Sunday, August 17, 2008


SCT Mythology

There sure are a lotta tall tales floating around about Unk’s favorite telescope design. Some patently ridiculous, some containing at least a grain or two of truth. Why all the urban legendry? I reckon it’s due to the fact that the SCT, despite having been around in commercial form for dang near fifty years, has never had a whole lot written about it. Until recently, there hasn't been much hard information available on the scopes outside the manufacturers' brief and poorly written manuals.

Not as much as the SCT's status in amateur astronomy as the most popular store-bought telescope of all time would seem to warrant, anyhow. You’ve probably heard plenty of these saws, old and new; my list is hardly all inclusive, but following, and in no particular order, are the most oft-heard SCT urban legends along with my Snopes-style debunking.

That glass on the front of the tube is nothing more than flat glass designed to hold the secondary up. It shouldn’t be surprising quite a few SCT newbies believe this one. The Schmidt Cassegrain’s big lens does look flat. It’s easy to see the MCT’s deep dish corrector is a lens of some kind, but not so the SCT’s imperceptible “4th order curve” (slightly higher in the center, lower halfways out, higher toward the edge). The only way, really, to tell this is a lens is to place it against an optical flat and observe the resulting “fringes.” Or you can get a practical lesson in correctors like my pal Bubba did.

Bubba managed to break his corrector plate—don’t ask how; it wasn’t a pretty story and involved an ex-wife, not a Fastar-mounted camera. Bubba sat and ruminated a spell, considering his options. He could send the scope off to Celestron, yeah, but that would cost at least 500 bills, seriously cutting into his Pabst Blue Ribbon buying. Or… “Elmo down to the autoglass place can cut me a piece of glass just right, I betcha.” First light was predictably a disaster. Despite his best collimation attempts, Jupiter looked like a custard pie. As most of us know, the SCT’s corrector has a vitally important function, eliminating the inherent spherical aberration of the scope’s fast focal ratio spherical primary mirror. Without the corrector you’ve got something, as Bubba found out, akin to the Hubble Space Telescope when it was first launched: a mess.

The Celestron 9.25 is radically different from other SCTs. I like the C9.25. It’s a great telescope. Sometimes I wish I had one. I’ll also be the first to admit that it may be a wee bit better than the C8. The field is slightly flatter if nothing else. Good scope, yeah. Magic? Naw. Nevertheless, the rumors began shortly after the 9 ¼ was introduced in 1996: this one was different. Celestron had abandoned the SCT’s normal spherical mirror for a parabolic one. That’s why it was amazingly better than the tired, old C8.

These stories are ridiculous on the face of them. If the mirror was parabolic, what was the reason for a corrector plate up front? The only reason for the presence of a Schmidt corrector on the front of a telescope is, as above, to “counteract” spherical aberration. “Maybe the corrector on the 9 ¼ wasn’t really a corrector, then? Maybe Celestron really was using a flat piece of glass?” OK, but given the scope’s normal, slightly aspheric, convex secondary mirror, a fast parabolic primary would produce a worse field edge, not a better one. “Well maybe the secondary is different, then, maybe a hyperbola or”—dagnabit! Do you think Celestron would introduce an R - C or a D – K Cassegrain or some such and then hide the fact? They’d be trumpeting it to the high heavens, just like Meade did with their recent aplantic SCT (which they initially advertised as an ADVANCED RITHCEY CHRETIEN DESIGN).

The truth about the 9 ¼ is less romantic but more realistic. It is a normal SCT with a spherical primary, a slightly aspheric secondary, and a complex-curve lens (the corrector). Yes, it may be slightly better than a stock C8, but the reasons are pedestrian and don’t involve parabolas, hyperbolas, or ellipses. What Celestron did this time out was go to a slightly slower primary (f/2.3 instead of f/2). That results in the scope’s somewhat flatter field and slightly longer tube. Good, not magic.

