Sunday, June 11, 2017
Issue #542: The Curious Case of the Schmidt Cassegrain Telescope
In the last few years, however, I’ve made it no secret that
I’ve somewhat turned away from SCTs and to refractors for a variety of reasons. What gives? Am I now fer ‘em or agin’ ‘em? Answer? It’s complicated.
In fact, just about everything concerning the SCT is
complicated except for its relatively simple design (other than that dratted corrector). Moreso than any other
telescope it raises strong emotions. It doesn’t just have fans and skeptics, it has lovers
and haters. It’s unusual to see a general discussion of the SCT online on
BBSes like Cloudy Nights that doesn’t degenerate into slings and arrows from
both sides.
That being so, I thought this Sunday we’d go over the
arguments of both the prosecution and defense in the Curious Case of the
SCT: Is it a Good Telescope or Not?
For the Defense:
The SCT is Portable
The Schmidt Cassegrain is just naturally easy to transport and set up given its
folded optics and short tube. The C8 packs two meters of focal length into an
OTA less than two feet long. Carrying around and mounting an 8-inch f/10
Newtonian is something of a nightmare, but an 8-inch f/10 SCT is practically a
grab ‘n go scope, to summon that overused cliché.
A modern fork mount model is virtually a self-contained
observatory. Not only is there goto, computerized pointing, to the tune of tens
of thousands of objects, some models now feature built in auto-guiding, wi-fi,
and can align themselves with little user intervention. An 8-inch SCT, a
battery, maybe a DSLR, a box of eyepieces and you are ready for either visual
or imaging work at the drop of a hat.
These Telescopes are Supremely
Affordable
The most amazing thing about SCTs? They’ve gotten cheaper. Back in the day, in 1995, I
paid over two-thousand dollars for my Ultima 8 SCT. It was a nice enough
telescope with very good—if not perfect—optics and a sturdy fork mount and tripod.
However, there was no goto or other computerization. The mount was powered by
a 9-volt battery. Turn it on and the telescope tracked, turn it off and it
stopped. It did have a Periodic Error Correction (PEC) feature, but when you
turned the scope off at the end of the night, your hard-won PEC “recording” was
lost, erased, and you had to re-do PEC all over again the next evening. Otherwise? There
wasn’t even a freaking auto-guide port.
 |
| Me and my new Ultima 8 circa 1995... |
Today, 1500 bucks in our decidedly smaller dollars will get
you a Meade LX90, a very competent SCT with a sturdy-enough mount and tripod
and a hand control with zillions of objects. If you can bump the budget up to 2700
George Washingtons, you can have an LX200 GPS, a telescope that doesn’t just
include just about every conceivable computer feature, but which has a mount at
least comparable to that of my old Ultima’s massive fork. Oh, and the LX has a
much better-looking field edge than the Ultima 8’s thanks to the telescope’s
ACF, “Advanced Coma Free” optics. While I haven’t done the computation, the LX200’s
current price is no doubt considerably less in real dollars than what I paid for that manual telescope in '95.
Meade and Celestron’s
Optics are Excellent Now
I’m not just talking about the dreaded Halley-scopes, the
SCTs produced during the comet Halley craze, when I say Schmidt Cassegrain
optics could be variable in the past. There were some excellent ones, but there
were also some dogs. The good news is that both companies are very
consistent today. There may be fewer stand-out scopes, but there are also far
fewer poor ones. Add to that advances like Celestron’s Edge optics which reduce
coma and field curvature, and Meade’s ACF optics, which reduce coma (and which
are available in f/8), and it’s no exaggeration to say that in general terms
SCT optics are better than they ever have been—and I include the legendary Celestron Pacific blue and white scopes in that equation.
The SCT is Well-suited
for a Variety of Tasks
Thanks to features like the moving mirror focusing system dreamt
up by the man who invented the commercial SCT in the 1960s, Celestron’s founder
Tom Johnson, few telescopes are so suited to such a wide variety of tasks. You
can take high resolution pictures of Jupiter one night, observe deep sky
objects the next, and do spectroscopy of distant galaxies the evening after
that.
