Sunday, March 19, 2017


Issue #535: The Final Piece of the Puzzle

In our pre-spring observing season drive to get novices (and maybe even a few not-so-novices) set up with a rig for deep sky imaging, we’ve addressed mounts, telescopes, and, last week, auto-guiding setups. This Sunday we’ll finish with suggestions for a low-cost camera. I’ve talked about imaging cameras with y’all fairly recently, but the difference is that this time I’ll try as hard as I can to keep the cost as low as possible.

So, you need a camera and a few accessories. Where do you start? The first question to answer is, “Do I want color?” While a monochrome CCD/CMOS astronomical camera can take color images by exposing successive frames through three or more colored filters, it’s not something you want to face when you are just getting off the ground in imaging. Unless you enter the ranks of the hard-core someday, you may never want to face it. In the beginning you will find just processing a “one-shot” color image enough of a challenge. Properly calibrating and combining three + separate frames into a color frame and then stacking and processing a bunch of those? Uh-uh.

So, it’s a color camera, a one-shot color camera, you want. How does one work? A color camera is different from a monochrome camera in that red, green, and blue color filters are built into the sensor chip. Software, either in the camera or in an image processing program, automatically combines the R, G, and B to produce a full color image. That is usually transparent to the user—with a digital single lens reflex (DSLR), anyway. You take a picture, you see a color image, end of story.

Some astrophotographers say a monochrome camera can produce visibly higher resolution images because it doesn’t waste pixels on the production of a color image. In truth, in the beginning at least, and especially on deep sky objects, you won’t notice any difference.

The next question is “CCD or CMOS?” That is not much of a question today. Unless you are interested in some special applications, mostly having to do with obtaining scientific data, there is no reason to choose a CCD chip over a CMOS chip. Today, the formerly preferred CCD has lost ground to CMOS sensors even for use in “astronomical” cameras. CMOS chips are now very sensitive and very low in noise. At any rate, almost all cameras in our price range, which I am topping out at 450 dollars, have CMOS chips, so the choice has already been made for you.

What a ZWO ASI120MC can shoot...
Next up, cooling. “Does a camera for taking long-exposure images need to have its sensor chilled to reduce thermal noise?” Today, probably not. With some camera/chip combos, an internal fan, at least, can be helpful to reduce the false stars of thermal noise, but the low-noise characteristics of today’s sensors usually means subtracting a dark frame is enough to deal with thermal noise.

And the Final Jeopardy Question… “Astro cam or DSLR?”  There are some interesting low cost astronomical cameras coming on line, like those from China’s ZWO, and I’ve actually taken credible deep sky image with one of their 1/3-inch cameras that cost a measly 200 dollars. However, I think for most of us a DSLR is just a much more sensible choice. A much more sensible choice.

Why is a DSLR better? There are several reasons, but there is one real big one:  when you’re not taking pictures of the night sky, you can be wowing everybody at your mother-in-law Margie’s birthday party with your snapshotting skills. There’s also that big elephant in the living room. Like many wannabe astrophotographers, a few nights wrestling with camera and scope may convince you you are actually more of a visual observer. If that be the case, you can still get years of use and enjoyment out of the DSLR, even if you never take another astrophoto with it.

Another big plus (for astro imaging) of the DSLR? Their relatively big chips. A less than 500 dollar camera will have an APS-C size chip. Lower cost astro-cams tend to have small chips that restrict your field of view, focal length for focal length, and also tend to make guiding more critical. 

Finally, while I control my DSLRs with a program running on a laptop (“tether them,” as we say in the photography business), which makes focusing and framing much easier, you don’t have to do that. You don’t have to have a computer out in the field when you are taking pictures. You can do just as we did in the SLR days:  telescope, mount, camera. You will, as in those SLR days, need a remote camera release (an intervalometer, preferably), but that is it.

OK, so which DSLR? The safe thing to say is still “Canon.” In some ways they still lead the pack in astrophotography. The Canons are remarkably low in noise over long exposures, and are easy to use in the field with a laptop if you choose to do that. Things are changing now, but until recently camera control software (like Nebulosity) was unheard of for other brands.

SCT Prime Focus Adapter
There’s also Canon’s longstanding involvement in our game. While Nikon and, now, Pentax are coming on strong for astrophotography, until the last couple of years only Canon acknowledged people were actually using their cameras for astronomical imaging and produced cameras with astronomy in mind.

Canon is a safe choice, in my opinion, but which one of their many DSLRs? If you are buying new and must keep the price tag low, the Rebel T6, which is available for about 450 dollars, is a remarkable value. Not only do you get a DSLR that will perform well for astro-imaging or anything else, you get a pretty good (zoom) kit lens for use in wide-field astrophotography or at Margie’s above mentioned b-day party.

Just don’t want a Canon for whatever reason? The equivalent Nikon is the D3300, which is even less expensive than the Rebel. And it can perform very well for astronomical imaging. BUT… Computer control options for this camera are (very) limited—it is not supported by the major Nikon astrophotography program, BackyardNikon—so if you want to tether camera to computer, a Canon is a far better choice.

How about buying a used camera? Is that a good idea? That depends. A fairly recent camera or seldom used older camera can push prices even lower. A perfectly serviceable older Rebel, like a 450D, for example, goes for 150 or fewer dollars with a kit lens and a few accessories. Be careful here, though. While the Rebels, Canon’s introductory DSLRs, and Nikon’s comparable models are well-made, they are not professional grade cameras and won’t stand up to real abuse. So, when considering an inexpensive camera it’s best to limit yourself to one that’s for sale locally so you can examine it in person and make sure it’s fully functional.


Prime Focus Adapter

Prime focus adapter (1.25-inch)...
Once you’ve got a camera, of course you’ll need accessories. You always need accessories in astronomy, you know that!  First off, you will need a prime focus adapter in order to connect camera to telescope. “Which” depends on your scope style. SCT prime focus adapters screw onto the SCT’s rear port. Those for other telescope designs, like refractors, typically have 1.25-inch or 2-inch nosepieces and slide into the scope’s focuser. I like the 2-inch models, not because you have to worry about vignetting or something like that with an APS-C size sensor, but because they allow me to dispense with a 1.25 – 2-inch eyepiece adapter and seem to provide a more secure mounting arrangement.


