Sunday, August 14, 2011

 

CATS AND KITTENS!


Which CAT, which three-legged, not four legged, feline is a good candidate for you to adopt? What the hell is Unk going on about now? One thing I get asked fairly frequently, muchachos, is “Which CAT, which catadioptric, which lens-mirror telescope, should I buy?”

It ain’t easy for a novice to decide. In addition to our time honored variants, the SCT, the MCT, and the MNT, the SNT has made a comeback, and some more exotic variants, KCTs and similar, are available, too. What in Sam Hill do all these pea-picking letters mean? Stay tuned and all shall be revealed, muchachos. Below are my thoughts on the different CATs, which I hope will make choosing one easier, assuming any catadioptric telescope is a good choice for your viewing habits and purse.

Schmidt Cassegrain Telescopes (SCTs)

When it comes to SCTs, it used to be easy: you decided whether to buy a Meade or a Celestron and you decided how much aperture you could afford and handle. There’ve been a few non M or C alternatives over the years, but none has made much headway. The Criterion/Bausch and Lomb SCTs were one, but they weren’t so hot and everybody (almost) knew it. Takahashi and Lichtenecker made SCTs, too, but only a few and for high prices. Today it is again M&C only, but there’s now more to it than just deciding “Which is better, pardner, the Meade or the Celestron?”

The traditional f/10 spherical primary and secondary SCT is still with us—and how—but beginning a few years back Meade kicked things up a notch. That began with their RCX-400 series, an f/8 with an aspheric secondary, which offered reduced coma as well as higher photographic speed. Alas, the RCXes were only available for a few years, but they begot the Meade ACFs and the Celestron HDs

The ACF was something of a breakthrough. The design wasn’t new and didn’t originate with Meade, but the ACFs improved on the SCT they had been selling for nearly 30 years. These f/10 telescopes do that with a differently figured secondary. While Meade says it is a hyperbola, the optics gurus I’ve talked to say it is a parabola.

Whatever it is, the effect is to reduce one of the two ills Schmidt Cassegrains are heir to. The stars at the edge of an SCT’s field are not perfect. They are not perfectly round and are a little bloated. In part, this is due to field curvature, but it is also because of coma, the same edge-of-field blurring effect that plagues Newtonians. The ACF still suffers from field curvature, but its optical design cleans up the coma and makes field edge stars look much better.

Celestron didn’t respond to Meade’s challenge immediately, but when they did, their new f/10 SCT, the Edge HD, kicked it up a notch. Not only does Celestron’s design, which differs from their other scopes in the presence a two-element corrector in the telescope’s baffle tube, reduce coma, it also flattens the field. I spend most of my time looking at the field center, not the edge, but I gotta admit the stars in an HD look real sweet all across the field.

Which, if either, should you choose? While the improvement is even more apparent with the HDs, the ACFs present lovely fields, too. If you want a fork, the choice used to be obvious, since Celestron did not offer HDs on the traditional SCT fork mounting. It was just announced, however, that HD tubes are coming to the CPC series. Meade? Unfortunately, what I’ve seen of Meade’s current production leads me to conclude that while the optics on the ACFs are indeed exquisite, the company’s output is still bedeviled by mechanical and electronic gremlins. Which is a shame given the superb optics in their ACFs

“So which one, Unk? ACF or HD?” For many of us, I must conclude the best choice is “none of the above.” The problem is that neither company has introduced a focal reducer designed for the optical prescriptions of these new telescopes. A standard SCT equipped with one of Celestron’s f/6.3 reducer - correctors can produce images with stars nearly as good at the edge of the field as these new telescopes. Yeah, you can take fine pictures with an HD or ACF at f/10, but who wants to shoot at f/10? Optec has introduced focal reducers (the Lepus series) for the new CATs, but I have not yet been able to determine how efficacious they are.

