Uncle Rod's Astro Blog

I see you out there, Joe and Jane Novice Amateur, looking all googly-eyed at advertisements for those pretty little refractors. You are not alone. More than a few veteran amateurs are not immune to the charms of 3 and 4-inch lens-type scopes. That’s not the question, muchachos. The question is, “What are they good for?” Are they worth the high prices (relatively speaking) commanded by color free apochromatic refractors or even the more modest sums achromatic refractors cost?

What’s the refractor story, anyhow? I ain’t gonna go through the whole nine yards concerning Hans Lippershey’s baby. I will just say the main threads of the refractor drama since telescope makers moved beyond the tiny lenses of Lippershey and Gallileo have concerned color and aperture.

The apertures of the early scopes didn't have to get much larger than an inch or two before astronomers began to be tormented by the color purple. What am I talking about? As you prob’ly know, refractors use lenses rather than mirrors to gather light. The big lens collects light and refracts the rays, sending them to a focus point. An eyepiece, a magnifying glass, if you will, can be inserted just behind that focal point to enlarge the image for inspection by an observer. Sounds simple. Much simpler than, say, a Schmidt Cassegrain telescope. It is, but, alas, there is a catch.

As objective lenses became larger, astronomers noticed brighter objects—the Moon, planets, bright stars—were ringed with purple halos. This “false color” was bad enough to reduce contrast and sharpness. What was happening? Glass refracts different colors (wavelengths) of light by different amounts. The same principle that allowed Newton to see the rainbow of the spectrum with a prism was ruining astronomers’ views of the sky. All the colors of light were not being brought to focus at the same point, resulting not just in color around objects, but, in the worst cases, a haze of purple that obscured objects.

The solution, at least a partial one, wasn't long in coming. Experimenters discovered long focal length lenses suffered less from this “chromatic aberration.” The longer the focal length of the simple single element objective lenses, the less obtrusive the color. Chromatic aberration was still there stealing sharpness, but it could be lived with.

The result was the infamous “aerial telescope” of the 17th century. Refractors were made in insanely long focal lengths—as much as 125 feet for a 7-inch aperture scope. A seven inch had to be that long. The larger the aperture of the lens, the longer its focal length had to be to keep chromatic aberration bearable. Double the aperture of the lens, and you had to quadruple the focal length. This effectively made a 4 – 6-inch telescope a big gun.

Tubes in these absurd focal lengths were impractical given the materials of the day—it was impossible to keep flexure from rendering the scope useless. The Huygens brothers dispensed with tubes altogether, keeping eyepiece and objective aligned with a taut rope. What is amazing is that they and others made some landmark discoveries with these crazy telescopes.

There had to be another, better way and there was. John Hadley and, later, Sir William Herschel and others, inspired by Newton, made the reflecting telescope into a powerful instrument for astronomy in the 18th Century. Without an objective lens, there was no chromatic aberration, and mirrors were not limited in aperture by the “two times larger, four times longer” rule. Still, the refractor had its fans. It was a more robust instrument compared to the reflectors, which used polished metal mirrors that began to tarnish as soon as polishing was done. If only there were some magical way of getting rid of the dadgum color purple.

The fix for the refractor, or at least a more practical partial fix than 100 foot tubes, came in the mid 18th Century. A London barrister, Chester Moore Hall, discovered that a two-element refractor objective lens made with two dissimilar types of glass with different indexes of refraction reduced chromatic aberration tremendously. Hall tried to keep the secret of his "achromatic" refractors, but it soon got out.

While the achromat was a big improvement over the single element objective, it was not a complete cure. An achromat will show considerable color on bright objects at faster focal ratios, and, as with single element objectives, the larger the objective, the higher its focal ratio has to be to keep color down. A 6-inch achromat needs to be near f/20 to be (mostly) color free.

The achromats were a revelation and the savior of the refractor. Nevertheless, astronomers still wished for that elusive More Better Gooder. For most amateurs, that had to wait till 1977, when Japan’s Takahashi began selling the first widely available commercial apochromatic "color free" refractor. Tak led the way with telescopes that used fluorite lens elements, but they were soon joined in the apochromat business by today’s big names is refractors, including Astro-Physics and TeleVue.