MCTs have darker backgrounds than SCTs due to their better contrast performance/better optics. Man, I dig MCTs, even if the only one I can afford is my little sweetie, Charity Hope Valentine, an humble ETX125. That doesn’t mean you should believe everything you read/hear about ‘em, though. A dark eyepiece field background is largely dependent on the MCT OTA’s baffles. The quality of these may be better than that in your average SCT. Or they may be worse. What most folks are actually seeing when they note the nice BLACK BACKGROUND in a Mak (including Sweet Charity) is the naturally smaller exit pupils generated by the scopes’ (usually) slower focal ratios and resulting longer focal lengths—the average MCT is around f/15; the average SCT is an f/10.

The Orange tube C8s were the best. Best what? Oh, I know many of us are emotionally attached to them purty OT C8s (and 11s and 14s). But were their optics better? Usually not. There were some extremely good ones—and some dogs, too. If nothing else, today’s Celestrons (including the new Chinese models) are more consistent. Perhaps Unk does not see as many wildly outstanding OTAs these days, but he doesn’t see as many barking dogs, either. Another factor? Coatings. Orange Tube C8s usually have either no coatings or minimalist coatings on their correctors (there were a few StarBrights, but mostly the “enhanced” coatings on those Orange Tubes that had ‘em were simple silicon monoxide). A modern XLT will blow the doors off one when it comes to light throughput (brightness) and reflections.

The Criterion Dynamax SCT’s cardboard tube was the reason it failed. When the first competitor for Celestron appeared way back yonder in the 1970s, the Criterion Dynamax, us boy and girl amateurs of the day were appalled to learn it had a cardboard tube. Remember, this was before the age of Sonotube Dobs, and we really expected more for a price only slightly smaller than that of a real C8. No doubt it was the Dynamax’s optical deficiencies that put the nails in that pore CAT’s coffin, but I’ve little doubt the cardboard tube story turned some folks away from Criterion despite the company’s assertion that the scope’s tube was STRONG ENOUGH TO FIRE ROCKETS FROM (comparing it to military anti-tank rocket launchers and foolishly focusing more attention on it).

Ground truth? Criterion was right, believe it or no. The dadburned thing really was strong enough to fire rockets outa, and all the Dynamax tubes I’ve seen are still in one piece. Turns out that, yeah, there may have been some paper in there, but the tube was essentially phenolic resin. It was still a dog of a telescope, but the tube was OK.

SCTs are no good for high power. Maybe yours ain’t. I know mine sure are. Many's the night I’ve ramped up to 500x on the C8 for a good look at Saturn’s rings. How did the scopes get this reputation? A couple of reasons, I reckon, collimation being the most important. Make that mis-collimation. A standard SCT uses a 5x magnifying secondary mirror, so any collimation errors are, well, “magnified.” Until fairly recent times, when some of us began to spread the word that SCT collimation is fun and easy, many SCT users seemed afraid to collimate their scopes despite the huge difference it can make at high (and low) power. Another reason? There is no doubt that Meade and Celestron produced quite a few sub-par scopes from the mid-80s to the early 90s (the Comet Halley era). These pore things, which were usually overcorrected and often have “rough” optics, too, do tend to fade away as they approach 200x.

Celestron/Meade use hand picked mirrors for their more expensive scopes. Nope; not as a standard practice, anyhow. Normally, the most expensive LX200-ACF’s guts are identical to those of the cheapest LX90, and a CGE 800 uses optics identical to those in a NexStar 8SE. The situation is maybe not as clear-cut as it used to be, with Celestron making some optics in China and some in California, and Meade selling both aplantic and regular SCT optics, but, in general, buying a more expensive scope doesn’t get you better optics.

I said “as a standard practice.” Over the years, Celestron has assembled a few OTAs with hand-picked, triple checked, super-worked-over optics. This has rarely had anything to do with the price of the scope being sold, however. These OTAs were put together for various reasons; sometimes as replacement tubes for customers who’d had bad/substandard optics delivered, sometimes for friends of the company and other “special customers.” Naturally, such scopes are rare and verifying their “hand-picked” nature may be impossible.

Collimating an SCT is difficult. Whoo-boy! is this one a howler! Consider the poor Newtonian telescope owner. This unfortunate soul must adjust the tilt of the primary mirror, the tilt of the secondary mirror, the rotation of the secondary mirror, and its axial position in the tube. If the scope in question is a truss tube Dobsonian, it’s almost guar-ron-teed that some of these adjustments will need to be made before every observing run. Contrast that with the lucky SCT user. There is only one adjustment to make, only one that (most) end users can make, anyway, the tilt of the secondary mirror via three screws. When properly collimated, SCTs may hold this collimation for months or years, even after being bumped over dusty backroads.

