ranger, and borrowed a Meade 221S 60mm achromatic refractor.The Meade 221S is quite similar in specifications and appearance to a number of other 60mm achromatic refractors now on the market.In addition, I borrowed one of those much criticized “department store” telescopes from a co-worker—a 60mm achromatic originally sold by Sears some years back, but purchased by my co-worker for $20 at a garage sale.(The accessories and packaging of the Sears reflector provide some examples of why these criticisms are valid.The telescope included 12.5mm and 4mm Ramsden eyepieces, and a 3x Barlow—for a telescope whose unsteady mount precluded using even the 12.5mm eyepiece.)
 
Name
obj. diameter
obj. focal length
focuser I.D.
Televue Ranger
70mm
480mm
1.25”
Sears
60mm
700mm
0.965”
Meade 221S
60mm
800mm
0.965”

So that we would be comparing the telescopes, not the eyepieces, I used orthoscopics of very similar design.I suspect that underneath the importer’s brand name (Celestron, University Optics) the eyepieces were if not brothers, at least cousins.

I paid $599 for the Ranger (though I’ve seen a few places offer it for $525). C.O.M.B. (a mass market liquidator of discontinued merchandise) sold the Meade 221S for a bit over $100 three years ago.The Sears probably cost something equivalent when first sold.

The Ranger, of course, did not include a mount of any sort (see the sidebar article, “Mounting Your Ranger”).The Meade 221S came with an unsteady, self-disassembling alt-azimuth mount.The Sears came with a mechanically similar alt-azimuth mount, but it was considerably steadier than the Meade.My goal, however, was to compare the optics, not the mounts.

I did the testing on a cold, clear, somewhat turbulent November evening. For reasons of convenience, I did the test from my suburban drive, but this is also the environment in which the vast majority of small refractors will be used (or at least that’s my rationalization for doing it in a convenient location).I chose Saturn as my test subject because the ringed planet is one of the most consistent sources of “Wow!” for beginners.Saturn also represents a realistic target for small refractors.Saturn is bright enough and large enough for a 60mm refractor to show a recognizable image.At the same time, Saturn provides a bit of a test for an optical system under less than ideal conditions—unlike the Moon, which provides a very satisfying image in almost any telescope.(Of course, Saturn’s low ring angle at the time I did this test made it a more challenging target than usual.)

Because the Meade and the Sears used 0.965” focusers, I bought a 1.25” to 0.965” adapter so that I could use the same family of eyepiece in all three telescopes.Because long focal length 1.25” eyepieces showed significant vignetting with the adapter, I limited the test to medium and high powers.

The Meade and Sears telescopes had more than a slight resemblance to each other, and were nearly indistinguishable in optical quality, with perhaps a bit more chromatic aberration in the Sears.They gave essentially identical images, except for the Meade’s slightly higher magnification because of its longer focal length.Because the Sears had insufficient focus travel to use the 1.25” to 0.965” adapter with some of the test eyepieces, and because the Meade and Sears images were essentially identical, I completed the test with the Meade and Televue telescopes only.

Because of the differing focal lengths, it was impossible to get exactly equivalent magnifications.For medium power, I used a University Optics 9mm orthoscopic in the Televue Ranger, and a University Optics 12.5mm orthoscopic in the Meade.These gave 53x and 64x, respectively.In both cases, the disk and rings were easily visible.But the Meade’s disk was fuzzy, with one side of the disk noticeably blue, and the other side noticeably red.The Televue image was crisp, with a sharp edge to the disk, and no color fringes.The rings in the Meade were noticeably thicker than those in the Televue, which at this low power appeared to be knife edges protruding from the sides of the disk.Titan was visible as a small bright dot, indistinguishable between the two telescopes.

At higher power, the differences between the two telescopes became even more obvious.With a 4mm orthoscopic (a “no name” brand), the Televue gave 120x.The closest equivalent magnification for the Meade was with a Celestron 6mm orthoscopic that produced 133x.The Televue’s image was still crisp, and I could now see what appeared to be the shadow of the rings on the planet.The rings and the disk were now distinct, with a clear separation between them.In the Meade, Saturn was a complete fuzzball.I could not distinguish the rings from the planet at all.