You’ll also need a t-adapter for your camera, aka a “t-ring.” This is a, yes, ring shaped adapter with T-threads on one end to screw onto the prime focus adapter, and a lens mount for your particular camera on the other end. These two things in hand, you can remove the camera’s lens, mount the combo of T-ring/prime focus adapter in its place, and then mount the camera on your scope by inserting everything into the focuser or screwing the prime focus adapter onto the rear port of an SCT.


As you may know, DSLRs, most of them anyway, and certainly all the Canons, can’t expose for more than 30-seconds without the addition of a remote shutter release. Even if your camera could expose for longer without a remote, you’d still want one as it allows you to trip the shutter without bumping the scope and causing trailed stars.

An intervalometer is a remote shutter release, but it’s also much more. Not only will one of these (usually) wired controls allow you to trip the shutter from a distance and expose for as long as you like, it will allow you to shoot sequences of images. Say 30 3-minute exposures, which is exactly what we want to do. An intervalometer allows you to do many of the things a tethered computer would allow you to do, but without the computer. How much? A Vello is about 50 bucks and a genuine Canon is about three times that. Guess which one I’d choose?

Memory Card

If you’re not using a tethered PC, you’ll have to have a memory card, digital "film" on which to store your images. An SD card (used by almost all DSLRs, now) with at least 64gb capacity is my recommendation—you’d be surprised how much space an evening’s images can take up. Get a good, decently fast card. I like the Sandisk ones. About 40-bucks.


If you’re going to use a battery, make sure you keep an extra, or, better, two extras in your gadget bag. During long exposures, the camera is drawing current from the battery continuously, and you’re unlikely to get a full evening out of one cell, especially on cold nights. There are lots of third party batteries available, but I have had noticeably better performance out of genuine Canon, so that’s what I recommend here, the real deal, for a change.

Power Supply

Yes, batteries are a problem during astrophotography, so don’t use one, or use a real big one. Hop on over to Amazon and buy yourself either a 12vdc or 120vac power brick for your Canon (or whatever). I do most of my shooting at locations with mains power, so I prefer the AC option. The DC supplies have cigarette lighter plugs that will plug right into your jumpstart battery pack.

What do you plug one of these things into on the camera end? These power supplies have little plastic (wired) widgets that take the place of the normal battery in the battery compartment and supply power to the camera that way. I’ve found one of the inexpensive—less than 15-dollars—units on Amazon to work just fine, but Canon will sell you one for considerably more if you like.

Anything else? Well, a few things, maybe. If you are new to DSLR photography, you probably want a camera bag, a gadget bag, to keep camera and lenses and, well, gadgets, together. A nice piggyback bracket so you can mount DSLR and lens on your telescope tube is a nice addition and you may find you like doing wide-field shots from dark locations. A lenspen is good to keep your lens’ surface pristine. A broadband light pollution filter can be helpful if, like me, you do some of your imaging from an at least somewhat light-polluted backyard. And that is really more than enough to get you started.

You’ve now got all the pieces to the complicated astrophotography puzzle, but how the heck do you put them together? We’ll talk about that, about getting started with all this stuff, next week.

Addendum:  How good can a VX be?

Auto-guiding wise, that is. Some of you considering a Celestron Advanced VX mount (or the similar mounts on the market today) have expressed grave concern about my statement last week that 2” (arc seconds) of RMS guiding error is about what you should expect of this group without some fine-tuning (of PHD’s Brain Icon settings, I mean).

Anyhow, while 2” is perfectly suitable for some image scale/camera pixel combos, naturally it would be nice to do a bit better with this inexpensive and highly portable GEM. So, I set about the other night to see how much and how easily I could tweak the VX.

Surprise! I really didn’t have to do much tweaking at all to get this modest mount’s RMS guiding error down. I did do a decent polar alignment, and I did spend some time carefully balancing the scope (east heavy with a little declination bias as well). As for the settings, I backed off on a couple of them. Cutting aggressiveness in half and reducing hysteresis as well. Oh, and, conversely, I increased Max Duration both for RA and declination.

The result? Despite OK but hardly great seeing, my errors were immediately halved with me getting just under 1” of RMS error most of the time. Even when my target got low in the sky, and seeing began to deteriorate, the error was just over 1”, easily good enough to yield round stars with an 80mm f/6.9 despite the fairly small (1/2-inch) sensor of the camera I was testing.

While I warned you not to start chasing lower and lower numbers with these GP/CG5 clone mounts merely for the sake of lower numbers, given the small amount of effort involved in this substantial improvement, the few minutes I spent was well worth it.

The other take-aways? People naturally worry about their guide-software settings, but what makes one of the very largest differences? Seeing. Without good seeing you will not see great guiding, so don’t start messing with your settings on an unsteady night. Oh, and good polar alignment is important for good guiding as well. Having to continually chase alignment-caused drift just muddies the water and makes guiding more difficult to get right. Finally, with this class of mounts, correct balance is just as important as polar alignment and seeing. If you want 1” or less guiding errors, you’ll likely need to rebalance if you move to a radically different part of the sky—cross the Meridian, etc. 

Sunday, March 12, 2017


Issue #534: Getting Your PHD

PHD2, that is, as in America’s premier auto-guiding software. I have written about the program, originally done by software wizard Craig Stark and now carried on as an open-source project, a time or two before, but lots of people still have lots of questions about it. It’s rare that my virtual mailbag doesn’t contain a missive pleading for help with PHD.

Before offering some of that help, I suppose I should explain what PHD2 is for the uninitiated. You’re probably more knowledgeable than I was when I began astrophotography.  Unlike me, you know you can’t just point your telescope and camera at a deep sky object, open the shutter, and walk away. You have to guide. The gears in most mounts are not precise enough to allow the scope to track precisely enough over longer exposures to keep stars round without some intervention.

To keep stars round, you watch a “guide-star” either with the main scope or a small auxiliary telescope, a guide scope, keeping it precisely centered. Or a little camera does that watching for you. There are some mounts that will allow you to dispense with guiding for long exposures, but you are talking about mounts in the 10micron class, expensive, top-tier mounts. Proletarians like yours truly guide their mounts throughout long exposures.

How exactly do you do that guiding? Well, back in the day, you monitored a guide star in a crosshair eyepiece in  the guide scope or in an off-axis guider, and pushed buttons on a hand-paddle—what we called our non-computerized telescope mount hand controls—to keep the star centered. Naturally, when computers and CCD cameras came along, we were more than happy to pass the onerous task of guiding to them.

A guide camera is used to watch that guide star, but most guide cameras cannot guide the telescope mount without the help of a laptop computer and an auto-guiding program. That program is the brains of the outfit, and that is what PHD2 is, auto-guiding software.