It’s not just that the moving mirror focusing gives the SCT
tremendous a back focus range that will accommodate eyepieces, cameras, and
other sensors of all types, it’s that it has become the PC or telescopes. The SCT has been around in commercial form
for over 50 years, and both companies have more or less retained the standards
Celestron implemented in the 70s. A visual back from the mid-1980s will still
screw on to any modern Meade or Celestron SCT. That means there’s a huge number of
accessories and add-ons for these scopes.
Those many accessories include focal reducers and extenders
(Barlows), and thanks to the Schmidt Cassegrain’s focus range, it’s easy to
make these things work on the telescope for visual or imaging use. Most SCTs are natively f/10 telescopes, but just a
few dollars gets you extenders and reducers that give you an f/20 and an f/6.3
too. It’s like having three telescopes for the price of one.
The Schmidt CAT is
Usually on the Cutting Edge of Technology
When some new telescope innovation is developed, it’s usually
developed for SCTs first. When goto came
to commercial amateur scopes, it came in the form of the Celestron Compustar
and Meade LX200 SCTs first. If you want the latest and the greatest—like
Meade’s Starlock System, which provides integrated guiding and goto—look for it
in Schmidt Cassegrains and especially fork-mount SCTs first. Why? In part
because Meade and Celestron SCTs are still the most popular commercial
telescope. The numbers keep the prices down despite ever increasing features. An impetus to this innovation is the fact you’ve
got two companies competing for one small market. It’s like Honda and
Toyota—the two keep innovating and adding more features in hopes of pulling
ahead of the competition.
 |
| High-tech observing in the 90s... |
For the Prosecution…
The Schmidt Cassegrain
May be Transportable, but it isn’t always Portable
Certainly, smaller SCTs are quite portable, but not quite as
portable as they’re often said to be. If you don’t mind carrying a telescope
out in at least two pieces and assembling it, a 5 to 8-inch SCT is portable. However, even A C5
stretches the idea of grab ‘n go. Oh, some people might pick one up in one
piece and waltz it into the backyard, but it’s more of a handful than you’d
think. Most Fork mount C8s? Few of us would want to carry one more than a few
feet. A fully assembled C8 on a GEM,
even on one of the relatively light CG5/AVX size mounts? No way, not unless
you are the incredible Hulk.
Get above 8-inches, especially with fork mount models, and
even “transportable” becomes dicey. A 10 – 12-inch fork SCT OTA/fork/drivebase will weigh in
at around 50 to 70 pounds. Even when setting up in alt-azimuth mode, more than
a few people will be challenged. You’ll have to lift that awkward
tube/fork/drivebase combo onto a tripod at least waist high and get it
oriented on and bolted to said tripod. Onto a wedge’s tilt-plate for use in
equatorial mode? At the 10-inch level that is sometimes a job for two, and at 11 and 12-inches it most assuredly is. The biggest CATs, the 14s and 16s, are telescopes for permanent or semi-permanent
installations where you can at least wheel scope out of storage and onto an observing
pad.
How about a GEM, then? Mounting a10-inch on one isn’t bad. At 11-inches, however, many normal adults will be stressed. It’s not so much the
weight as it is the awkward bulk that has to be held steady as the dovetail is
slid into or tipped into the mount’s saddle. A 12-inch is entering the realm of
scary for most, and a 14-inch—which is like wrestling with a full garbage
can—is a daunting task. Mounting a 16-inch is a serious undertaking for at least two
men.
SCTs in smaller sizes are
transportable and convenient, but as aperture increases, they become
surprisingly less portable than some other designs. Today, there are 20-inch
(ultra-light) Dobsonians that are far
more portable than a 12-inch LX200.
These Telescopes are
Affordable, but You Do Get What You Pay for
There is no denying Meade and Celestron Schmidt Cassegrains
are supremely affordable considering their apertures and features, but there is
a reason they are cheap. While both companies have thousands of satisfied
customers, the road to satisfaction is sometimes a rocky one. One of the reasons
SCTs are inexpensive is that they are made and sold in (relatively) large
numbers. Another, more concerning, reason is the Chinese companies’ QA programs
are not exactly robust. Even scarier is that some of the materials and parts used
in these telescopes are of lower quality than they should be.
 |
| Me and the bigun a "few" years ago... |
A case in point was Meade’s much heralded RCX400. The one I
used (loaned me by a Meade rep at a star party) worked well and had some
amazing features. I did note the fit and finish was rather poor even for a mass
produced SCT, especially given its 4000 dollar plus price tag. Internally, it
turned out, the story was even worse. Many of the telescopes arrived DOA, often
thanks to the inexpensive motors Meade used in the focusing and collimation
system—and that was just the tip of the iceberg.