OK, a standard SCT, then? My comments about Meade’s problems apply here, too. For example, one of the BEST SCTs I have ever used, the Meade LX90, whose production went years without problems, developed declination motor faults after Meade moved their opeeration to Mexico. It is perfectly possible you will get a problem free Meade, but not quite as possible as with Celestron.

Not that Celestron has been perfect, either. They have had more problems with the CGEM German equatorial mount (a design based on the venerable EQ-6) than they should have. Far more. If I wanted a fork mount SCT? I’d choose the Celestron CPC. An inexpensive GEM SCT? The Celestron Advanced Series CAT which is equipped with the time-tested CG5 mount. Still, I must admit a buddy’s new 10-inch LX200 ACF was one of the finest fork mount telescopes I’ve ever used—till its mount quit working.

I get asked how Meade’s move of production Mexico and Celestron’s move to China have affected optical quality. The answer is, if anything, it has improved it. Frankly, I believe the telescopes are at least a little better optically on average than they have ever been. Electronically/mechanically? Yes, Meade’s problems seem somewhat worse than they were before, but there has always been the chance of getting a problem scope when you are talking a complex computerized rig at the price territory Meade inhabits. The same goes for Celestron.

How big an SCT should you contemplate? That is a sticky subject, but your Old Uncle Rod has you covered. Have a look rat cheer.

Maksutov Cassegrain Telescopes (MCTs)

Quite a few newbies and not so newbies who have decided they want a CAT also decide they want More Better Gooder than a mass-production SCT. The first place folks usually turn to for that is the Maksutov Cassegrain.

Why is the MCT better? Without sounding like a dadgum optical textbook (which Unk couldn’t, anyway), the combination of an easier to make corrector plate and a slightly higher native focal ratio primary (as in the SCT, in the MCT a low focal ratio primary is “magnified” into a higher focal ratio by the secondary) combine to offer what some folks, including yours truly, believe to be somewhat better images. I know my Mak, Charity Hope Valentine, an ETX-125, offers images that are very sharp and contrasty and visibly superior to those of a Celestron C5 SCT despite Charity’s somewhat sizeable central obstruction (do not tell her I mentioned that).

“OK, what’s the catch, Unk?” You are right: TANSTAAFL, “there ain’t no such thing as a free lunch.” Firstly, the MCT is usually a higher focal ratio instrument. While we’ve seen some faster ones over the last decade, the de rigueur focal ratio is usually around f/15. Is that bad? It’s bad if you fancy getting the entire Pleiades or some other big object corralled in the field. Truthfully, though, most of the deep sky objects we look at are not large, and, because of its superior contrast, an MCT can be a superb deep sky scope. When somebody says MCTs are only good for the planets, look ‘em straight in the eye and intone, “NO SIR, BUDDY!” Just don’t go looking for the Cat’s Paw Nebula with one.

Then there is the aperture problem, which is more serious. Oh, you can buy MCTs in large apertures, as big as 16-inches or even larger. But you would probably not be able to afford one. When you pass 6 – 7-inches, the price of an MCT takes off like a skyrocket. Why? Part of it is the deep dish corrector. Finding a good, big blank in the thickness required is an expensive proposition. 7-inches is the limit for most of us. Which can be just fine. Depends on you.

Finally, MCTs are notorious for long cool down times. I used to think that was due to their thick corrector plates, but that optical guru of gurus, Uncle Roland Christen, schooled me on that. According to him, and he if anybody should know, having made superb MCTs himself, it is not the corrector, but the closed tube of the scope and the heat absorbing materials within. In truth, the cool down characteristics of an MCT, how long it take before it adjusts to outside temperature and begins offering the best images it can, are the same as those of an SCT. The difference is that the MCT with its higher focal ratio delivers higher magnifications eyepiece for eyepiece, which tends to exaggerate cool down effects.