If you need direction on getting PHD2 downloaded, installed, and initially configured, please see this (fairly) recent article. Today, we’re going to focus on what you need to do to get PHD2 performing by fine-tuning its default parameters. What you have to do to get those pesky stars round.

What does “PHD” stand for, anyway? It ain't “doctor of philosophy,” but instead, “push here dummy.” Mr. Stark’s original goal was to produce an auto-guiding program that was as simple as it could possibly be. One that would allow you to hook everything up, push one button and guide your way to round star heaven. That’s actually possible in some cases, but due to the nature of the beast, often not.

The Guiding Tab...
There are so many different possible configurations of telescope/guide scope/guide camera/main camera/telescope mount, etc., etc. that making a no-set-up auto-guide program is a near impossibility. Oh, if you stick to shorter focal lengths (500mm and down) on a decent  (VX and up) mount, and don’t insist on longer than 300-second sub-frames, it is possible all you will have to do is push that button and guide. Most of us will have to mess with PHD’s guiding parameters, which are accessed with the program’s famous brain icon. Before we attack that, though, a couple of preliminaries: “What is the best way to guide?” and “What is the best guide-scope to use?” 

I am frequently asked by newbies how they should guide. Should they use an ST-4 connection, a direct connection from a camera to a mount’s auto-guide port, or should they guide through the hand control’s serial port?  I asked myself that very thing years ago when I first essayed auto-guiding.

Some people think serial port guiding, particularly “pulse guiding,” a feature of some ASCOM telescope drivers, is better since each guide message going to the mount contains not just the direction the telescope needs to move, but also for how long. With ST-4 guiding, once the software decides the mount needs to move, it will cause the camera to close an electronic “switch” to move the mount. When the move is done, the switch is opened. With pulse guiding, there is no (possible) time-lag resulting from ST-4 mode guiding having to send an additional command to open the switch. On the other hand, ST-4 fans say that since no back and forth computer talking is needed with ST-4 mode guiding, it must be inherently more responsive.

The ground truth? With my mounts/scopes/guide-cams, there was absolutely no difference in accuracy between the two methods. The pluses for each have more to do with convenience. If you are controlling your mount with a computer, why not pulse guide? If you are using EQMOD in particular, that seems a natural—everything, goto commands and guide commands, is routed to the mount over a single cable. On the other hand, while ST-4 guiding requires an additional cable run from camera to mount, there’s no fooling around with serial connections and USB to serial adapters, which is a good thing. I normally do ST-4 for that reason.

Calculating cal step size...
The other question concerns the guide-scope or lack thereof. What sort of a guide-scope should you use? In my opinion, the answer is one with a focal length of about 400 -500mm. That provides a fairly wide field for small guide-cam sensor chips, but also has enough image scale for precision guiding. The venerable Short Tube 80mm is a good choice as long as you can lock the focuser down firmly and mount the whole thing securely to prevent image-destroying flexure.

Me? I use a short focal length 50mm finder-guider. One of these will work up to about 1200 – 1300mm of imaging scope focal length, and is small, light, and easy to mount firmly. For anyone who doesn’t top 1000mm of imaging scope focal length, a finder-guider is a natural. Having that wide field is often a blessing when it comes to choosing guide stars.

There’s always the option of doing without a guide scope, too. Using an off-axis guider (OAG) which intercepts a small amount of the light coming out of the main scope for guiding. Obviously, since you are guiding through the main scope, there is no flexure to worry about. If you are running an imaging telescope at over 1500mm of focal length, you may find an OAG is your only workable option. The downside? You only have access to stars at the edge of the main scope’s field, and for that reason it can be quite difficult to find a good guide star. Luckily for me, a long time OAG hater, I rarely image at a focal length long enough to require one.

One final thing to discuss before we do “brain surgery.” How good does your guiding have to be? How much error is acceptable? The answer is, “that depends.” At 1000mm or less with an APS-C sized camera sensor chip, an RMS error of around 2” or so is good enough. Stars will be round and small enough to please. You can even get OK (if sometimes not perfect) stars at that error level to about 1500mm of focal length.

It’s a good thing this degree of error is acceptable at the focal lengths I use, since the plebian mounts I have in my inventory, GP clones like we discussed last week, and the EQ-6 and CGEM mounts a step above them, will deliver 2” of RMS error with fair ease. Getting guiding much tighter than that with these sorts of mounts isn’t always easy and will often take considerable experimentation.

Alright, click PHD2’s brain icon and let’s get started entering some guide parameter values in place of the defaults, parameters than will bring us round stars (we hope). With the brain window displayed, skip its first two tabs, “Global” and “Camera,” since I’m assuming you’ve gone through them in the initial program setup. Which brings us to…

Guiding Tab

The Algorithm Tab...
The first entry here is “Search Region.” This is the size of the tracking box PHD2 draws around a star. Normally you should leave this at the default value. If you have so much drift between guide exposures that the box needs to be larger, you aren’t going to get anywhere with guiding anyway. The accompanying “Star Mass Detection” has to do with PHD2 monitoring the star’s brightness as compared to the sky background. Leave this as is as well. Likewise, leave the tolerance setting for Star Mass Detection alone.

The next part of the window is quite important, “Calibration.” Enter the focal length of your guide scope (you should already have entered the size of the guide-cam’s pixels in the “Camera” tab), push the button labeled “Calculate,” and PHD2 will figure out how long guide pulse duration should be during calibration. The main concern here? If you have a short focal length guide scope like I do, you need to enter a much higher calibration step size than the default. I have a value of 1350 here. Given the short focal length of my 50mm finder-guider, I need that large a setting. Otherwise, calibration would take all freaking night to complete. Leave the other stuff here alone.

The final part of the window contains things you don’t have to worry about in the beginning. Well, except for one thing. Make sure “Enable Guide Output” is checked, otherwise PHD2 will not issue guide commands to the mount. It will be like that goober in the TV commercial, “I’m not a dentist; I’m a DENTAL MONITOR.”

Algorithms Tab

Here’s where we get down to the nitty gritty, the place where you can change the settings that really and truly affect guiding. You’ll see that the window is divided in two, with one area for right ascension and one for declination. Let’s begin with RA.

The first thing to set is Hysteresis. PHD2 is pretty smart; it can remember what the last RA correction was like and use that information in formulating the next correction. The number here is a percentage. It is how much the remembered previous correction affects the next one. At 40%, the next RA correction will be 40% based on the magnitude of the previous correction, and 60% on the star movement PHD2 is seeing at the moment.