The principles of the “APO” refractor objective had been known since the late 19th century: lenses with more exotic glasses than crown and flint, sometimes including fluorite crystal elements, and objective configurations sometimes consisting of three or even more elements. What it took for the apochromatic refractor to become a practical reality was modern materials and fabrication techniques. And, most of all, an audience, serious amateur astronomers, who began to appear in numbers in the mid 20th Century—professionals had long since discarded the refractor.

The APO’s downsides? Cost and aperture. Perfection in the form of exotic and perfectly figured glass don’t come cheap. It is difficult for manufacturers, even given an unlimited supply of dineros, to find decent glass blanks of the special glasses good enough to make an APO lens. For the well-heeled amateur, a 7-inch APO is purty much the limit, and, in today’s terms, a 7-inch is a small scope. For the less affluent, like your old Unk and many of y’all, even smaller, 3 or 4-inches, is more like it.

Which brings us back ‘round to that sixty-four dollar question, “What can a reasonably priced 3 or 4-inch refractor do for you?” From a suburban backyard or from a semi-dark club site? Before I could refresh my memory as to the facts in that case, I had to locate the 80mm f/7 APO refractor that has lived with me for many a year, but which hasn't gotten much use lately. She's a beautiful little scope, a William Optics Megrez fluorite, but with the freaking Herschel Project going pedal-to-the-metal for several years, this little person, Veronica Lodge, had sat in her case for a weary old time.

Out to the shop went Unk to hunt up Miss Veronica; I remembered stowing her case out there during the move to the New Manse. There she was. Opening up the nice WO aluminum box revealed my little gal pal, who now has nearly a decade of miles on her. Her paintjob ain’t what it once was, and her poor dewshield ain’t exactly round anymore, the result of a fall to the floor of an observatory she took when she was new (don’t ask). Still, she looked good, with her pretty off-white tube, gold trim, retractable dewshield, and hefty Crayford rotatable focuser. All the parts and pieces was in the case, too, including the 2-inch WO extension tube necessary for almost anything to reach focus with her. Even the stalk for the red dot finder was in its accustomed place.

All I had to do to get Missy ready to go was gently clean her objective, which had a little gunk from who knows where on it, but which cleaned up nicely and soon looked pristine, and mount the finder, a selectable-reticle red dot job that I normally use with my Zhumell100mm Tachyon monster binoculars.

The mount? For the quick backyard run I contemplated, that would be my SkyWatcher AZ-4. I bought this alt-az mount (which you can still get in the U.S. of A. from Orion, if at a higher price than the SkyWatcher-badged version) for the C90, but it’s come in handy for everything from refractors to C8s to Unk’s homebrew f/6 6-inch Newt. There is a Vixen compatible dovetail screwed onto the scope’s “foot” that would allow me to mount Veronica on the VX GEM or the Atlas EQ-6, but I demurred. It would be a relatively hazy evening more than like, and I’d be a little tuckered from teaching my late afternoon astronomy lab, I figgered “AZ-4 on the deck” would be just about freaking perfect.

One good thing:  here in the second week of September, darkness is now arriving at a more reasonable time, DST or no, with it dark enough to do some gazing by 8 p.m. When that hour arrived, I waltzed Veronica onto the deck, and went back inside to hunt up the eyepiece case and to give the scope a little while to adjust to the steamy outdoor temps. While a small refractor will normally present almost perfect images without acclimatization to outdoor temperature, it was so hot and muggy I didn’t dare remove the aperture cap for a while—the objective, coming from the air-conditioned house, would have fogged-up in a heartbeat.

Anyhoo, by the arrival of astronomical twilight just before 8:30, both scope and eyepieces had warmed up and plenty of stars had winked on. Yes, there were a few passing clouds early on, but a look north showed magnitude 4.3 Zeta Ursae Majoris was easy to see despite its relatively low altitude from out latitude. I know that ain’t exactly dark sky heaven, but it’s better than what I’ve had to deal with at home for the last quarter century and I know it’s better than what a lot of you put up with. On the other hand, the sky was certainly bright enough to provide a test of what a small lens-scope can do from a light-polluted backyard.