You want one? Where do you get one? If you are cheap like me, you look to Synta. Their MCTs, including a very sweetly priced 7-inch, perform very well indeed and most assuredly will not break the bank. If I were doing more visual observing I’d have the Orion/Synta 180. Why “visual”? Can’t an MCT take good pictures? Sure one can, of the planets certainly. Deep sky? One could, but who wants to take pictures at the focal ratio of f/15? Even with today’s sensitive cameras, exposures become disgusting long at that slow speed. Focal reducers can work, but I’d have a problem getting down to the f/3 – 4 I need for deep sky video.

Want something a little fancier? Alas, renowned Russian MCT maker Intes has left the business. So has Lomo, who made some fine scopes, including an MCT with a fork and drive base that was almost as friendly for noobs and casual observers as a fork mount SCT. There is one Russian company left in the Mak game, Intes Micro, whose Alter M703 and Alter M615 certainly deserve a look.

MO’ money? There is Questar. I’ve always questioned the wisdom of forking out the bucks Questar asks for a telescope whose design is stuck in the 50s, but I don’t own a Rolex watch, either, and obviously just don’t get it. The basic scope still comes with an AC synchro drive. The drive base limits what you can view in the south. Forget about go-to, though you can fit digital setting circles from JMI. On the plus side? Beautiful glass from Cumberland Optics and a build quality that ensures the scope will last a lifetime—if not several lifetimes.

It all comes down to $$$, though. A Questar 3.5 simply won’t show a whole lot of the deep sky. In fact, a 3.5, no matter how fine, won’t show much of the shallow sky, the Solar System. One is fine for casual grab ‘n go observing, but who wants to pay 4K for a grab ‘n go? Not moi, that’s fer dang sure. No, for serious work you want the Questar 7. B-U-T. There is a reason this used to be called “the doctor’s scope,” and it’s not just that it looks like a shiny piece of operating theatre kit.

The Questar 7 was so expensive back in the day that folks joked that you had to be a wealthy physician to own one. Today, the Q7’s price doesn’t seem quite that daunting, but it’s still way high for a 7-inch. The out? You don’t have to buy the fork mount version, which isn’t always available anyway. The OTA-only ain’t exactly cheap at about 7500 buckeroos, but it is at least a much more modern proposition, with a “normal” finder and star diagonal. Yeah, some folks love the 3.5 and 7-inch forks’ built in finder/diagonal/control box. Me? I have never cared pea-turkey for that stuff.

'Course there is that elephant-in-the-living room MCT, the consideration of which involves going from the sublime to the ridiculous. Or maybe not so ridiculous. I am talking about Meade’s ubiquitous Questar clone, the ETX. Yes, it has plenty of plastic. No, it will not last several lifetimes or probably even one, but the little scopes do have their charms. The biggest of which is their optics. Questar owners don’t like to hear it, but the optics in the ETX-90 are almost indistinguishable from those in the Questar 3.5. On some occasions when I’ve run an ETX against a Q, I’ve been able to tell the Questar was slightly better, but only slightly and only sometimes.

Meade’s little scope actually has some advantages over its inspiration. Yes, the ETX go-to system can be finicky, as you know if you’ve read my adventures with Charity Hope Valentine, but it almost always works, and I suppose I usually see way more with my ETX over the course of an evening than the average Questar owner sees by star hopping with that weird dadgum finder.

It’s not just go-to where Meade pulls ahead. They offer a 5-inch MCT, the ETX-125. Yeah, I wish Meade still sold their 7-inch, but 5-inches is enough to allow me to see almost as much of the deep sky as I’d see with a C8 on the sorts of nights when I drag out Charity, and a lot more than what I’d see with 3.5-inches. On the planets? Miss C is exquisite. And guess what? You can buy 4 – 5 ETX-125s for the price of one Questar 3.5. No the ETX is not as finely made as a Questar, not even close, but in my opinion it is a much more practical telescope.

There are other good MCTs out there as well, especially smaller scopes. The word on the street, for example, is that Celestron’s new C90 is amazing for the price. Unless you are just after grab ‘n go, however, I advise strongly against a smaller than 4 – 5-inch MCT. To really take advantage of this kitty’s strengths, fine views of the planets and small – medium deep sky objects, you need more light than a 90 can give you.