Guiding Assistant...
What should you set it at? More Hysteresis yields smoother guiding. Too much, however, and a sudden guide star movement will not be adequately compensated for. I have my value at 40%, which seems OK.

Coupled with Hysteresis is “Aggressiveness.” That setting is how much (as a percentage) of the calculated necessary movement PHD2 actually sends to the mount. The reason for this is to decrease the chance of the mount overshooting the star, going back the other way on the next guide command, and overshooting in that direction too, “ping-ponging.” Normal settings rage from about 70% to 100%. I am set at 85%.

Next is “Minimum Move.” This is the amount the star is allowed to drift without PHD2 issuing a guide command. The reason for this is to reduce unneeded guiding corrections caused by non-tracking related star motions due to seeing or other momentary events like mount vibration, wind, etc. The default is .15 and that’s where I’ve left it.

Max RA duration, the last setting on the RA side, is similar to the above in that it’s meant to smooth out guiding, to prevent herky-jerky guiding. This figure is in milliseconds, and limits the duration of the RA guide command. I’ve settled on a value of 1200 for RA through trial and error. I am thinking that is low, however, and might try a higher value next time out. 

Now for the declination side of the house…

First up is “Resist Switch,”  which means PHD2 tries to avoid reversing the guide direction in declination. That is always a good thing, since in many cases issuing a guide command in dec to go back the other way will be a problem. Star movement in declination opposite the constant slow (you hope) drift caused by polar alignment errors is usually caused by seeing, vibration, mount flexure, wind, etc., and as with RA, we want to avoid issuing guide commands for these things. Most of all, many mounts have considerable backlash in declination, which would create a considerable time lag between command and movement if the mount reversed direction in dec.

Also on the declination agenda are aggressiveness, minimum move, and backlash compensation settings. I have the first two at the same value I have for RA. The backlash compensation option determines whether PHD2 will use a backlash compensation value it has computed if a declination correction opposite the previous one needs to be issued. I have this off, since I don’t seem to be having any major dec problems.

Max Dec Duration has the same purpose as in RA, to smooth guiding. I have my value set a little higher here than I do in RA, 1500, but it could probably be higher still.

Finally, there is “Dec Mode.” Normally this is set to “Auto,” which tells PHD that the occasional declination reverse guide command (caused by whatever) is permissible. Why would you want to disallow this by selecting “North” or “South”? If your mount has really bad declination backlash, trying to make a “reverse” correction may cause serious problems—the cure may be worse than the disease. I am set to “Auto.”

And that is it, folks. The other Brain tabs cover use of adaptive optics guiders and are of little interest to most of us.

Getting round stars with an import mount is fairly easy at 900mm...
How do you fine tune your mount if these values don’t work for your particular setup? Trial and error, which was what I did to arrive at the numbers I’ve given here. There is one alternative, though, PHD2’s “Guiding Assistant.” Theoretically, invoking this tool should allow the program to decide what your guiding values should be. When the procedure has completed its work, it will make suggestions, which you can implement or ignore at your discretion.

Alas, when I tried Guiding Assistant some time back, one night at the 2015 Peach State Star Gaze, the figures PHD2 came up with seemed to make my guiding worse rather than better. However, that was over a year ago, so the Assistant may have been improved by now. If you invoke it and use the suggestions, make sure you’ve written down your old numbers so you can get back to the way things were if Guiding Assistant doesn’t work for you.

I hope all this stuff didn’t put you off too much. Again, with a halfway decent mount and a reasonable focal length, you might not have to do much with anything beyond basic setup other than just setting your calibration step parameter. And remember, if your stars are round your stars are round. Don’t start chasing lower and lower error values just for the sake of lower values, “The Only Enemy of Good Enough is More Better.”

Sunday, March 05, 2017


Issue #533: A New Way to Polar Align

If you are using an equatorial mount, fork or German equatorial, for imaging, that mount has to be accurately polar aligned. The right ascension axis has to be pointed precisely at the North Celestial Pole or South Celestial Pole. If it’s not, longer exposures will suffer from a phenomenon called “field rotation,” which makes stars trail no matter how accurate the guiding. Heretofore, there were basically two ways to polar align a mount, the easy way or the hard way.

The easy ways? One was to use a polar borescope on a GEM. Once you figure out how to set it up, a polar finder can yield alignments ranging from excellent to usable depending on the borescope’s manufacturer and your expertise in using it. For many folks kneeling on the ground to peer through that dim little telescope in quest of a sometimes-rough polar alignment (unless you have a Takahashi mount and its excellent polar finder) is a bummer. Also, no truly accurate polar finder has ever been produced for fork mount telescopes, though some people, like the late Roger Tuthill, have tried.

Another fairly easy polar alignment method is “Kochab’s Clock.” That involves lining up the RA axis with the help of one of Ursa Minor’s stars. Kochab’s can potentially yield a good alignment  if done with care, but in most cases, not a sub 5’ – 10’ alignment.

Finally, there is the sure thing, a declination drift alignment, which, unfortunately, most of us don’t consider overly easy. Or at least not overly quick. You observe a pair of stars near the Celestial equator, and watch their drift in declination (through the main scope) as the telescope tracks, adjusting the mount’s altitude and azimuth controls until there is no significant north/south drift of either star over at least five minutes of time.

“Drifting” is not hard once you get the hang of it, but it does take time, and you have to be able to acquire suitable stars, one near the intersection of the Celestial Equator and the Local Meridian, and one near the Celestial Equator and about 15 - 20 degrees off the eastern or western horizon. That’s not always possible at every observing site.

And there things remained for years. In the 1990s, I used a fork mounted SCT, and did a two-step polar alignment. First, I’d rough it in using a 50mm finder scope with a polar alignment reticle. That was, as above, not a recipe for a good alignment on a fork mount scope, but it at least got me in the neighborhood. Then, I’d go on to drift, which took about half an hour or so once I gained some experience. I never liked drifting, though, and for that reason I usually quit before my polar alignment was quite good enough for the long exposures required in the film astrophotography days.