Why the bad collimation rap, then? One reason is that, for novices, the physical act of doing anything to that intimidating looking OTA is scary, and fiddling with something positioned on the surface of a pristine corrector plate is even more scary. Another reason is that some folks don’t understand what twitching the three screws does and get confused and frustrated when they finally decide to attack collimation.

Most modern SCTs use a very simple secondary mirror collimation arrangement. Three screws are threaded into three holes on a secondary mirror backing plate/assembly. The center of the backing plate rides on a central pivot; tightening a screw tilts the mirror. The nature of this arrangement means that after a certain amount of adjustment of one screw, its opposite number or numbers must be loosened to continue in the same direction. It also means that if the secondary is to hold collimation, the screws need to be left snug. Normally, you collimate only by tightening screws, and that will ensure the SCT holds its collimation for a long time.

You can use a laser to collimate an SCT. Yeah, you can—if you want a mis-collimated telescope. Lasers work fine on a Newtonian where you have control of all optical elements and everything is (supposedly) centered on everything else. In an SCT, the eyepiece tube (visual back, rear port) may not be precisely concentric with the rest of the optical train. This is compensated for at the factory by shimming and other adjustments. This (likely) lack of concentricity means adjusting the secondary so the laser spot returns exactly back upon itself will probably yield optics that are misaligned in relation to the factory adjustments.

How about SCT lasers, collimators especially designed for our scopes? They work differently from a Newtonian model. What most have you do is perform a precise alignment of the scope the old fashioned way (by observing the rings of a slightly out of focus star). You then mark the return position of the beam on the laser with a sticker or some other device. Supposedly, then, you can precisely re-collimate in the future by adjusting the secondary with the laser in place until the beam returns to this spot. In the real world? Various factors prevent this scheme from working perfectly. Polaris is better and cheaper, still.

Meade invented the go-to. That’s what a lot of folks “remember,” anyway, and there’s no denying the first contact most of us had with a computerized telescope was with Meade’s much-loved LX200 “Classic,” which debuted in late 1992. Meade did not invent go-to, however; neither did Celestron. It was tinkered into life by some dedicated amateur and professional astronomers/electronics hobbyists/computer whizzes. OK, well, then, Meade must be credited with the first mass-produced commercial go-to scope, then, right? Negatory, there, Good Buddy. Seems folks have forgotten that wonder of the late 1980s, the Celestron Compustar.

The Compustar was a fully functional go-to SCT available in 8, 11, and 14-inch apertures. It worked fairly well, and featured what is still probably the best hand controller ever seen on an amateur scope. Then, why the heck is it all but forgotten? Why was it a failure? I don’t know if the word “failure” is exactly right. Celestron kept it alive into the 1990s. But there is no denying that they did not sell many of ‘em. Howcomesthat?

The biggest strike against the C-star was price. The Compustar 8 sold for about 4 grand in big 1980s dollars. You can imagine what they wanted for a C14, even when the list price was heavily discounted. These were also scopes that were best suited for permanent installations. They needed a hefty power source, benefitted from a good polar alignment, and did not like being bumped over country roads on the way to your favorite dark site on the Macon County Line. Shame? That Celestron just ditched these telescopes and went off on another tangent when it developed its Ultima 2000, instead of finding ways to reduce the price and improve the performance and portability of the Compustars. Be that as it may (sigh): Celestron was there before Meade in the go-to race.

The Autostar was the first Meade computer controller. Have we forgotten the Classic LX200 and the LXD series go-to GEM mounts already? It would seem so. I’ve heard the Autostar referred to as the “first Meade go-to system;” not just by casually chatting amateurs on the ‘net, but in a recent Astronomy Magazine article on the history of amateur gear. Meade introduced the Autostar for its first ETX 90 go-to model in 1999. The Classic had been on the market for seven years by that time.