Obviously, I had exceeded the useful power of the Meade (at least for Saturn).What about the Televue?Saturn was still crisp, so I put in the Celestron 6mm with an ancient 2.8x Edmund’s Scientific Barlow, giving 224x.Saturn was still crisp, though turbulence was beginning to cause “burbling” of the image.More magnification would not have been useful, at least under the current viewing conditions.(On other nights, I have found 267x in the Televue still provided a reasonably crisp image of Saturn—but 336x degenerated into fuzz.)

What are the lessons to be learned from this comparison?First of all, there is a difference in image quality.Yes, the Ranger has a 20% larger objective, and we should expect 20% better resolution, and 44% more light.The Ranger, however, provided a far better image than the Meade at almost twice the magnification, with similar eyepieces, the same observer, and the same viewing conditions.Unlike some other consumer products, where more money often means little or no difference in performance, the difference between the Ranger and telescopes that might at first glance appear “similar but cheaper” was dramatic.

Mounting Your Ranger

The Televue Ranger and Pronto don’t come with a mount.Neither do a number of other premium small telescopes.The Televue Ranger includes a “balance bar” that allows you to mount the telescope on a standard ¼-20 camera tripod.Unfortunately, the weight of the Ranger, and its length, mean that when you point the telescope high into the sky, even the best camera tripods become a handful to use.Holding a steady position while tightening the tripod’s clamps turns into a battle.At high power, the very minor adjustments that you need to track an object are impossible.(And the Ranger is one of the lighter small refractors!)

Televue and Celestron both sell very high quality—and very expensive—alt-azimuth mounts.These certainly provide one solution, but at high cost, and with no astrophotography potential.The Great Polaris equatorial mount provides another solution, but even more expensive, and much heavier than a lightweight telescope like the Ranger needs.

In digging through the Orion Telescope and Binocular Center catalog, I found what seemed to be an interesting alternative: the EQ-2 mount.It is cheap, reasonably compact and light, and quite adequate for a small telescope.This is the same mount that comes with the Celestron Firstscope 80 EQ.It’s manufactured in China, and has the materials and finish that you might expect from a Chinese product.But it only cost $129, and it’s compact and light enough to throw in the car on a moment’s notice.

The slow-motion controls in right ascension and declination are smooth enough, and precise enough, that even at 224x, I can easily track objects across the sky.The right ascension and declination axis clamps apply smoothly, without perturbing the telescope’s point of aim.(The clamps on some mounts that I have used apply torque to the axis as they tighten, causing a slight but irritating rotation.)The mount is rigid enough to quickly dampen any tube motions or vibrations.

The mount has provision for a clock drive ($84.50 + S&H), but I have found the slow-motion controls so delightful that unless I am doing astrophotography, it just is not worth the effort of running an extension cord.One initial annoyance was that the slow-motion controls do not work consistently unless you clamp the corresponding axis.Unfortunately, Orion sells this mount without any sort of manual, and it took a bit of experimentation to figure this out!

The setting circles are black with white markings.They are not engraved, but the markings seem to be baked on, so I’m not worried about losing the markings anytime soon.The RA circle is marked in ten minute intervals, and the declination circle uses one degree divisions.With a little effort, most observers will be able to interpolate to half a degree of declination and five minutes of right ascension.One nice touch is that the RA circle can be locked to the polar axis with a thumbscrew, so that the circle is driven by the clock drive:A number of equatorial mounts don’t provide this option.They only allow you to use the RA circle for relative positioning.This seems like a very poor alternative to a driven RA circle.

The polar axis adjusts from 0º to 90º, with one degree markings on the polar adjustment scale.The polar adjustment clamping is precise, even with a telescope mounted on it.(I cannot say the same for some other mounts I have used over the years, where I needed large wrenches, and there were no degree markings at all!)The RA circle is marked in both directions, so that you can use the mount in the Southern Hemisphere, assuming that the clock drive is cooperative, and provides a North/South switch.Unlike some of the Polaris family mounts, there is no provision for a polar axis alignment scope.It would have been nice if the mount had some sort of sights (like a rifle) to ease eyeball alignment of the polar axis with Polaris.