While the RCX is an extreme example, the same sort of thing,
the same low-balling of parts and lax QA, is evident all across Meade's and Celestron’s
product lines. Wise advice? Don’t be an early adopter of one of the companies’
scopes. While both do tend to get
their products right, it often takes a while—that “while” extending even to
“years.” If you get a DOA SCT and have to ship it back to your dealer or the
maker, or if your scope develops continuing problems, it may not seem like such
a bargain after all.
Meade and Celestron
Optics are Pretty Good Now, but are Still a Compromise
Both companies have come a long way since the 1980s and
early 1990s when it comes to optical quality. And, certainly, credit where
credit is due for them introducing improved designs like the Edge and ACF.
However, SCT optics with their 30% range central obstructions will always be a
compromise. Their contrast characteristics are never going to be as good as
those of unobstructed or minimally obstructed scopes. Also, thanks to their
mass-produced nature, these days you will likely get good optics but you will rarely—if ever—get great optics.
Surprisingly, both companies produce great refractor optics and often excellent
MCT optics. It seems SCTs are just a little more difficult to get to that
level, and it appears they always will be.
The Schmidt CAT is a
Jack of All Trades, but Master of None
The SCT is indeed good at many things, but there are often
other designs that are better at any one of those things. If you are an
astro-dilettante like Uncle Rod, that may not matter, but if you have a special
interest area in our avocation, it might. A Newtonian or a refractor, for
example, is a better instrument for planetary observing. A large and portable
Dobsonian is better for visual deep sky work.
A refractor, an APO refractor, is arguably a better choice for deep sky
astrophotography (although an SCT can certainly shine when it comes to imaging small
DSOs or planets).
Sometimes We’d be
Better Off Without All the High-Tech Gimmickry
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| Even "just" a C11 is a handful... |
The Verdict?
That is up to you. I’ll let you make up your minds about
it and would love to hear your decisions in the comments section. Me? These
days I am perhaps not quite the Schmidt Cassegrain evangelical I once was, but I still use them. My stable may have shrunk to a single
C8 (my Edge 800) and a C11 (whose days may be numbered), but, yeah, I still use
‘em.
When I do pull my C8, Emma Peel, out of her case, it’s like coming home. I know, Mr. Wolfe said you can’t go home again, and that is somewhat true. I am more aware of the design’s warts than I used to be (or would admit), but I still believe “right tool for the right job,” and that right tool is still sometimes the good old Schmidt Cassegrain.
When I do pull my C8, Emma Peel, out of her case, it’s like coming home. I know, Mr. Wolfe said you can’t go home again, and that is somewhat true. I am more aware of the design’s warts than I used to be (or would admit), but I still believe “right tool for the right job,” and that right tool is still sometimes the good old Schmidt Cassegrain.
2023 UPDATE
What's the story with SCTs an' me six years down the line? I still love and use them. Hell, I even wrote a 2nd Editon of my Choosing and Using a New CAT book in the intervening years. But...I love 'em every bit as much...but I don't use 'em as much as I used to. My Edge 800 SCT, Emma Peel, is, today, my only SCT.
Due to the physical limitations facing me in these latter years, I have sold off all the rest. I have Emma, and I have my li'l ETX-125, Charity, and I have a wee little orange tube C90 and those are my only CATs. I just can't manage them like I used to. 20 years ago, when I'd want a quick look at the Moon, I thought nothing of lugging a C8 into the backyard. No more. Now, it's an 80mm refractor. Even the ETX is sometimes a challenge.
I still think for many people, including novice amateur astronomers, a Meade or Celestron goto SCT is the best choice. In the beginning, you may not know which "branch" of amateur astronomy is for you. An SCT will let you try out all of them.
