Wednesday, January 31, 2007

 

The End of the Road?

For the Schmidt Cassegrain? After all, this venerable telescope design has been around basically unchanged since 1965 or thereabouts. The short answer is “yes” and “no.” Certainly, there is still and will continue to be a market for inexpensively priced SCTs in the classic all-spherical design used by Celestron and Meade (Celestron does do some minor aspherizing to its secondaries—MINOR). These telescopes are easy to make in a mass production setting, and produce good results for most amateurs. Yes, stars do tend to gain weight and lose shape as they reach the field edge, but this has until now been a minor annoyance, and folks bothered by it have found a combination of high quality eyepieces and the ubiquitous f/6.3 Reducer/Corrector does a good job of fixing the SCT’s field edge faux pas.

That said, there’s a growing number of amateurs who want Schmidt Cassegrains, but who want better Schmidt Cassegrains. Some of these folks are driven by the fact that CCD chips are getting bigger and bigger. When SBIG introduced its amateur-oriented big-chipper, the STL11000, I wasn’t concerned. Who would want (or need) to use such a big chip with an SCT? But people do want to use this camera with SCTs, it seems, and these people are producing great images with the STL11000 on their SCTs, but there’s no denying that the big chip makes the “field edge problem” more serious.

Frankly, though, only a relatively few amateurs are going to equip themselves with the 11000 (or comparable cameras). The entry level price of almost 9000 dollars sees to that. B-U-T…lots and lots of amateurs are beginning to use digital SLRs like the inexpensive (sorta) Canon Rebel with their SCTs, and the CMOS chips these cameras are equipped with are, you guessed it, BIG.

Add to these two groups a younger generation of amateurs who’ve cut their teeth on small apochromatic refractors and who, though they want to move to the larger apertures offered by SCTs, simply demand “more-better-gooder.”

What’s the answer? Initially, I thought it would be a new design generation of Reducer/Correctors. Many of us shot pleasing 35mm deep sky images with the R/Cs. Not perfect, maybe, but good. And the f/6.3 R/C does do a good job on the C8. Unfortunately, though, it doesn’t do as well on larger SCTs, and vignetting is an annoyance even on an 8-inch (and becomes more of a problem with sensitive CCD chips). I thought it would be natural, then, for somebody to come out with a better R/C, maybe a larger aperture one that would use the larger rear ports on the larger than 9.25-inch SCTs, and that everything would again be rosy in spherical SCT land. That doesn’t seem to be happening. Other than the Meade/Celestron 6.3, there ain’t much out there—nothing at all, really, that’s designed specifically for SCTs or that improves dramatically on the “standard” f/6.3 R/C.

Then, all of a sudden, everything changed. It appeared an improved R/C would be a moot point. Meade, you see, had introduced an improved SCT design. This momentous occasion was, unfortunately, overshadowed by the company’s decision to call this (aplantic) SCT design a “Ritchey – Chrétien” (for marketing reasons, I guess), and by the criticism and lawsuits that followed from this decision.

Look, it’s pretty clear what Meade’s new design (used in the RCX400 and in the LX200 replacement, the LX200R) is, it’s an SCT. The scopes use a CORRECTOR PLATE (somewhat but not a lot different from a normal corrector, I assume) and a SPHERICAL PRIMARY (the same as in their other SCTs). The major difference is in the secondary, which is not a sphere in this design but (I’m told) a slightly overcorrected parabola. It has nothing in common with a “real” R/C as far I can tell other than that both telescopes are descendants of the original Cassegrain design.

Be that as it may, the new Meade telescopes offer a very real improvement over all-spherical SCTs. NO, this improvement is not dramatic visually, though you can tell that stars look “smaller;” especially at the field edge, but it’s real, and does make a difference when you’re imaging with large chips.

So the spherical SCT is doomed? Maybe. Probably not. In addition to the fact that spherical SCTs will likely always be cheaper to make and sell, Meade appears to have bungled the introduction of the RCX. Not just in their (in my opinion) very unwise decision to refer to it as an “Advanced Ritchey-Chrétien” design, but in their QA failings. Far too many of the scopes seem to arrive DOA, and are shortly on their way back to Irvine, California. These problems rarely involve the optics, but they do tend to create the perception that the RCX is a lemon (the R, in contrast, on its LX200 mount, appears to have had a relatively problem-free introduction). I’m afraid this may, at the very least, discourage Celestron from introducing an improved SCT design of their own. At worst, Meade may decide to throw in the towel on the RCX if not the R.

And that would be a shame. Because the Meade telescopes really do take us to the next level SCT-wise. It is time for a change in my judgment, and the RCX and the R do show that the SCT can be done better.

Comments:
Ammo Box!? he he

Stupot
 
What about the Faststar option with Celestron?
 
Don't really need to tell people this, but ignore the spam url in the last comment. It's a site run by a guy pretending to be someone else who hates Uncle Rod and uses this Ian Hill Smith alias to hide the fact he's making abusive statements about Unk on various lists and chats
 
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