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 (spelled "Cloudy Nights"), 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.

Celestron Advanced VX

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. Down-check? 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.

SkyView Pro

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 unarguably better VX.

Bresser Exos-2

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. 

iOptron CEM25

This is the different kid. It’s one of iOptron’s “center balanced” equatorials. That offers several advantages, its light weight, great polar scope, and quiet (stepper) motors. Down-checks? 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 <ahem> +/- 10-seconds of periodic error.  

Orion/SkyWatcherSirius/HEQ-5

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.

Balancing

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.

Guiding

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, pay attention to balance, 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  somewhat smaller DSOs like the larger Messier globs, M13, M22, M10, M12. If you are using a larger higher-resolution chip, like the sensor on a DSLR, you can also “enlarge” your images somewhat by cropping and still retain smooth-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).

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 rodmollise@southalabama.edu). 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 1 - 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 bigleagues. 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.