As for the SCT manufacturers, the water is a little muddy right now. One company, Meade, is once again American owned (by Orion). All their manufacturing is still overseas, of course, and it's not clear to me the direction the company will take. We live in interesting times, I suppose.
Comments:
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I have been using the same Meade 10 inch LX50 OTA since 1998. Now mounted to an LX200GPS fork mount, it has travelled across the country from New England to Califonia, from the seashore at Long Beach, CA to 8,000 feet above sea level at Mt Pinos, CA. Used at countless astronomy outreaches, it has proven to be a robust, reliable and faithful telescope. As long as my degenerative disk disease riddled back can stand picking it up and mounting it to the tripod, I will continue to use it.....
At the end of the day, my $100 C8 is still my favorite scope. 😃
May all enjoy what they have.
Paul Lennous
May all enjoy what they have.
Paul Lennous
# posted by 
: 3:08 PM
: 3:08 PM
At the end of the day, my $100 C8 is still my favorite scope. :)
May all enjoy what they have.
Paul Lennous
May all enjoy what they have.
Paul Lennous
# posted by : 3:09 PM
: 3:09 PM
While I really only use my scope in my yard now and mostly to using it for stack imaging, I have mine since 2001 (along with a trusty C90 that I use for astro and spotting) and have never really wanted another scope. Put the FR on it since day one and even imaging never take it off (probably will for some of the Herschel's imaging). It and the old LDX75 keep on keeping on. If the 75 dies I'll just get an AVX. it is a jack of all trades and does thing very good if not perfect, but the cost/value ratio of the 8" is very high. For me it has been a life time scope and I've looked through enough 5" refactors and 12 to 20" Dobs to say not for me. Great scopes just too big or expensive in most cases. In fact Rod gave me advice on getting an 8" and it was money in back way back when and as far as I am concerned still is today.
Whether or not you prefer it over other designs, depends also on your observing conditions and priorities. I was 100% sure my 1st priority was portability and a 6" accomplishing it very well. Thanks for the article Uncle Rod!
I think I paid over $2k for my C8 Celestar in the early 1990s... I have since deforked it and its just one of many scopes in my stable... but there is a special place for that scope, its optics are excellent and it was a great workhorse over so many years of non GOTO and it puts up some impressive planetary images too. So sure I have the top shelf APOs and other scopes, but its still an all around great scope.
$ for $ and pound for pound the SCT is a great first real scope for an aspiring amateur.
$ for $ and pound for pound the SCT is a great first real scope for an aspiring amateur.
# posted by : 7:57 PM
: 7:57 PM
Good post, and I heartily agree ... mostly. My main scope is a C-11, originally on a G-11. But more recently I've had it on an FTX with a light but stiff tripod. The mount can be carried one-handed. That, plus the C-11, a few eyepieces and an iPad with SkySafari hooked up to the encoders, and I'm good to go (visual only, of course). I don't even use a finder.
# posted by : 10:17 PM
: 10:17 PM
I had a small refractor, and a small Newton. Once I bought a Celestron C9.25 plus my Barlows and 6.3 reducer I could sell them happily. Cheers
"...It's unusual for a discussion... On Cloudy Nights... Not to degenrate into slings and arrows from both sides" ... This was your comment regarding discussions on the pros and cons of SCTs. That is an understatement! However, I usually follow these discussions with amusement, and afterwards go and enjoy sub-optimal views, enjoy the poorly designed, mass produced hardware in my trusty Celestron 6" SCT. Not quite grab'n'go, but up and running in less than 15min. I've had it for over ten years, and I will never out-observe that scope.
And for all of you SCT sufferers out there, do yourself a favor and do a google search and download Rod's Guide to Collimating and SCT. Easy for me to follow, even as a newbie (after 10years, I still consider myself that).
Keep up the excellent writing, Rod!
Regards,
J.F.
And for all of you SCT sufferers out there, do yourself a favor and do a google search and download Rod's Guide to Collimating and SCT. Easy for me to follow, even as a newbie (after 10years, I still consider myself that).
Keep up the excellent writing, Rod!
Regards,
J.F.