First stop was bright Arcturus, now getting low in the west. I wanted to check Veronica’s collimation on a bright star, given the abuse she’s taken over her life. Defocused the yellow star and, yep, alignment was still dead on. That is one of the beauties of refractors, y’all. You’ll often read a refractor will only need “infrequent” collimation. Actually, assuming your scope is of decent quality, it’s unlikely you will ever have to worry about collimation. Despite the only so-so seeing, I couldn’t resist giving Miss a quick star test, too. The patterns in and out of focus were, to my eyes, identical.

“What about color, Unk?” What color? I didn't even think about that. There was simply none. Not on Arcturus. Not on Vega. Zip. Zero. Zilch. It’s been so long since I’ve looked at the Moon with the scope that I can’t remember what Luna looks like with the Megrez, but I am guessing it is every bit as good as these bright stars color-purple-wise.

Now for rubber-meets-road time: “Can an 80mm refractor, no matter how good, satisfy in the backyard? On the deep sky?” Off to M13 we went. Given the wide field of the scope with my 100-degree 16mm Zhumell Happy Hand Grenade in the (Intes) 2-inch diagonal, about 3-degrees, finding M13 or anything else, even in relatively poor skies was trivial. There was Daddy-O Messier 13. Even at 34x, I had to admit he was looking dang good. Better than I remembered him being under similar circumstances in my 4.5-inch StarBlast Newtonian, and certainly better than he’d ever been in my good, old Short Tube 80 80mm f/5 (who now lives with Unk’s son, Chris). But I was not prepared for just how much better.

The view sure was nice in the 16mm 100-degree eyepiece. The globular looked surprisingly grainy, and the field stars was just so purty. So small. Yes, I know, stars just naturally look smaller in a small scope at relatively low power, but I ain’t gonna lie to you:  stars are sharp and oh-so-pretty in a lens scope. Period. Anyhoo, in my quest for More Better Gooder, in went the 8mm Ethos.

DADGUM! (This is a family-friendly blog, y’all.) The silly little scope was actually resolving the Great Glob. No, it wasn’t a great big ball of stars, but it had gone from “grainy” in the 16mm to “real grainy” in the 8mm and, yes, as I stared I could see—with direct vision—cluster stars winking in and out. That, campers, is at least as good as what my 4.5-inch StarBlast can do from a dark site under better conditions and with higher magnification.  The magic wasn’t just due to Veronica being a refractor, however; it had to do with her being a very good refractor. The Short Tube 80, much as I loved the little guy, couldn’t make M13, which is a fairly tight glob, look like anything more than a fuzzball from the darkest skies.

The results were similar for every DSO I pointed Veronica at. The Dumbbell Nebula, M27, showed form and substance. M92, Hercules' “other” glob, was small but grainy, almost wanting to resolve. M12, the loose globular over in Ophiuchus, didn’t just want to resolve; it broke into stars despite its low altitude. M57, the Ring Nebula, was obviously a ring. Under poor skies, M57’s donut hole is sometimes difficult or impossible to make out with a 4-inch reflector.

By 10 p.m., the bugs was biting, Unk was sweating, and clouds was moving it. One last look. How about Polaris? As most of y’all know, Polaris is double star. It's easy with a C8, but can be a challenge with a smaller scope. While Polaris' buddy is a sizable 19” from the primary, the differences in magnitudes—9 vice 2—can make it difficult to see. Veronica? She laughed, giving me a glimpse of the companion star just as clouds moved across the field.

And, with that, it was time to pull the Big Switch. Luckily, an 80mm APO on an alt-azimuth mount equates to “little switch.” In 5-minutes, I was back inside enjoying a cold one and watching a DVD of Star Trek the Animated Series. What was next? While I have a lovely little WO SD 66mm APO (or maybe “semi-apo,” whatever that means) refractor who performs admirably under dark skies, I believe she is just a bit on the wee side for a bright backyard. Up next would be a pair of scopes that have a little more aperture, 4-inches, if considerably less ritzy pedigrees.

With my Short Tube 80, Woodstock, gone, there are “only” three achromats left in the stable, an Explore Scientific AR-102 (mm) Miss Dorothy won at last year’s Deep South Regional Star Gaze, an older Celestron (Synta) C102 f/10 4-incher, and an 80mm SkyWatcher f/11. I decided to leave the 80mm SkyWatcher out of the running for now, since I was purty sure she couldn't compete with the Megrez, and concentrate on the 4-inchers. The ES would get first crack at the New Manse’s backyard.

