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.

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 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 rmollise@bellsouth.net). 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.

Mounts

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.

Accessories

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:  asalati@gmail.com 

Sunday, February 12, 2017

 

Issue #530: Get Connected


Keyspan USB-Serial Converter Cable
It’s been—wow—the better part of two decades since laptop computers began to appear on our observing fields and observers began to use them to send computerized telescopes to sky objects. Unfortunately, a stroll through the Cloudy Nights mount forums reveals a lot of you still have trouble getting mounts talking to computers. That is not something to be ashamed of. There are several gotchas involved, gotchas that can drive those of us who don’t do much with computers in our personal or professional lives absolutely MAD.

Luckily, it’s fairly easy to get even the most computer-phobic person going with connected astronomy. Well, most of the time. On the PC side of the house, there are so many computer hardware configurations and variations that anything is possible. There can be inexplicable difficulties that defy reason and stymie the most PC savvy person. Luckily, that is not usually the case, and it often takes no more than 15-minutes to get a rig working. That’s because if there is a problem it usually has nothing to do with telescope or computer; it’s the connection between computer and telescope mount that stops people before they get started. That is what we will address first.

Well, we’ll address that after you have the two items needed to make a computer-telescope connection work (in addition to your fave astronomy program). First you'll need a serial cable. This must be a cable wired specifically for your mount. Why the scope makers didn’t just adapt the standard RS-232C cable, I have no idea, but they didn’t and there is no use worrying about that at this late stage of the game.

A Meade cable won’t work on a Celestron, and a Celestron cable won’t even work on a SynScan (SkyWatcher) mount despite the fact that both are made by the same company, Synta. Get the specific cable for your mount/telescope from an astronomy dealer, or, if you are handy with RJ crimpers (most telescope cables use an RJ plug for the scope end), make one according to the pin-out for your particular mount. Most telescope/mount manuals will give the design specs for a serial cable.

Next, you’ll need a “USB to serial converter.” What? To this point, most telescopes only “speak” RS-232C serial. While that is a somewhat outmoded data communications standard, it has its pluses for astronomy. You can run very long runs of serial cable without a problem. If you want to control a telescope thats' in an observatory or set up in the yard from inside your house, RS-232 serial makes sense.

Com 3 is assigned...
Unfortunately, modern PCs (and Macs) don’t have serial outputs. That's long since gone the way of the buggy whip. Luckily, the above-mentioned converter cable is an easy solution. The converter takes a USB signal coming from a computer and changes it to the serial data understood by a telescope/mount. And all is well, right?

Not necessarily. Most of the time, any USB to serial converter you can find (they are now scarce in local computer stores) will work OK  with a scope mount. But some do work better than others. That can be important if you want to do more complicated things than just sending a mount on gotos with a PC. If, for example, you want your laptop to take the place of a hand control (NexRemote and EQMOD). If you do, I recommend the Keyspan USB-serial converters (available from B&H Photo). In my experience they are the most reliable and problem-free.

First Steps

With serial cable and converter in hand, it’s time to get connected. The first step is getting the PC squared away with that serial converter. You don’t need to fool with the telescope or cable yet. Just plug the USB – serial widget into the laptop. If the PC (or Mac) is anywhere near recent, it will automatically install a driver for the USB-serial device (if you have a very old computer, say a Windows XP machine, you may have to download and install a driver for it manually).

What’s a driver anyhow? That’s a term that will come up again and again in the world of computer – scope interfacing. In short, it’s a little program, a little app, that tells the computer about a particular device. What it is and how to talk to it. When you plug in an external device, be it a USB – Serial converter or a printer, the driver is accessed and tells Mr. Computer what to do. This works the same whether you have a PC, or a Macintosh.

Once the driver is installed and the computer declares the device (the USB-serial cable) ready for use, there’s one more thing to do. On a PC, you need to open "control panel"/"hardware and sound"/"device manager" and see which com port number (serial port number) the computer has assigned to the converter cable. This is very important. Not do doing this or doing it incorrectly is what gives most people problems.

Selecting one of TheSky's internal drivers...
To check the serial port assignment in recent flavors of Windows, right click the Start button and choose “control panel.” Click “hardware and sound,” and then “device manager.” A “tree” will appear, with “com and LPT ports” on it. Expand that entry, and you should see a com port number. The PC has a serial port now and has assigned it a number. If you’re a Mac user, you’ll need to do the same, and I hope you know how to do that, since I sure don’t. At any rate, remember the com port number; you will need it. If you always plug the converter into the same USB port, the same number will normally be assigned to it. If you plug into a different USB port, a different com port number may be assigned, and you may have to check it again.