Typical polar borescope finder reticle
Flash forward ten years or so to the coming of the computerized GEM mounts like the Celestron Advanced GT CG5. One of the big breakthroughs with the CG5 (and also a few other brands) was an automated polar alignment routine. With the CG5, you did a three-star (no 2+4 in the firmware’s early days) goto alignment. You then requested “Polar Alignment.” The mount would then point at Polaris, and over the course of a couple of steps would slew away from the star. You’d then re-center Polaris using the altitude and azimuth adjusters. While you would be centering Polaris in the eyepiece, what you’d really be doing would be offsetting the RA axis to place it on the true Celestial Pole about ¾ of a degree away from Polaris (the routine also worked in the Southern Hemisphere).

This procedure didn’t produce a great polar alignment, but it was a little better than what I could do with the CG5’s (pitiful) polar borescope, and it was definitely quicker. It was sufficient for the short exposures at short focal lengths I was doing with my Meade DSI CCD camera at the time.

Then came Celestron’s new polar alignment routine, AllStar Polar Alignment, ASPA, in late 2008. This alignment procedure was different mainly in that it allowed you to supposedly use any star (other than Polaris) for polar alignment. We eventually found out a good ASPA star was not really any star, but a star due south and on or lower than the Celestial Equator. Get a good star, do two iterations of ASPA, and you’d have a close enough polar alignment for most imaging tasks.

While AllStar was not inherently more accurate than the old Polaris system, it was coupled with the new and much more accurate 2+4 goto alignment in Celestron’s updated firmware. With these types of polar alignment routines, the better the goto alignment, the better the resulting polar alignment. How accurate was/is ASPA? You’ll wind up about 10’ away from the pole or a little better, usually, with one iteration.

The downside? If you wanted better than that 10’ or thereabouts, you needed to do two ASPAs.   That could be a bummer since you’d normally want to do a new goto alignment after each ASPA (or at least “replace” the last goto alignment star). If you chose to do a new ASPA after each iteration, by the time all was said and done you’d have centered as many as 18 stars for goto alignment. The automated StarSense alignment camera made doing two ASPAs a little more palatable, but you’d still be spending around twenty minutes doing goto and alignments.    
Nothing changed for nearly another decade, till the enterprising Chinese CCD camera maker, QHY, came up with a new idea, which they called “Polemaster.” I was skeptical at first. A tiny camera not much different from my QHY-5LII guide camera save for the addition of some wide-field optics would be mounted in place of the mount’s polar borescope on the forward end of the RA housing. You would point the RA axis roughly toward the pole, toward Polaris, and the cam would plate solve the star field and tell you how to move the mount for precise polar alignment. That seemed like a pretty tall order to me.
Polemaster camera...

How would the alignment of the Polemaster camera affect the resulting polar alignment? How would you mount the cam if your RA axis didn’t have provision for a polar borescope? Or you didn’t want to remove or block the polar finder? Even if everything was perfect, how precise an alignment could a small-chip camera like the Polemaster produce?

When I had the chance to see the Polemaster in action at the Maine Astronomy Retreat last summer thanks to my friend Bruce Berger, all my doubts were dispelled. The camera was completely sufficient unto its task, producing more than enough stars in short exposures to allow it to do its job. The real key, however, was the software. Once I had a good understanding of the process, it was obvious what you had to do to move the mount’s RA axis to the pole. Not just obvious, but quick. If you are in a hurry, you could probably the entire Polemaster polar alignment in five minutes.

Further, I later learned the mounting of the camera was not critical. As long as it is attached to the mount somehow, someway in reasonably secure fashion it will work. I’ve seen people use it successfully, for example, just by duct-taping it to the mount head. Alignment is also not an issue. The camera does not, repeat, does not have to be finely aligned with the right ascension axis.

Watching Bruce polar align his CEM 60 quickly and precisely, I decided this was just the solution I had been looking for. Well, it would have been save for one thing:  the price. While the Polemaster is not overly expensive, about $300.00 with an adapter for one telescope mount, that was more than I wanted to pay given that ASPA was working pretty well for my purposes, with its main problem being it was time-consuming and occasionally annoying.

Annoying? Yes. There’s a bug in the Celestron StarSense firmware that sometimes causes the auto-align process to fail after the ASPA (and StarSense requires you to do another goto alignment after ASPA). It’s not a big deal to turn the mount off, reset it to home position, and start another StarSense align from scratch, but it is annoying.   

Oh, and I would have liked a little better accuracy than what ASPA produces, especially after only one iteration. For my (mainly) short focal length, short sub imaging, I can get away with less than perfect polar alignment, but it would still be nice to have the option of being able to expose longer thanks to a better polar alignment.

Initially, I was hoping QHY might have pity on us and sell their software separately. I figured my QHY-5LII would work just fine for polar alignment in conjunction with my wide-field 50mm finder-guider. Alas, they have not seen fit to do so; the software will only work with the Polemaster cam. So, I continued ASPAing it. What else could I do?

Then one day a couple of weeks back, I began to hear about Sharpcap’s polar alignment tool. I was well aware of Sharpcap itself, Robin Glover’s fantastic camera control program. Despite its somewhat nondescript and generic name, Sharpcap is a well-respected piece of astronomy software. It began as a tool for planetary imagers using webcams and webcam-like cameras, but has evolved into a program that can do long exposure deep sky work easily and well. Sharpcap is compatible with just about any camera out there as long as there is an ASCOM driver for it. Best part? Sharpcap is free.

Screen 1
Anyhow, I was told the latest release of the program, version 2.9, included a polar alignment routine similar in concept to that used on the Polemaster. A visit to the Sharpcap website revealed I had everything I needed to give this Polar Alignment Tool a try:  a compatible camera (the QHY-5LII is supported natively by Sharpcap), and that short 50mm guide scope. All I needed was one of those increasingly rare clear nights to give it a try. I read over the instructions a time or two in preparation, but, frankly, there isn't much to the procedure once the camera is connected to Sharpcap. Press an onscreen button a few times, move the mount once, and adjust the polar alignment with the mount’s altitude and azimuth adjusters.

That nice night finally came, and saw me setting up my AVX mount and Celestron Edge 800 SCT in the backyard. Why the AVX? It’s light and I am lazy, as I admitted not long ago. The SCT? I figured the scope’s long focal length would serve to reveal how good Sharpcap’s polar alignment results are. Further, I needed to take a few Moon pictures for a magazine article I am writing, and 4000mm (with a 2x Barlow) is just right for high resolution lunar vistas.

I put the telescope in normal “home” position, that is, pointed north with the counterweight “down.” The QHY was inserted into the guide scope and connected to the computer, which I positioned (temporarily) right next to the scope so I could adjust while watching the indications on Sharpcap’s screens.