Unlike when you take your first glance at the Megrez, your initial impression of the AR 102 will likely not be “pretty,” but “odd,” or “gawky” or maybe, if you are being charitable, “different.” That’s not because of its beautiful gleaming white tube, but because of the peculiar dewshield—short and fat—the AR’s Chinese maker, JOC, bestows upon its refractors.

Look deeper, however, and I think you will agree this is an extremely nice scope. The focuser is a large and smooth Crayford (though not rotatable like that of the Megrez) and, like the APO, includes a slow motion/fine adjustment on one of its two large (metal) focus knobs. Put a 35mm Panoptic in the diagonal and point the tube at the zenith and there is no slipping whatsoever. The included tube rings are heavy and solid. Finder? One is in the box with the scope, a good 50mm. There’s also an outstanding 2-inch diagonal. Is the build quality of the scope as good as that of the Megrez? Of course not, but it’s surprisingly good anyway. How the hell Mr. Scott Roberts and company do this for 500 bucks, I do not know, but I hope they keep on doing it.

That’s not the true test of a scope, however; that’s images. I had no illusions that the optics of the AR could keep up with the Megrez's. The APO's fluorite objective alone is worth considerably more than the whole AR102. I did, however, remember the AR had done an outstanding job on the deep sky from the dark observing field of DSRSG last year. The question was how well it would do from a semi-punk backyard.

To find out, I removed Veronica from the AZ-4 and secured her in her case, replacing her with the AR102, who hasn't yet told me her name. While lighter than a C8, the AR102’s tube is somewhat longer, but is not a problem for the AZ-4. The telescope looked good on the SkyWatcher mount, I gotta say.

As night fell, it was apparent the AR102 wouldn't enjoy quite as good an evening as Miss Veronica had. It was clear, with the aforementioned Zeta UMi shining steadily. And if I held my mouth just right, I could even see the dimmest bowl star, magnitude 5.0 Eta, without much difficulty. However, there were bands of clouds passing through all night. I had to pick and choose my targets, and even the “clear” patches were not as clear as the sky had been on the previous evening. Be that as it fracking may, after five decades behind the eyepiece, I thought I could give the scope a thumbs up or down, clouds or no.

I’ll cut to the chase, y’all:  the extra inch of aperture helped. If maybe not quite as much as I’d expected. That M13 was better in the ES was apparent as soon as astronomical twilight arrived. In a 7mm eyepiece (94x), it was everything it had been in the Megrez and a little more. It simply looked more like a ball of stars. I had little doubt that from a dark site, the Great Glob would have been looking awful good. At least as good as it is in my 5-inch MCT, Charity Hope Valentine. Part of the reason for that was that the 7mm eyepiece yielded slightly more magnification in the 4-inch than it had in the 80mm (80x), but that wasn’t the whole story. The extra inch of glass was picking up more photons and that made the star cluster better.

The results were the same with M92 and M12. They looked just a little better. M12 was slightly more resolved, and M92 actually showed some stars now and again. M15, while not at all resolved that I could see, was considerably superior to how it looks in the 4.5-inch StarBlast from better sites. Its halo was larger and the core was brighter than in my little green gremlin of a telescope.

Maybe not too surprisingly, the 1-inch advantage was not as apparent on nebulae. Oh, M27 looked right nice, but better than it had in the Megrez? Not really. The same went for M57. I thought it might be a step up in the AR102, but it was not. The little smoke ring’s appearance was identical to what it had been in Veronica the night before.

The color? There was color, you betcha, but really only on Vega. At 41x, Alpha Lyrae was surrounded by a considerable (blue, not purple) halo. At 94x, the halo went from “there” to verging on the disturbing. While careful focus minimized it, this is a short focal length achromat, and you will not banish the color. You can mask it with a filter, but it will still be there reducing contrast. That said, none of the deep sky objects showed any color whatsoever. Neither did bright Arcturus. This would not be a scope for looking at the brightest stars, but I am unlikely to do that with it anyway. Caveat? Some people (especially those with younger eyes with less yellowed lenses) are more troubled by excess color than others.

