Sunday, February 19, 2017


Issue #531: The SCT Now

Wow! A magazine just for users of Schmidt Cassegrain Telescopes? Is it for real? No, it ain’t for real. I made it up out of whole cloth the other morning. Could such a magazine come to be, though? Perhaps. SCT users are hungry for reliable information about their telescopes and ancillary systems. And they don’t always get that reliable information online, as a voyage through a certain popular amateur astronomy forum will quickly show.

There is so much to the SCT world these days, so many gadgets and add-ons (the Schmidt Cassegrain has become the PC of the telescope world), that it’s hard to keep up. So, I do think a magazine like the above actually could make it if done right. At least as an e-zine as opposed to a “real” print magazine. In fact, if I were ten or twenty years younger and had more wherewithal, I’d do it myself. But I am not and I don’t. If somebody wants to do SCT Magazine, though, I gift you with the idea, no strings attached. Have fun.

Until such a publication exists, however, you can at least read about your favorite telescope design here, on occasion anyhow. Yeah, as you probably know, I’ve sorta pulled away from CATs, being more of a refractor person now. But I still like Schmidt Cassegrains and want to and will keep my hand in.

Improved SCTs

This isn’t exactly news; Meade’s ACF telescopes and Celestron’s Edge telescopes have been with us for years. But people still want to know, “Which is better? Is either one really much better than a standard SCT?”

The Meade ACF

Meade’s Advanced Coma Free design hit the streets in 2006 in the form of the company’s “Advanced Ritchey Chretien” the RCX400. I’m not going to go back and cover all the old ground concerning the spurious R-C claims and the ensuing controversy and lawsuit. Google is your friend, and I wrote about the whole episode years ago. Bottom line? The RCX (and the ACF) have nothing to do with R-Cs. They are of a design that’s been known for many a year, the “aplantic SCT.”

Meade RCX
The main difference between the ACF and a standard SCT is the ACF’s non-spherical secondary mirror (contrary to what you may have heard, it is a parabola rather than a hyperbola). The net effect is a telescope that features reduced coma compared to a standard SCT, which has coma of the same magnitude, roughly, as an f/6 Newtonian. The practical effect is that stars approaching the edge of the ACF telescope’s field look like stars rather than little comets.

How well do these ACF telescopes perform? In my experience, very well. The field edge is noticeably better that that in a regular Meade or Celestron telescope. That’s not the whole story, though. The real news is how good Meade’s ACF optics seem to be at the moment. Naturally, I can’t vouch for every OTA coming out of their Mexican factory, but those I’ve tried have been outstanding. Particularly a couple of f/10 LX200 tubes.

“F/10? Aren’t all SCTs sold today f/10?” No. The original RCX was an f/8, and today you get f/8 OTAs on Meade’s top of the line LX600 and LX850 rigs. You can also purchase 8 – 16-inch f/8 OTAs without mounts. With an f/8 ACF SCT, you get a wider field and reduced coma without the need for a reducer/corrector. The f/8s also have a much improved focusing system that eliminates focus shift and features dual focusing speeds.

Celestron’s Edge

Despite the two companies now being owned by the Chinese (perhaps by the same Chinese company, Synta; it’s hard to work out the lineage of Mainland Chinese corporations), the Meade vs. Celestron SCT arms race continues. Not long after the RCX debuted, Celestron announced a new SCT design of their own that offered even more improvement.

The Edge’s draw is that in addition to reducing coma, it also flattens the SCT’s curved field. The stars at the field edge of an Edge really are close to perfect visually and photographically. This is not accomplished by a new optical design, per se, but by the addition of internal corrective optics mounted in the telescope’s baffle tube.

Edge 800
Other than the built-in correctors, the Edges are pretty much standard SCTs. The only change is a slightly different optical prescription that moves the focal plane farther out from the rear cell (helpful in some imaging setups).  While the focuser is the same old flop/shift arrangement as ever, Celestron has added primary mirror locks to the OTA to eliminate problems with mirror flop during imaging—locks don’t help with focus shift. There are also vents on the rear cell to aid with cool-down, which is a good thing.

How are the Edges? I have the 8-inch version, the Edge 800, and, as I have said before, if ever the term “refractor like” could be applied to the images of an SCT, it is with the Edge. Optically the scope is just beautiful. The only slight downer? The Edge’s corrective optics require a specially designed and expensive reducer on the rear cell if you don’t want to image or observe at f/10.

Celestron’s reducer, which takes the f/10 scopes down to f/7, works well visually and photographically. Unfortunately, though, the reducers proved hard to design, expensive to produce, and had to be tailored to each aperture. There are reducer models for the C8, C11, and C14, but one has not appeared for the C9.25 and it doesn’t appear one ever will—probably not enough 9.25s are sold to make a reducer for them financially viable. Other companies, like Optec, are producing reducers for use when imaging with the Edges, but unlike the Celestron reducers, they cannot be used visually.