First task was getting an image, a focused image. That was easy enough to do (well, after I remembered to remove the lenscap from the guide scope). Once I was close to focus, the sensitive QHY was producing more than enough stars to meet Sharpcap’s requirements in a mere 1.5 seconds of exposure. To work, the program needs 15 stars within 5-degrees of the pole, and according to the information on the first polar alignment screen, I was getting more than twice that many despite a crescent Moon and the usual backyard light pollution.

Ready to go, I clicked Sharpcap’s Tools menu and selected “Polar Align.” I was then presented with Screen 1, shown here. Stars marked in yellow are the ones Sharpcap is using for plate solving the star field (figuring out which star is which). I didn’t worry about that, just let the program think for a little while as the frames rolled in. Shortly, the “Next” button was enabled, meaning I was ready for step 2.

After pressing “Next,” screen 2 was presented and I was instructed to rotate the mount 90-degrees in right ascension. I did, so, moving the mount roughly 90-degrees to the east. Sharpcap then studied a few more frames in order to determine where the Celestial Pole was and what I needed to do to aim the mount there. Once it knew these things, the Next button was enabled again.

Screen 2
After pressing Next for a final time, a star was highlighted in yellow and there was a yellow arrow connecting it to a circle, my target . The task was to move the mount in altitude and azimuth so as to position the star in the little circle, not unlike what you do with a polar borescope (by the way, you don't need to return the mount to home position before adjusting; leave it rotated 90-degrees). As you move in the proper direction, the yellow arrow gets shorter and shorter and eventually disappears. It is then replaced with a pair of brackets around the target to allow fine tuning. As you center the star in the target circle, the brackets will move closer and closer together.

How easy was this to do? Quite easy AFTER I understood exactly how to do it. In the beginning, I was fairly far from the pole, with the arrow extending off screen. I’d been told that at this stage it was best to adjust while watching the error numbers Sharpcap displays instead of worrying about the arrow. These numbers (degrees, minutes, and seconds) indicate how far you are from the pole. They aren’t labeled as altitude and azimuth; instead they read “Up/Down” and “Left/ Right.” Sounded easy to me. I’d adjust the mount’s altitude until the Up/Down number got smaller, and the azimuth till the Left/Right went down. Alas, that didn’t work at all.

It turned out there was a catch, and until I understood what it was, I was all at sea. Up/Down does NOT mean the mount’s altitude, and Left/Right does NOT equal azimuth. Instead, these error numbers relate to directions onscreen (that's what I thought, anyway; see the addendum at the end of the article). At first I was mightily confused by the fact that moving in azimuth changed the Up/Down distance instead of Left/Right, and vice versa. As soon as the light went on in my head, that moving the mount’s altitude control changed the Left/Right distance, and adjusting azimuth affected “Up/Down,” the rest was duck soup.

In just a minute or two, I had the program indicating my distance from the pole as under a minute in both directions, which was where I left things. If your mount has precision altitude and azimuth adjusters, you can get the distance lower, but the AVX’s controls, while OK, are not exactly precise.

How long does a Sharpcap polar alignment require? Next time out, I doubt the procedure will take any longer than the few minutes required by Polemaster. Most of my time was, as above, spent scratching my head wondering why adjusting altitude moved the darned Left/Right numbers.

Screen 3
The accuracy? Some night soon, I need to fire up PHD2 and find out exactly how good Sharpcap’s polar alignment is. I know one limitation is that I am a little close to the equator at 30-degrees north, and that since the program does not currently take refraction into account there will be a limit to how close it will get me. However, I will tell you it looked darned close on this first night given the declination drift (or lack of it) of the Moon and stars at f/20. It was obvious the alignment was at the very least as good as two iterations of ASPA, and likely better.

Ground truth? I doubt I’ll use ASPA anymore. Now that I understand Sharpcap’s procedure, its Polar Align Tool is just easier and, I believe, more accurate. Sure, to do it you have to have the guide scope and guide camera mounted on the telescope, but if you are after a precise polar alignment you likely will be imaging and will want to guide with that guide cam and scope anyway.

So, friends, why not bop on over to the Sharpcap website, download the program and give it a try? Don’t cost nuttin’, and its polar alignment feature is only one of the many good things this wonderful program offers. At the very least, it’s made me stop wishing I had a Polemaster, and has allowed me to keep 300 George Washingtons in my hot little hands. 

Addendum:  Just heard from Robin (see the comments) concerning the "direction" issue that I and some other people are having. He says that moving the polar axis up or down should indeed affect the up/down numbers. At any rate, the program works great despite the direction reversal, and what's important is to shorten that arrow, which I found easy to do once, as above, I understood what was happening.

Sunday, February 26, 2017


Issue #532: Astrophotography with Inexpensive German Equatorial Mounts

Can you? Should you? You can and you might want to for several reasons. Will it ever be as easy taking pictures with a 500 – 1000 dollar GEM as with a 10,000 dollar telescope mount? No. Not always, anyhow, but it is certainly possible to get deep sky photos that will please with one of these comfortably portable rigs. 

Reading the telescope mounts forum on a certain popular amateur astronomy website, you might come to the conclusion that to just get started in imaging requires at least a mount in the 4K price range, and that actually getting decent pictures means you go to the 10K tier. Not so, not hardly.

As I have often said, what matters most is still the woman or man behind the camera, not the pedigree of the equipment. High dollar gear can make things easier, but as many, including one friend of mine, have found out, you cannot buy your way into deep sky imaging. This person has gone from Bisques to A-Ps searching for the elusive telescope mount that will take pictures for him without him having to endure the astrophotography learning curve. He has been disappointed. Mastering long exposure imaging takes blood, sweat, and tears, and no matter how modest your rig, you can get beautiful pictures if you understand your mount’s capabilities and limitations.

What do expensive mounts buy you? The payload capacity and precision to allow imaging at longer focal lengths more easily (if not always easily) than with lower priced GEMs. If your goal is to photograph smaller objects over long exposure times, certainly a high-dollar mount can help. But in the beginning, you need to learn the basics, which are easier to learn with a 500mm scope and a VX than with a 3000mm scope and an AP1100. Best of all? You won’t be out 10,000 George Washingtons if you decide astrophotography ain’t for you (not uncommon).

After gaining some experience, you may find your modest mount suits your needs perfectly well. That it is more than adequate for taking pictures at 500 – 1000mm (the sweet spot for the mounts we’ll discuss this morning), and you actually like the wider fields offered by this range of focal lengths. 