How about the Moon and planets? The Moon wouldn’t be above the trees till late, and the planets, Mars and Saturn, were down behind the treetops to the southwest. I have never tried the AR on the Moon or the planets. My guesses would be:  Venus purty much unbearable, Jupiter bearable and showing considerable detail, Mars OK, Saturn pretty, the Moon fine on the terminator with a loss of contrast on the disk. In other words, this scope would be alright for casual inspection of the planets, but not what you’d want if that is a regular pursuit. I will turn the AR to Luna, at least, one of these nights and report back.

One last impression:  while the AR102 had a little more light gathering oomph, its images were never quite as nice as those of the Megrez. The stars weren't quite as sharp, and the contrast was not as high. There just wasn’t as much “pop” in the images. This was not quantifiable, but was my impression, nevertheless, and one that was constant throughout the evening with the AR.

Hokay, then, only one contenda remained, the old Synta-Celestron, the “C102,” given me as a house-warming gift by my old friend, Pat Rochford. There is no question it is a good-looking scope. No, the tube ain’t as long as that of a freaking Unitron, but it's long enough and hearkens back to that classic “long refractor” look. That’s enhanced by the nice faux brass paintjob Pat applied to gussy up a slightly worn OTA. The dewshield is a normal long and skinny one, and the Synta tube rings, while minimalist, work fine and look OK.

The telescope’s focuser is a second-generation Synta rack and pinion. No, it is not nearly as smooth as the AR’s Crayford, but it works well and smoothly enough, and a rack and pinion has the advantage that it will not slip under most conditions. It is a 2-inch job, unlike those on some of the earlier Synta/Celestron 4s, and the pretty brass focus knobs Pat added complete the attractive picture. This sucka looks like a refractor.

The C102’s down-checks were few. The 30mm finder would have to go. Hell, I hadn’t much liked using the 50mm finder on the AR102 despite its bright images. This old boy prefers zero power finders. Luckily, I had a Synta red-dot sight that slid right into the mounting shoe on the scope. The original Synta dovetail was kinda pitiful, but it only took a couple of minutes to replace it with a nice ADM Vixen bracket that was sitting unused out in the Shop. The Celestron’s objective cell is not adjustable, but it’s unlikely you’ll ever need to adjust it, so that is a non-issue.  Finally, the coatings on the objective aren't as good as those on the AR102, whose lens tends to disappear in room light.

Back in her box went the AR102 and onto the AZ-4 went the Celestron. The scope’s tube is, obviously, considerably longer than the AR’s and not much, if any, lighter.  I wasn’t so much worried about weight as tube length, which is what really tends to cause trouble for a mount. I needn't have worried; the 4-inch f/10 was at least OK on the AZ-4. In fact, SkyWatcher used to sell the AZ-4 with a similar 4-inch refractor.

Yeah, I know us Baby Boomers still lust after the f/15 Unitron 4-inch Photo Equatorials of our youth, but dealing with the f/10 Celestron made me almost lose my desire for a true Long Tube. This was the only refractor I had to remove from the mount to get out the door easily. It was just too long and awkward otherwise.

It was also the only scope that impelled me to extend the AZ-4’s tripod legs to their maximum length. Even then, the tripod seemed too short. I can scarcely imagine what life would be like with an f/15. What’s the problem with pulling the legs all the way out? Any mount performs better with its legs at least partially collapsed, and that was sure true with my AZ-4 (which is, unfortunately, the model with extruded aluminum legs rather than tubular steel legs). The other two refractors were rock solid; the C102 was, yeah, shaky. Not fatally shaky, but shaky. 'Course, I was operating from the wooden deck; the scope would have been more stable on the ground.

Verdict under the stars? Celestron has been selling this scope in one form or another for over twenty years, and it is easy to see why. The optics are amazingly good. Not that I thought so at first.

Arcturus was shining bravely in the gloaming, so we went that-a-way so I could get the red dot finder aligned. In focus, the star was showing much more chromatic aberration than the AR had. Not a tremendous amount, but a noticeable amount compared to the wide-field refractor, which had shown no appreciable color around the star. I was puzzled, natch, but, paying more attention, I saw how much Alpha Boötis was dancing around. I decided seeing and/or tube currents were the culprits and went inside to await darker and, I hoped, steadier skies.

At 8:30 p.m., astronomical twilight had arrived and another look at the star showed it to be free of that nasty colored halo and shining steadily rather than hopping around. Next up was a collimation check. Alignment was dead on. How about a little star test, then?