Nice analysis Rod. Having had a 6" Newt as a teenager in the 70's my dream scope was always an 8" Celestron SCT. I finally fulfilled that dream in the late 90's and have the same scope today. It's not perfect. Mirror flop and shift can be a pain depending on what you are doing. The closed tube currents are sometimes very troublesome and unpredictable. But on most nights it delivers the goods. I've used it for visual, planetary imaging and EAA over the years and only felt the need to expand my arsenal when i started dabbling with wide field deep sky imaging and obtained a lovely 72mm Semi apo frac. That little frac also gave such nice sharp contrasty visual views that I got a 100mm Tal RS (admittedly an achro) but that has been a bit of a disappointment - not enough aperture to provide anywhere the resolution of the C8 (without any advantage in portability) and unable to reach high mags without tiny exit pupils. Sort of neither one thing nor the other for me. I dont think a 100ED would float my boat either. My feeling is that a 150 ED frac might do it for me, but at what cost and size? Anyways, I must go back over some of your older posts about the joys of fracs...perhaps I can be convinced to get a 120ED. :o)
# posted by : 5:28 PM
: 5:28 PM
Well, Rob...here's my take...
For imaging, a 100 - 120mm ED refractor is just about perfect.
For visual use? I find my 120mm very competitive with my Edge 800. HOWEVER...for visual use, I prefer my 150mm f/8 achromat. If anything, to me, it seems a step ahead of the C8 visually. http://uncle-rods.blogspot.com/2016/11/issue-516-big-ethel-gathers-photons-at.html
For imaging, a 100 - 120mm ED refractor is just about perfect.
For visual use? I find my 120mm very competitive with my Edge 800. HOWEVER...for visual use, I prefer my 150mm f/8 achromat. If anything, to me, it seems a step ahead of the C8 visually. http://uncle-rods.blogspot.com/2016/11/issue-516-big-ethel-gathers-photons-at.html
Thanks for the link Rod, an interesting read and quite compelling. I think I will have to see if I can track down some examples of 120 ED and 150 Achro at a forthcoming star party and have a look through.
# posted by : 11:58 AM
: 11:58 AM
Looking at history SCT's became so popular due to a 'perfect storm' of circumstances.
First, telescopes were popular for a while due to the space program. People who were kids and teens during the moon landings were becoming adults in the mid 1970's and 1980's. Many of us dreamed of a 'real' telescope growing up, now we could get one.
At the same time light pollution became a real problem. If you wanted to see any deep sky stuff you had to travel. Exacerbating this was that cars were getting smaller. Since the gas shortages 1874-75 compact was the way to go. Loading an 8-inch or even 6-inch Newtonian in a sub-compact was almost impossible but an 8-inch SCT, no problem. My wife bought me a Celestron 8 for Christmas in 1982 and the whole affair, scope (in it's case), wedge, tripod, power cord, small eyepiece case and star chart all fit in the hatchback boot of a Volkswagen Rabbit. I know, because that's what I drove. Aside from the stafglation of the late 70's the economy was overall better and those new adults tended to be upwardly mobile and have fewer kids, so more disposable income. All these facts made the SCT the most versatile, portable and usable telescope bigger than 3-or-4 inches diameter.
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First, telescopes were popular for a while due to the space program. People who were kids and teens during the moon landings were becoming adults in the mid 1970's and 1980's. Many of us dreamed of a 'real' telescope growing up, now we could get one.
At the same time light pollution became a real problem. If you wanted to see any deep sky stuff you had to travel. Exacerbating this was that cars were getting smaller. Since the gas shortages 1874-75 compact was the way to go. Loading an 8-inch or even 6-inch Newtonian in a sub-compact was almost impossible but an 8-inch SCT, no problem. My wife bought me a Celestron 8 for Christmas in 1982 and the whole affair, scope (in it's case), wedge, tripod, power cord, small eyepiece case and star chart all fit in the hatchback boot of a Volkswagen Rabbit. I know, because that's what I drove. Aside from the stafglation of the late 70's the economy was overall better and those new adults tended to be upwardly mobile and have fewer kids, so more disposable income. All these facts made the SCT the most versatile, portable and usable telescope bigger than 3-or-4 inches diameter.
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