The tripod legs are wooden, adjustable for height so that the telescope can be anywhere from 37.5” to 53” off the ground.The leg clamps are surprisingly positive in their application, considering how simple their materials and construction are.They are also reasonably convenient, even while fumbling around in the dark.On some small telescope mounts, a chain prevents the legs from splaying all the way out (until they are parallel to the ground).That’s not a terribly rigid design, but it does simplify folding the legs inward to carry away the telescope.On the Orion mount, a metal accessory tray holds the legs in place.This seems to improve rigidity, but at the cost of easy folding of the legs.This is not to say that you can’t fold the legs—but the first time I unthinkingly tried this, the accessory tray crumpled, though it straightened out again.(The accessory tray has a mixture of 1.25” and 0.965” eyepiece holders—meaning that for most amateurs, this tray will be always be half unusable.)For those of you with very small cars, the legs detach quickly from the base of the mount.Unfortunately, the accessory tray attaches to the legs with screws and wingnuts, precluding rapid assembly and disassembly of the three legs into a more compact state.

This mount has a couple of very nice touches to it that show some serious thought went into its design.The end of the declination axis has a flange on it that prevents the counterweight from slipping off the end.Another nice touch is that the counterweight section of the declination axis quickly unscrews from the mount, helping to compact the mount down to a very small package indeed.

Upon putting the mount together, I discovered one part missing: the thumbscrew that holds the declination slow-motion control knob to its screw mechanism.Orion shipped me the replacement part within two weeks—but it was the wrong thumbscrew.Fortunately, I was able to find a replacement screw at the local hardware store (5mm, standard pitch, in case this happens to you).

As Orion sells the mount, there is no tube attachment hardware.They sell a ¼-20 camera adapter plate for $23.95, and a couple of different ring assemblies.Because the ring assemblies were for 3.5” and 5.5” diameter tubes (both too large for the Ranger’s 3.06” tube), I bought the ¼-20 camera adapter instead.This mount with clock drive and the ¼-20 camera adapter would make a fine camera-only astrophotography tracking platform.However, the ¼-20 camera adapter has a few deficiencies when used with the Ranger, and probably any other ¼-20 threaded telescope. Because the adapter uses a single screw to locate the telescope, you can’t be guaranteed that the telescope’s position relative to the declination setting circle will be the same each time you attach it to the mount, rendering the declination setting circle of questionable accuracy.A somewhat more serious problem is that the adapter plate feels as if it could, in the normal course of use, loosen its grip on the Ranger’s balance bar, because of the Ranger’s weight twisting the balance bar around the ¼-20 stud.This might be followed by the sound of shattering glass, and then the sound of me, screaming.It never happened that the adapter lost its grip on my Ranger, but still…

The 3.5” ring adapter that I purchased next is for the Celestron Firstscope 80mm telescope.At the local hardware store I purchased a couple of 1/8th inch thick sheets of hard rubber, commonly used for making custom washers for plumbing.By layering two sheets of this material, I was able to nestle the Ranger inside the 3.5” ring adapter in a way that was secure, without applying excessive pressure to the Ranger’s tube.

The 3.5” ring adapter provides a very secure mount, though to make it fit the Ranger, I had to remove the Ranger’s balance bar.This isn’t difficult, since only a 1/16th inch Allan set screw prevents the balance bar from sliding off the scope.The telescope can be quickly removed from the ring by loosening a single thumbscrew on the ring mount.Loosening the same thumbscrew lets you easily reposition the scope within the ring.As an added benefit, the 3.5” ring adapter also has a ¼-20 screw on the top, for piggyback astrophotography.

Orion’s small mount isn’t a Great Polaris, of course—as even a brief encounter with a Great Polaris mount will remind you—but at roughly 1/5th the street price, it’s hard to beat it for equatorially mounting a small telescope.

By day, Clayton E. Cramer is a software for a northern California manufacturer of telecommunications equipment.By night, he writes history books and articles.Praeger Press published his fifth book, Concealed Weapon Laws of the Early Republic: Dueling, Southern Violence, and Moral Reform, in 1999.His web page is http://www.claytoncramer.com.