Which should you choose? The ACF or the Edge? I own an Edge and am quite content with it. However, I find the field edge of the ACF to, frankly, look every bit as good as that of the Edge to my aged eyes. The ACFs I’ve used have been impressive, and if I were to buy a new SCT, which doesn’t seem that likely at this juncture, it might well be a 10-inch f/8 ACF.

The deeper question is, "Should I get an improved SCT at all?" That depends on you and your agenda. While these telescopes produce fine images, are they worlds better than those of a standard SCT equipped with a reducer corrector? No. I recommend an improved SCT mainly for imagers using at least APS-C sized if not 35mm full frame sensors.


Things have changed in SCT land. For nearly 30 years “SCT” equaled “fork mount.” That’s no longer the case, with many prospective SCT users refusing to consider the time-honored fork configuration, and instead drooling over sexy and expensive German equatorial mounts.

While it’s true GEMs have some advantages, especially when it comes to making large aperture CATs more manageable, the fork has its advantages too, like making imaging near the Meridian more practical.  While the fork may not be sexy anymore, probably more SCTs are still sold in fork mount packages than as bare OTAs or GEM configurations (excepting the Celestron C14, which hasn’t been sold on a fork for many years).

The Meade LX600

Yes, the two manufacturers have continued to sell forks with wild abandon, but only Meade has offered anything new (well sorta) in this arena in recent times. That new fork is the LX600.

What makes the LX600 a prime choice for an SCT user wanting a fork mount scope? It’s not so much the excellent f/8 OTA, or even the StarLock system which handles pointing and guiding chores (and operates full time), it’s that somebody finally did something about the SCT weight problem.

One of my favorite SCTs of all time was my old fork-mount NexStar 11 GPS, Big Bertha.  I used her happily for more than a decade, but recently I had to admit she was becoming just too much for me. Too heavy that is. Even when I was a decade younger, lifting her 68-pounds onto a tripod (or loading her into and out of a vehicle in her huge case) was a not inconsequential task. I could still do it at the time I removed the OTA from the fork and bought a CGEM to use as her mount two years ago, but I no longer wanted to—and hadn’t really wanted to in a long time. What good is a scope you don't want to use?

Do I like Bertha on a GEM? Yes, but. The fact is that the fork was more convenient and comfortable, especially for visual observing in alt-azimuth mode. If only she had been a little easier to set up and transport.

Years ago, Celestron’s enormous old fork mount C14s could be removed from their mounts. It wasn’t easy to do, but it could be done, and you didn’t have to remove hordes of screws to do so. It made setting up that huge scope at least somewhat easier, if not easy. I wondered for years why M&C didn’t revisit that idea for larger aperture SCTs.

Enter the Meade LX600. The tube can be removed and replaced on the fork with some ease. Not only does the tube come off the fork, so do the upper fork arms, which go into alignment pins on the lower fork assembly. I won’t try to tell you that that will make mounting the 12-inch and 14-inch scopes trivial, but it is easier.  It’s the 10-inch that really benefits from this, set up. The removable OTA turns the scope into something at least doable for the broken down among us like your correspondent. Even with the removable OTA, trying to get the telescope set up in equatorial mode is not a safe job for one person, but it does make assembling the 10-inch in alt-azimuth fashion at least thinkable for a lone observer.

Celestron Evolution

EVO and friend...
When it comes to forks, most of Celestron’s recent releases have been incremental improvements. For example, their CPC Deluxe Edge scopes use a fork and drive base much like that of the standard CPCs, the successor to the NexStar GPS scopes. The only advance is that the Deluxe has (somewhat) improved gears and motors. Yep, I found the CPC Deluxe to be kinda ho-hum, but that didn’t mean Celestron didn’t have an interesting new fork idea up their sleeves: the Evolution.

The Evolution, available in 6, 8, and 9.25 inch apertures, at first glance doesn’t look much different from the light single arm-fork equipped NexStar SE. Like the NexStar SE, the Evolution mounts the tube to the fork using a Vixen compatible dovetail, making these small – medium aperture CATs quite portable indeed. That is not all there is to the “Evo” story, however.

The innovation here is that the Evolution comes with built-in wi-fi control. That’s right. You align and operate the telescope with your iOS or Android phone or tablet. The Evo comes with a hand control, but most users will never have to mess with it; they will prefer to run the Evo with their phones and SkySafari. I would guess the Evolution is the shape of things to come and that it won’t be long before the company’s larger fork mounts include wi-fi.

Which fork mount telescope would I choose if I were to buy one today? If I were wanting to do astrophotography, it would be, hands down, the LX600. That F/8 OTA and built in autoguiding system make a task that can often be daunting, imaging with a larger aperture SCT on a fork, much less frustrating. If I just wanted a portable CAT for looking, planetary imaging, and perhaps dabbling in deep sky photography, it would probably be the Evolution. Certainly, though, both companies’ older setups, particularly the Meade LX90, also deserve a look if you want a general use Schmidt Cassegrain.


Where there are SCTs, there are accessories. Celestron and Meade still offer plenty of stuff to trick out your CAT, if not quite as much as they did in their salad days. What’s out there now? Focal reducers…GPS receivers…SCT style diagonals, yadda, yadda, yadda. None of it too inspiring. Well, with one exception, which happens to be from Celestron.