“Flocking” improves images dramatically. Why do people flock their SCTs? Why do they cover the inside of the tube with light absorbent flocking material (contact paper) and paint their baffle tubes a flat(er) black? Supposedly to reduce unwonted reflections in the field of view. Or more accurately? Because their bubbas on the Astromart Forums, Cloudy Nights, and the Yahoogroups did and they want to too. How much good will flocking do? It can do a little when it comes to reducing scattered light effects such as those that occur when the Moon or another very bright object is just outside the field. Darkening the field background? I’ve never been able to see any difference between flocked and unflocked.

There’s also a purty huge caveat associated with this supposedly simple procedure. In the process of removing the corrector, applying contact paper to the tube interior, and painting the baffle tube insides, all too many tyros and a substantial number of veterans have made a mess of an innocent OTA. A tear forms at the corner of Unk’s eye at the thought of broken correctors, scratched secondaries, contact paper stuck to primaries, and paint spots on a formerly pristine main mirror. Yeah, I know you don’t want to hear it one more time, but here it is, Unk Rod’s Number One aphorism: “The Only Enemy of Good Enough is More Better.”

The Orange Tube C8 was the first commercial SCT. NO SIR BUDDY! The original “OT” C8 was a ground breaker, but it was not the first mass-produced (semi, anyway) SCT or the first Celestron SCT; that honor goes to the storied Celestron White and Blue Tube Schmidt CATs of the 1960s. If you want to learn all about these beauties, Unk insists you hunt up a copy of Bob Piekiel’s e-book, Celestron, the Early Years. But I will at least say these things worked as good as they looked. They derned shore shoulda; in 1965 the Celestron C10 sold for about the same amount as a brand new Volkswagen Beetle!

Focus shift makes imaging impossible or difficult with SCTs. Focus shift happens when the primary mirror of an SCT tilts a minute amount as it’s pulled down or pushed up the baffle tube to focus. Even at lower powers, this shift is all too visible in the eyepiece—objects move across the field as they are focused. This is annoying, but it doesn’t do anything to prevent imaging per se. It does make focusing a hairline-reducing experience if you’re usin’ small CCDs, however. Turning the knob to focus Jupiter at f/30 on a webcam chip can make the planet slide right off the edge of the frame. There is a fairly simple cure; however, a rear cell Crayford focuser. This is a standard in/out focuser that screws onto the scope’s rear port. Use the main focus knob to get in the eyepiece’s or camera’s focus “range,” and do fine focusing with the Crayford thereafter. Voila! No shift.

Mirror flop makes imaging impossible or difficult with SCTs. Mirror flop, which is often confused with focus shift, can have serious implications for imagers, but obviously it does not make Astrophotography impossible, since tens of thousands of great long- exposure deep sky images have been taken with SCTs over the years. What is flop? It’s a little like focus shift in that it involves the primary mirror and its carrier tilting slightly on the baffle tube and making an image move in the field. The difference is that in flop this happens without the focus knob being touched. The mirror/carrier can be in a slightly off-balance condition after focusing, and a substantial change in scope attitude—crossing the meridian, for example, can make this off-balance mirror FLOP, shift slightly on the baffle tube. If you are doing a long exposure image, and are not monitoring a guidestar through the main scope (with an off axis guider or an on-camera guide chip) the guider will not know the image has moved in the field (the mount didn’t move) and the stars in the image will trail, ruining the shot.

The good? This doesn’t happen all the time, and can be avoided. Unk Rod has had exactly one image (a nice M15) ruined by flop over the years. How do you prevent it? Easy: don’t image something that will cross the Local Meridian during the exposure. With today’s short CCD integrations, that should not be a big deal. You can also reduce the chance of flop by ending your focusing “uphill.” Make your final focus move a counter-clockwise turn of the focus knob. That will move the primary up the tube against gravity, lessening the chance of it being left off-balance. A Crayford focuser, by the way, will do nuttin’ to lessen the chance of mirror flop. What will? The only true cure is locking down the primary mirror, either with the recent Meade OTAs’ nice mirror locks or some jerry-rigged mirror “stabilization” bolts of your own.