M22 with an 80mm f/6.9...
Which telescope is best for these GEMs? To begin, I suggest a short focal length refractor, an 80 - 100mm aperture one with about 500 – 600mm of focal length, something in the f/6 – f/7 neighborhood. As we’ll see, this doesn’t mean you can’t kick it up a couple of focal length notches with the mounts in question—even to 1500mm—but make it easy on yourself in the beginning. Not only does a refractor in this focal length/aperture range make guiding/tracking easier, it lessens other problems. At 500mm, your mount’s goto accuracy is much less critical, for example.

As you probably know, f/ratio for f/ratio on extended objects—nebulae and galaxies—all more aperture gets you is a larger image scale, not fainter details. A 6-inch f/5 won’t go any deeper on extended objects than a 3-inch f/5; the object will just be larger.

OK. Which “500 to 1000” dollar mounts am I talking about? The Celestron Advanced VX and its predecessor the CG5, the Bresser (Explore Scientific) Exos-2 and its predecessor the Meade LXD75, and the newer and somewhat different iOptron CEM25. We might even stretch our budget range a couple of hundred dollars in order to include the Orion (Synta) Sirius (HEQ-5). The Sirius is a little heftier than the rest of the group, but has more in common with them than with the next group up (Atlas/CGEM, etc.).

One thing all these mounts have in common is that they are equipped with reliable goto systems. That is indispensable for imagers. Who wants to waste those increasingly rare clear nights (down here, anyhow) trying to find and center objects? They also all have acceptable tracking error figures, usually around 30 – 40-arc-seconds max, and the errors are smooth enough to guide out successfully. Exactly which of these GEMs to choose, though? Pick one. These mounts are all more alike than different. Here is the short and sweet.

An 80mm refractor is great on a light GEM...
The modernized CG5. Its big plus is Celestron’s phenomenally accurate goto system. It also includes the AllStar Polar Alignment procedure in its hand control, which makes getting polar aligned well enough for the kind of imaging that is these mounts’ forte remarkably easy. Downcheck? Mainly the declination axis. No ball bearings there. Despite the fears of some novices, however, the VX guides well enough in declination. There’s some declination backlash, too, but less than with the older CG5.

Celestron CG5

The CG5 is robust and reliable—mine was working as well as it ever had when I sold it after nearly 10-years of service. I never had problems taking my (modest) deep sky astrophotos with it. There is no doubt the AVX rounded off some of the CG5’s rough edges, however. As above, my CG5 had a fairly large amount of declination backlash. Nevertheless, my guiding software, PHD, would always calibrate and guide successfully. While the CG5 has been out of production for several years, it is common on the used market, where it often goes for 400 dollars or less.

This Synta mount, sold by Orion in the U.S., is basically a CG5 with the SynScan goto system. In other ways, it is the CG5. The Synscan HC is fine, if not as full-featured and accurate as that of the CG5. Its goto targeting ability is quite sufficient for short f/l scopes, however. Unfortunately, from Orion this GEM is nearly as expensive as the arguably better VX.

This JOC made mount is very much like the AVX or the CG5 as far as payload capacity (see below), but it does have one plus:  ball bearings on its declination axis. The minus? A somewhat primitive goto system. Accuracy should be OK for the smaller refractors we’ll use, at least. The mount also lacks a working serial port, so no interfacing to the computer. It does have an ST-4 port for auto-guiding, however.

The Exos-2
Explore Scientific, the U.S. seller (and JOC subsidiary), is promising a version with its new PMC-8 computer system (at a price similar to that of the VX, 900 dollar range), but that mount has not appeared yet. At any rate, the Exos-2 is one heck of a bargain despite its computer faux pas. You can get one for an amazing $599.99.

Before the coming of the Exos-2, the mount was available as the Meade LXD-75. JOC OEMed it for Meade, who installed their own goto system driven by the Autostar computer. If you can find a deal on a used LXD-75 in good working order, go for it. The Autostar is superior to the Bresser HC. While a few mechanical rough edges have been cleaned up for the Exos-2, the LXD-75 is usually a reliable performer much as the CG5 was. Do avoid the previous JOC made mount Meade sold, the LXD-55. The less said about that one, the better. 

This is the different kid. It’s one of iOptron’s “center balanced” equatorials. That offers several advantages, but the main advantages of this mount are its light weight, great polar scope, and quiet (stepper) motors. Downchecks? The mount's payload is less than that of the other GEMs in this group. It will handle a C8, but just barely. Also, the mount, which began as the servo motor equipped ZEQ-25, has had its share of teething problems. The latest version, the CEM25P, is, I am told, a fairly substantial improvement on the earlier versions, with iOptron guaranteeing +/- 10-seconds of periodic error.

The Sirius is an improvement over the VX and the others in some ways. There are ball-bearings for the declination axis, and it offers slightly higher payload capacity (as long as you get one with a 2-inch legged tripod) than the VX or Exos 2. The downside is that SynScan goto system, which, while OK, is kinda ho-hum regarding both accuracy and features. That’s not the whole story goto wise, though. You can use this mount (and the SkyView Pro) with the EQMOD ASCOM driver, which can offer much-improved accuracy at the expense of having to use a laptop with the mount every time.

Payload Capacity

The CEM 25 and the Sirius are the outliers here, with somewhat less and somewhat more weight handling ability respectively. The rest? For imaging, they are perfect with around 10-pounds or less at modest focal lengths. Astrophotography is a breeze with my 80mm f/7 APO. And the mounts are also OK with my 120mm f/7 refractor, which weighs in at a modest 11-pounds.

For any of these GEMs, a C8 is the practical upper limit for picture taking. The increasing weight and, moreso, focal length see to that. By the time you add camera and guide scope, you are really pushing any of them. Sure, you can use a C11 on a CG5 for visual, but that’s for visual. The bottom line for imaging? The less weight you can get away with, the better.


Cropping allows you to zoom in a bit on an 80mm image...
What is one of the main things that can cause problems with tracking in this tier of mounts? They are a little sloppy gear-wise. In certain orientations, like when nearing the Meridian, loose gear mesh can mean the gears in the RA drive are not always fully engaged. The solution is simple:  balance slightly east heavy.

“East heavy” is something needed by almost all mounts in this tier, and even the next group up, but I note considerable confusion as to what “east heavy” actually means. It’s really simple. You want the mount to always be slightly heavy to the east. That ensures the gears are always engaged; the RA motor pulls the scope along. This is not necessary for visual, and it won’t hurt the motors or anything if you are not slightly east heavy. It just helps the scope track better.