To be accurate, the Celestron StarSense alignment camera/system is not specifically an SCT product; it’s usable on most Celestron mounts, fork or GEM. Nevertheless, it’s often purchased for SCTs, and Celestron even offers some CAT configurations that include the StarSense in the package. Be that as it may, it’s one of the more impressive and useful add-ons it’s been my pleasure to try.

I first used the StarSense a couple of years ago. I was skeptical this little camera and replacement hand controller could really do as good a goto alignment as I could do manually. Frankly, I didn’t believe it would work at all.

I was completely wrong. Despite an early firmware release in the unit I tried, the goto alignment it produced was easily as good as what I could do myself, and it sure was a lot easier than centering up to six stars (or sometimes more) manually.

In the last couple of years, Celestron has cleaned up the firmware, and the StarSense is better than ever. One of the great benefits of it isn’t just the time/labor saving, but that it encourages astrophotographers to make best use of the Celestron All Star Polar Alignment Procedure (in the hand control).

To get the best polar alignment possible with ASPA, you really need to do two iterations of it. Unfortunately, you also need to redo the goto alignment after each ASPA. That means that when you are done you’ll have centered a total of 18 stars, not that much fun. StarSense takes away all that pain. It handles the alignments (in about 2 – 3 minutes each). All you have to do is center the ASPA star with the altitude and azimuth adjusters.

I also think StarSense has some untapped potential. Integrate it with a guidescope, and you’d have something like Meade’s excellent StarLock system. But one you could buy aftermarket and use with your Celestron scope.

Anyhow, that’s some of what’s happening on the current SCT scene. As new products and technologies arise, I promise to keep you updated, even if the telescopes I’m usually using out on the observing field are (choke!)  refractors.

How do You Focus?

Well, you twitch the focus control until the image is sharp. That’s fine for visual observers, but attaining good focus for imaging can be and often needs to be a little more complicated. Your eyes can compensate for slightly out of focus images, even stars, when you’re observing visually, but slightly, just slightly, out of focus stars in images look absolutely dreadful. How can you ensure you are in dead-on focus?

There are various ways of achieving exact focus. More than a few camera control programs like Nebulosity have a fine focus routine that will get you there. You kinda need to get close to focus before those are effective, however, and I hate going out to the scope, twitching focus, going back to the computer, squinting at the images, and—well you get the idea. One way to achieve close focus quickly is with a Bahtinov mask.

What’s that? If you haven’t heard—they’ve been in use by imagers for some years now—it’s a plastic (usually) mask with slots cut in it. It fits over your objective, corrector, or the end of your reflector’s tube and produces a peculiar diffraction pattern on a star as seen here.  You change focus until the two horizontal spikes are precisely centered between the diagonal spikes on each side. I find the Bahtinov sensitive enough that I usually don’t even have to worry with Nebulosity’s fine focus routine. Set the camera for 1-second exposures, get the spikes centered, and I am done.

While I have a couple of Bahtinov masks for my SCTs, I didn’t have one for my 5-inch refractor, and decided I wanted to make focusing less onerous with the lens scope. I could have made a Bahtinov mask easily enough. There are routines on the Internet that will draw a template for you. But the idea of fumble fingered me messing around with a sharp Exacto knife sounded like a recipe for disaster. I’d buy instead.

I have never been that happy with the SCT Bahtinovs I have. I just don’t like their mounting system or lack thereof. You lay them on the corrector, which isn’t really that great an idea in my mind. But who makes better ones? Coincidentally, one recent morning I got a Facebook message from a friend of mine, Andrea Salati. He mentioned therein that he had begun producing Bahtinov masks and wondered if I’d like to try one, “Sure.”

Andrea’s mask is great, well-made from sturdy plastic. But, let’s face it, a mask is a mask is a mask. What makes his different is the mounting system. It uses three adjustable pins in slots that allow you to size the mounting precisely for your scope (on mine, the pins go outside the dew shield). Neat. Elegant. A pleasure to mount and remove. I recommend Andrea’s mask highly and suggest you get one from him for your scope ASAP (he sells Bahtinovs for a range of apertures). Tell him Uncle Rod sent you: 

Unk great article for a beginner back yarder. I was a little surprised you said you would go for the Evolution since your Edge came with the AVX. If you could go back would in time would you have bought the Evo over the AVX?

I like the VX very much, but if I were more into "just visual" would be the 8-inch Evo all the way.
That was mean, Uncle Rod. I got excited for a new magazine for SCT users....
Unk, as I am about to overcome my collimation-phobia and dip my toe into the SCT pond, this article couldn't have come at a better time for me. Thanks.
I actually have a 9.25" and I absolutely adore it. Less futzing with unreliable USB adapters and it's quick to go. Just wish SkySafari was on Windows as well as Mac, since I'm using two computers to run everything right now.
Great article, very helpful in decision making, loved my LX200 years ago. However I find myself loving the refractor more when it comes to imaging. However you have given me thoughts on the LX600. Thanks
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