Asymmetrical star images in a star test indicates faulty SCT optics. Bad diffraction patterns in a star test (comparing the inside and outside focus ring patterns of a bright star) mean bad optics. Not necessarily. That esteemed personage, Roland Christen, is on record as saying that this is not a reliable way of judging the quality of a “compound” (e.g., catadioptric) telescope. Much better is comparing the size of the secondary shadow on either side of focus. The closer that is to being the same, the better your optics likely are. A Ronchi test is another good indicator: straight lines good, curved lines bad, hooked lines extra bad. Perhaps the best gauge of how good, though? The appearance of a planet at high magnification under steady seeing. If Jupiter shows plenty of detail, lots of little squiggles and whorls in the belts, you can be assured your scope is a wiener—err, "winner.”

An SCT is, by nature, a very portable telescope. One can be. Certainly a C8 comes close to that long sought goal, a “portable observatory.” But, let’s face it, a fork mount telescope becomes massively large and unmanageable quickly as aperture goes up. A 10-inch is uncomfortable, an 11 is a little disquieting to mount, a 12-inch is crazy, a 14 is insane. Big SCTs are more “transportable” than “portable.” One thing I’ve been careful to point out to novices, and which I say in both my SCT books, is that the scopes look way, way larger in person than they do in those pretty magazine ad layouts.

The SCT’s field edge is comatic. The Schmidt Cassegrain field edge is not perfect, that’s for sure. I know you’ve noticed that stars at the edge of the eyepiece field are never quite in focus when those in the center are sharp and vice versa. The cause is not coma, though, not mostly, anyway. While SCT optics do display some coma, just like Newtonian scopes that use parabolic primary mirrors, that’s not the main cause of the CAT’s slightly icky field. The reason is that the SCT has a naturally curved field. Why?

Without wading too deep into technical waters likely to drown your Silly Old Uncle, suffice to say this is a characteristic of all Schmidt-type telescopes and cameras. The original Schmidts were designed to be cameras only, not visual instruments, so this problem was taken care of in a very simple manner: these telescopes use/used curved film holders to sharpen up the field edge in images. If you want the real why, however, I commend to you Telescope Optics, Evaluation and Design by Harrie Rutten and Martin van Venrooij. It can be difficult going, but is probably the best resource for CAT fanciers at this time (I don’t mind letting slip that the author of Celestron, the Early Years, Bob Piekiel, is working on a book on SCT optics).

Knowing the reasons for this SCT characteristic doesn’t do anything to fix the problem, though. Can anything be done? Well, I’ve never found the Schmidt CAT’s slightly fuzzy field edge a problem; I tend to focus my attention on what’s in the middle of the eyepiece’s view. But I know it does bother some folks. What to do? Good eyepieces can help. TeleVues and other high-quality oculars eliminate or reduce eyepiece aberrations that make the field edge look all the worse—things like astigmatism. How about a Paracorr or other coma corrector? One might make the field look a little better by eliminating some true coma, but will do nothing to flatten the CAT’s curved field.

The only real solution is a reducer/corrector. These f/6.3 “r/cs” sold by Meade and Celestron do a good job of field flattening and are inexpensive. Drawbacks? They tend to cause vignetting in 1.25-inch eyepieces over about 30mm of focal length, and do even worse with 2-inchers. That’s a problem for imagers using all but the smallest chips, too, but can be cured with a good flat-field frame.

Well, Pards, that’s my rogues’ gallery of CAT myths and misconceptions, but no doubt there are many more floatin’ around on the Internet and down to the local club. I’d sure be interested in readin’ about the ones y’all have heard if you wouldn’t mind makin’ a post or three on the subject.

Hey there,

What about the myth about the Ultima 8'' PEC being the BEST non-goto SCT?

Or is that one true?

My bias as a U8 owner and lover prevents me from answering!


Unk Rod

The picture of VenuSCT is verry similar to my U8...

So I guess I know your answer
Sorry Rod, but SCTs from Meade and Celestron do come up short in the contrast department when compared to a well-made MCT of the same focal ratio. I owned an f/10 Russian Mak and it routinely outperformed 8" SCTs even though the aperture was smaller.

Example: M1 just after dusk, 7" MCT (Intes-Micro), 8" SCTs (Meade & Celestron), same TV 12 mm Radian eyepiece. M1 is visible in the Mak but one SCT shows only the faintest smudge, and the other nothing at all.