Maksutov Newtonian Telescopes (MNTs)

What’s an MNT? It’s the marriage of an SCT and a Newtonian, which, its fans will tell you, is one made in heaven. Unlike SCTs and MCTs, the design does not incorporate a magnifying secondary. Instead, a medium focal ratio primary mirror, maybe about f/6, a mirror with a spherical figure just like those of the other CATs, is paired with a Maksutov corrector plate at the end of the tube and a Newtonian style diagonal flat. The secondary’s housing is mounted on the corrector and the mirror sends images out a hole in the side of the tube, just like a Newtonian.

What are the advantages of this design? The high native focal ratio mirror and the flat secondary, mean it is potentially capable of delivering images superior to those of an MCT. As compared to a similar focal ratio Newtonian, the field edge is better. One advantage for visual observers with some MNTs is that the secondary obstruction, the size of the secondary mirror, is very small, maybe improving planetary performance a bit. Alas, small secondaries make the scopes impractical for photography. The human eye doesn’t notice the small secondary does not fully illuminate the field, but a camera with any but the smallest sensor does. There is no reason MNTs have to be made with tiny secondaries, though, and some are not.

Should you want one? Maybe. These are good telescopes, but they do have a strike or two against them. Primarily, that they tend to be heavy and are necessarily equipped with long tubes, since there is no optical folding and secondary amplifying as there is with an MCT or SCT. That means you are going to need a substantial mounting for a seven or eight inch MNT.

If you want a Mak-Newt, where do you get one? The usual source in recent years has been Intes Micro, whose Alter MH 76 is a good bet. This is an 8-inch f/6 with a 20% central obstruction optimized for visual performance. After suffering a decline in popularity at the end of the 90s, the MNT design is getting hot again, and there are some new ones appearing, including the 6-inch f/4.8 David H. Levy Comet Hunter sold by Explore Scientific. The David Levy has secondary mirror which presents a 32% central obstruction, making this MNT a good choice for imaging.

Schmidt Newtonian Telescopes (SNTs)

If you can marry a Newtonian and an MCT, why can’t you do the same with an SCT? You can. Meade and Celestron have done it a number of times over the years. All you do is replace the MNT’s corrector with a Schmidt corrector, and you are done. Advantages? An SNT delivers considerably less coma than a Newtonian without the need for corrective optics. Given their fast primaries, which range from f/4 to f/5 (in the Meade scopes) they deliver nice wide fields, too. Optical quality is good enough for wide field imaging and visual observation at relatively modest magnifications. Is that it? Yeah, purty much. But that is not really the fault of the design.

It is more the fault of the fact that nobody does a real top of the line SNT. The major purveyors have been mass-producers Meade and Celestron, and they have always relegated the SNT to their low - mid level product lines. Today, as a matter of fact, only Meade is producing amateur grade SNTs.

How are Meade’s Schmidt Newtonians? These scopes, part of the LXD-75 line, are OK. The optics are, in fact, pretty dern good. It’s the mechanics where they fall short. The problems include a minimalist mirror cell and poor focuser (which can be fairly easily replaced). Their major fault, though, is the mount they are shipped with, the LXD-75 GEM. This EQ-4/CG5 class go-to rig is barely adequate for Meade’s 6-inch SNT and a joke for the 10. Or was.

I’ve heard the LXD-75 line is being discontinued and have no idea what that will mean for the SNTs. Will they be moved to another, better mount? Discontinued as the 1980s Meade SNTs were discontinued (for over a decade)? We shall see. I’d hate to see them go because (optically) I find these telescopes fairly impressive when properly mounted and a good bargain.