Amazingly, this old warhorse threw up an essentially perfect star test, with the in and out of focus diffraction patterns being identical far as my old peepers could tell. The results were on a par with those with the Megrez, and a hair better than the star test of the AR102, which, while quite OK, had shown just a hint of undercorrection. I hate to be the breaker of dreams, but the C102’s optics tested as good/better than those of any Unitron I’ve examined over the years.

There was one last bit of drama—or comedy, more like. I wanted to see what the C102 would do with Polaris’ companion. Turned to the North Star and saw nothing—nothing but the yellowish primary. That was all. Even with the 7mm it was just a single star. Huh! Then, staring up at the sky in puzzlement, I realized I not looking at Polaris, but at Kochab. Doh! When I had the real Polaris in the field, you coulda drove a truck between primary and secondary.

What I really was curious to know was whether would this telescope’s fine optics and 4-inches of aperture would put it in the lead over the other two refractors. There was little question M13 was a smidge better in the C102 than in the AR102 and the Megrez, but only a smidge. And I think that improvement was mostly because the Celestron delivered more magnification eyepiece for eyepiece, 143x in a 7mm vice the faster 4-incher’s 94x, for example. Anyhow, in the C102, M13 was one more click closer to being a ball of stars. Frankly, what really amazed me was that all three refractors made something of M13 from less than perfect skies. That’s something my comparably sized Newtonians—including my beloved long focal length Palomar Junior—simply cannot do.

The story was much the same on the other objects I looked at. M92, M57, M27 and the rest looked very good, but not appreciably better than in the AR102 or Megrez. Yes, there was some gain over the Megrez attributable to that extra inch of aperture, but it was not a slam-you-in-the-face kinda thing. As with the AR102, the stars looked good and tiny in the C102, but the images just didn’t have the somewhat ineffable breathtaking quality of those in the APO.

My conclusions regarding 80 – 102mm refractors are that they can and will do a good job for semi-casual backyard observing. They can indeed satisfy. How do you choose between a smaller (and much more expensive) APO and a larger achromat, though? That is simple. If you just want to look—at the deep sky—you won’t go wrong with one of today's inexpensive medium-fast achromats. If you want a more versatile scope that can do the Moon and planets impressively well and is useful for imaging, however, you want an APO. Forget using an achromat for long exposure work unless you like purple halos and bloated stars. While you can get acceptable images with an achro, processing will be more involved and demanding. 

If you decide on an achromat, fine, I salute you. Don’t go hog-wild, though. A 6-inch achromat can be had for around 600 dollars these days, but don’t imagine it will necessarily give you a big dose of More Better Gooder. The light gathering advantage of a 5 or 6-inch achromat is at least somewhat offset by the increase in color and concomitant decrease in contrast.

My specific thoughts on the refractors I used over this series of nights are that all three amazed me with their capabilities. The true star was the little Megrez, though. Its images were just freaking terrific. While William Optics doesn't sell that particular model anymore, they have comparably good or maybe even better 80s for around 900 – 1000 dollars. Which, while it might sound high to the uninitiated, is a bargain once you start comparing prices for similar scopes from folks like Takahashi and TeleVue. The only surprise to Unk is that more observers don’t consider WO when buying refractors and eyepieces. Their products are still easily available in the states, including from one of Unk’s favorite dealers,  Agena Astro Products.

What’s next for our three stars? You’ll hear more about Veronica, the Megrez, in Part II when we look at small-refractor imaging, one of the biggest reasons for getting an 80-100mm APO. The achros will get their place under the stars in the near future, too. The AR, will be going with us to DSRSG next month. The C102 (who just whispered to Unk that her name is “Amelia”) will take over from the ETX when Unk continues his Messier Album series. Charity did a good job on that, but Amelia is much closer to the spirit and goals of that project.

The C102 back inside as clouds rolled in—as they tend to do in mid evening in late summer down in the Swamp—Unk grabbed the Rebel Yell bottle and ruminated. I have never been a refractor guy, muchachos. In fact, I have often laughed about the obsession for small and exquisite lens scopes. Maybe in part because of my advancing years, however, I have to admit small is becoming more and more beautiful. Especially when coupled with “real good.” I will never abandon my love for CATs, but, yeah, in his golden years, Unk just might change horses—once in a while, anyhow.

Next Time:  The Refractor Way Part II...