IMO this isn't a collimation issue or a difference in the focal ratios as all three scopes were well maintained and all were f/10. The better baffling in the Mak and higher contrast inherent in its optical design makes the difference between win place and show for difficult objects.
That may be--for some scopes, anyway. But contrast was not what I was addressing, but the issue of the background's darkness.

Excellent debunking of SCT myth. 3 of my 7 scopes are SCTs and they perform quite well. The 14" allowed visually seeing the central star of M57 - from the midwest no less! Proper collimation is absolutely key - as it is on newtonians and refractors.

I thought that for a given aperture, final f/ratio and FOV the slower the primary (i.e. the lesser the amplification of the secondary) the greater the diameter of the secondary. You seem to state the opposite discussing C9.25 as against C8. Do I read you right?
Not necessarily. The 9.25 _is_ a very good telescope, if not a magical one. I'd still rather have a C11. is an improvement of the 8 in light gathering and edge of field sharpness.
I don't know what you're talking about Rod. The C14 is a small portable scope. Greg N
Rod, what is the most portable mount that can take the C11 for visual only? can I use the advance VX? or is it pushing it too much even for visual only? The CEGM DX is a monster! :(
Rod, can I use the AVX for the regular 11" for visual only? the CEGM DX is a monster! :(

Yes, Celestron even sells a VX with an 11-inch OTA. You probably will want a set of Celestron's vibration suppression pads, but this will work.
Thank you for clearing things up! I love my Nexstar 11GPS and I always will. It's a great scope. just loved to read your blog...I am currently deciding between Celestron c9.25 and c11... I will have it on a double star mount adjacent to a Meade 5" the SCT is more for deep sky viewing. Given the longest FL eyepiece I own is a 40mm...I'm concerned about the longer FL of the C11. I have also heard that the sharpness and design of c9.25 performs relatively better in terms of higher contrast and sharpness compared to the c11... You input will be appreciate.
The 9.25 is a good scope. Is it that special compared to the other Celestrons? No. If you want to go all the way, get an Edge 800 or 1100 (you can't get a reducer for the 925 Edge that will work for both visual and imaging and that is a drawback). with the f/7 reducer the Edge 1100 does well. Me? I am very satisfied with the Edge 800.
Oh man. I just finished cleaning my Meade SC Scope (f/10 D=102mm F=1000mm). NOW I read about indexing marks on the Primary, Secondary, AND Corrector lens. Just wonderful. So I see what looks like an actual notch in the side of the Primary, a ghost of a perma-marker line on the Secondary, and two parallel etched lines on the surface close to the outer edge of the Corrector lens - I have no clue which surface goes in or out. I should have known better. I really like the Scope because it makes for awesome daytime photos of passing aircraft. I suspect this scope is very old. In any event, Thank you Unks for the great information, and yes, as I was reading your blog, my shoulders started sinking. Alas, at least I followed a prescribed cleaning technique so that turned out well. I'll try to figure out a way to see the actual surface difference on the Corrector. And, maybe even be successful at collimating?
The was a Sky And Telescope (Gleanings For ATMs) Back in the 70's? that had an article on making an SCT. This telescope was F12 from memory, with a flat field. The reason? Primary and Secondary were ground as a pair, and figured together resulting in equal radii of curvature of opposite value. Its Petzvil Ratio = 0, so it would appear with a less compact design we could have much flatter field SCT's and if the tube length was double with the corrector at RC much less coma. One thing that left me wondering, is, if the f ratios are equal and opposite, wouldn't the overall F ratio be infinity? I had the Sky and Telescope mag with this article lying around for years. Where is it now? :)

I need a manual focus knob for a de-forked silver tube Nexstar 8. The OTA came with an electrical Celestron (JMI) focuser that I now don't need.

About mirror flop, for my C11 this problem is not limited to meridian but is present on all the sky, after moving it for more then 10 degree in AR you need to recollimate it for high resolution shot .. due to mirror flop .. just to say that the problem is not so well defined, each scope is a different story
Post a Comment

<< Home

This page is powered by Blogger. Isn't yours?

stats counter Website Hit Counters