Klevtsov Cassegrain Telescopes (KCTs)

I’ll lump all similar scopes into this category, though not all of them are technically of the Klevtsov design. What they have in common is that they are Schmidt/Maksutov Cassegrain-like telescopes that replace their full aperture corrector plates with sub-aperture ones. There are several places the correctors can go, including in the baffle tube, but the usual spot is behind the secondary mirror (closer to the primary), and usually in contact with it. The genu-wine KCT uses a Maksutov like meniscus lens and a mirror/lens combination, a “Mangin” corrector, but other designs are possible and in use.

Drawbacks? Several. First, since the big corrector on the front of the tube is gone, a spider must be used to support the secondary mirror, just as in a Newtonian. That yields diffraction spikes around bright stars, which you either love or hate. Also, the corrective optics are substantially more difficult to make properly than the mass produced corrector plate of other CATs. Since light passes through the KCT’s lenses twice instead of once as in “normal” designs, they have got to be good.

Advantages? Well…hmmm… One is that you do not have the big corrector to gather dew. The other is that theoretically subaperture corrector lenses, while difficult to get right, can be better than an SCT’s corrector, with the system suffering less from field curvature. Trouble is, nobody is making a KCT that lives up to that potential in my opinion.

Those I’ve tried, including the Russian TAL KCTs and similar designs from Vixen, have one thing in common: they are not noticeably better than an off the shelf Meade or Celestron Schmidt Cassegrain. Add to that the fact that the Vixens are more expensive than Meade and Celestron SCTs, and that the TALs are not cheap either and are not easily available in the U.S. at the moment. The TALs also tend to hearken back to the Russian scopes of the early 1990s; they are built like T-72 tanks with both the good and the bad inherent in that.

Finally, Meade and Celestron are now producing SCTs, the ACFs and HDs, that address the designs drawbacks of the standard SCT and make off-the-wall variations like the KCT less attractive. I do note that, while I have not tried them, I have heard some good things about the Orion (UK) sub-aperture MCTs (their larger aperture Maksutov Cassegrains are all of this design)...

So there you have it, a rundown on the best of the breed for the CAT fancier. Which do you choose? That’s up to you. What I find works for me is a standard SCT equipped with a Celestron reducer/corrector. I supplement that with the 5-inch Meade MCT for grab-n-go use. But your choice is your choice, and I will admit that at times and for some unknown reason I still lust after a Tal Klevtsov. Why? The appeal of the exotic for a distinctly non-exotic old hillbilly, I reckon, muchachos.

Next Time: Gone Fishin’.

Comments:
Great blog inspired me to put my Super C8 Plus back together again and go after some summer Faint Fuzzies,
Once the weather clears!
 
Great! Rock on!
 
Is it possible to have a Schmidt Newtonian Telescope
that is optimized for visual performance, i.e F6 focal
ratio and 20% obstruction? The thinner corrector of
the Schmidt should allow for brighter images along
with being cheaper to manufacture.

Thanks,
Richard
 
It's possible, but no one has done it lately. The thinner corrector won't offer noticeably brighter images, and while a blank would be cheaper, the difficulty in making a Schmidt corrector might outweigh that.
 
Rod - while I agree that a 5" aperture matters a lot over a 3.5", I just recommended (again) an ETX-90 to a frustrated newbie because of its remarkable convenience due to its size/weight.

He'd been frustrated with a small newt, and was told by others to get a refractor (no collimation). His kids were awkward with the long tube (he was going to get a f/11 refractor and run into the same trouble again). Kids also have difficulty with the newt observing position, understanding the "spyglass" of a refractor - they see the small mak as a cute/tiny refractor. The lack of widefield doesn't bother them.

Generally its a big success, although Meade / Celestron never seem to understand the success of this - although with the ETX 60-80 they get pretty close - the CA and lack at high magnification annoys many here. Yet this is my second best recommendation for families getting in to astronomy.
 
I'm very happy in visual and photo with my SCT C9.25, with and without Celestron f6.3 reducer/corrector :)
Maybe it is the f2.5 primary and I must care the collimation
Regards
 
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