How do you do east heavy? If you are imaging on the west side of the Meridian, balance the scope and then move the counterweight up the shaft about ½ - ¾ inch or so. So the mount is just slightly telescope heavy. If you are imaging an object east of the Meridian, balance and move the counterweight down the declination shaft by that ½ - ¾-inch. The mount is now just slightly counterweight heavy. Yes, its’s best to re-balance if you switch which side of the Meridian you are imaging on. I usually find it easy enough to confine one evening’s run to either “east” or “west,” however.


Can I tell you a story? One night I was out in the backyard imaging with the VX and my 80mm f/6.9 refractor. The brightness of the sky background on that evening due to slight haze was enough that I really had to keep my sub-frames, my individual exposures, down to about a minute. I sat at the computer and watched the subs begin to roll in. “PHD2 sure is doing a nice job of guiding tonight,” I thought. Then it hit me:  I’d forgotten to start PHD2.

If you keep your weight and focal length down and can settle for 30-second to 1-minute exposures, you may not need to guide. The tradeoff is that if you want to avoid guiding you need to take extra care during polar alignment—I do two iterations of AllStar with the VX. Also, at 1-minute of exposure, you will likely have to throw out the occasional frame.

An 80 also has enough field for the big subjects...
If you do want to guide, the key is, again, keeping the weight down. Try to minimize the weight added by guide scope and guide camera. I use one of the 50mm finder – guide-scopes that Orion and KW telescopes sell. My guide cam is the sensitive but tiny (about the size of a 35mm film canister) QHY 5L-II. The 50mm finder is fine at 500mm of imaging telescope focal length, and does a nice job even at 900mm with my 5-inch APO.

Targets:  What can you image at 500mm?

Actually, a 500mm – 600mm focal length telescope can be quite versatile. It’s equally at home photographing big objects like M33 or M45, or imaging somewhat smaller DSOs like the larger Messier globs. M13, M22, M10, M12—all are nice at 500mm or so. If you are using a larger high resolution chip, like the sensor on a DSLR, you can also “enlarge” your images somewhat by cropping and still retain nice-looking resolution.

Putting it All Together

Yeah, let’s put it altogether. First thing is setup. Get the scope and guide-scope and cameras on the mount, obviously. Not so obviously? Make darned sure none of the cables—you’ll have three usually:  camera USB, guide camera USB, and ST-4 guide cable—can snag on the mount or the tripod.  Double check and dress the cables as necessary after you go to the first target. These mounts are light enough that a cable snagging even momentarily will ruin the sub-frame.

Next, do the goto and polar alignments. If you are using a Celestron mount, think about doing two iterations of the ASPA procedure (with a new goto alignment after each). Celestron’s StarSense alignment camera can make that easy, doing the onerous goto alignments for you. iOptron? Their polar scope is excellent. If you are running the Exos 2 or one of the Synscan (Orion/SkyWatcher) mounts, I recommend the Polemaster polar alignment camera or the new polar alignment routine in the free program Sharpcap (which uses your guidecam and guide-scope, I am told).

Double Cluster with an 80mm f/6.9...
When you are goto and polar aligned, fire up the computer (or just the DSLR if you don’t use a computer with your camera), and focus on a field with a bright star in it. As I said last week, a Bahtinov mask makes that easy.

I like to control the mount with the computer (usually with the free program, Stellarium). Being able to sit comfortably at the PC and fine tune image centering with the little onscreen (ASCOM) hand control allows me to go longer than if I am constantly getting up and walking out to mess with the scope. Obviously, you can’t control the current Exos-2 mount with a computer since it lacks a serial port, but you could no doubt wire up a hand control cable extension and at least have the HC there with you at the computer.

Then…well, you just start taking sub-frames. How long should each be? That depends on the quality of your sky and the quality of your guiding. A bright sky can limit you to as little as a minute (or even less) of exposure. If your guiding tends to wander off, you may have to use shorter exposures as well (if you’d like the settings I use in PHD2 with my mounts, send me an email at Seeing, atmospheric steadiness, can also limit the efficacy of your auto-guiding.

The 50mm guide-scope will usually deliver an RMS guiding error of about 2” in my experience. That is more than good enough for the image scale delivered by 500mm or even 900mm of focal length, as long as your declination and RA guiding corrections are of similar magnitude. I generally find myself doing 4-minute and shorter exposures depending on the target and sky conditions.

Going Longer

Will one of these mounts support imaging with more focal length and weight? Say with a C8 (reduced to f/6 or f/7)?  Yes. I wanted a new C8 at the time I bought my VX, and ordered it with the Edge 800. It seemed natural to try a little imaging with the new scope on the new mount. I’ve been able to attain pleasing if not utterly perfect results with the f/7 reducer. If I’d been more careful with polar alignment and balance, my results would likely have been even better. One important thing? Don’t consider shooting at 1500mm with the VX (or the other mounts) on any but calm nights. A strong breeze will wreck your photos tout suite.

VX + C8:  it can work...
Would the AVX or any of the rest of these mounts be my choice for imaging with a C8? No. Not at all. For that, you really want the next group up, the Atlas/EQ-6 (or the Pro variant), the CGEM, the CGX, or the iOptron iEQ-45 Pro. You can shoot at 1300 – 1400mm with the AVX group, but it will never be as problem free as at 500 – 900mm.

There is one thing that encourages me to use a C8 on my VX, though:  I’m lazy. I’ve had excellent results with the SCT on my CGEM and Atlas, but they are so darned heavy compared to the VX that whenever possible I prefer to use the lighter mount. That’s the trade-off if you can’t afford to play in the GEM big-leagues. You can get lower priced mounts with good payload capacity like the EQ-6, but in order to increase the payload, the mount head’s weight goes way up as compared to something like the Astro-Physics Mach One.

I can’t—or at least won’t—afford a Mach One, and I can’t always convince myself to drag out the Atlas or CGEM. So, I’m willing to put up with a little hair pulling when I think I need a C8 to image what I want to image. But you know what? With a little care, these humble mounts, the VX, the Exos 2, the CEM25, and their kin, can still bring home the bacon in the form of beautiful pictures.   

Better still? One of these mounts and an 80 – 100mm refractor makes for a setup that is so light, easy to transport, easy to assemble, and effective, that even jaded old me doesn’t mind heading out to the dark site occasionally for an evening of relaxed picture taking.

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: 

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