Getting Telescope and PC Talking

The next thing to suss is the telescope driver type question. Does your astroware use external or internal? Telescope drivers work the same way as the drivers the PC uses to communicate with the USB-serial cable, or printer, or anything else. The difference is that they are accessed by the astronomy program instead of the computer itself. The PC doesn’t know anything about telescope mounts. As far as it is concerned, the scope is a generic serial device, end of story.

The fine points of goto commands and such vary from mount brand to mount brand and even sometimes from model to model, and the astronomy program in use has to have an appropriate telescope driver—Celestron, Meade, Losmandy, etc.—in order to know how to command the telescope and how to interpret the data coming back from it over the serial interface.

There are two general types of drivers in use by astronomy software, internal (“built-in”) drivers and external drivers. Internal drivers come with the astronomy program, and are written by the people who wrote that software. Many programs have moved away from internal drivers. Most software authors or even development teams don’t want to be saddled with writing drivers to support every new telescope/mount that comes out, as you can imagine.

Despite the above, there are still some well-known PC programs that come with internal drivers including TheSky X and Stellarium. Macintosh software invariably uses internal drivers, since a system of external ones has never been developed for the Mac. If your astronomy program uses built-in internal drivers, you simply choose scope  brand and model from a list in the software’s “telescope” menu, fill in a few items, and are good to go.

ASCOM Chooser in Cartes du Ciel...
External drivers are the norm for the PC world these days. The beauty of them is that the authors of astronomy programs don’t have to worry about drivers at all. All they have to do is provide a link to a 3rd party driver system. In the PC world, that is ASCOM, “Astronomy Common Object Model.” While ASCOM provides drivers for more than just mounts (focusers, cameras, etc.), its most common role is as a telescope mount driver system.

The way it works is this: download and install a program called the “ASCOM Platform.” It handles communications between an ASCOM compatible astronomy program and a driver for a particular telescope, which is also downloaded from the ASCOM website.

While there has been talk about porting ASCOM to Macintosh over the years, that has never happened. A few people have tried to come up with ASCOM-like external driver systems for Apple, but none has caught on. ASCOM has never come to Linux either; in part because Linux users have their own system called “Indi.” Indi is, like Linux itself, not quite as user friendly to install and use as ASCOM, but the main reason you probably haven’t heard of it is that there aren’t that many non-professional astronomers using Linux/Unix for telescope control.

Configuring the Telescope Interface

One thing many beginners miss? Unless you are using specialized software like NexRemote or EQMOD, the first thing you do when interfacing computer and telescope is not start playing with the laptop. The first thing you do is align the telescope/mount with the hand control the old-fashioned way, just like always. Trying to interface the scope and computer before the telescope is aligned will cause nothing but problems.

If your astro-software uses built-in (internal) drivers, interfacing to the telescope will differ somewhat depending on the software in question, but all programs require similar things to be filled-in in the telescope set up window. The example I’m using is TheSky 6, which normally only works with built-in drivers (but can be “tricked” into using ASCOM).

ASCOM Chooser in Stellarium...
The first thing to do is select the telescope or mount brand/model. While this can vary a bit, most programs that use internal drivers will list individual telescope models. In the picture above, I’ve chosen the good, old CG5 German equatorial mount. After that, enter basic communications settings. With TheSky 6, press the “settings” button. With other programs, the com setup and other options may all be on the same screen. Anyhow, enter the com port number found in Control Panel (or in the appropriate place on a Macintosh). If the software wants baud rate, enter/choose “9600.” A few older programs (like the still-popular Megastar) will ask for data bits, parity, and stop bit. You don’t have to understand these serial communications arcana; just enter “eight, one, and none” (8-1-n).

Most programs will offer some additional options, as TheSky 6 does. Do you want telescope crosshairs on the screen? Should the software automatically switch to night vision mode when a telescope is connected? When everything is selected or entered, click a connect button or, as with TheSky, go back to the telescope menu and choose “link/establish” (or with other software "connect," “enable interface,” or similar). The documentation that came with the astronomy software will make clear how to proceed.

That’s it for built-in drivers. Using ASCOM is a little more complicated, but not much. You don’t (can’t/shouldn’t) start the ASCOM program; the astronomy software you are using will start it for you. The beauty of ASCOM is that the telescope/mount setup windows are the same no matter which astronomy program you use. Everything will look the same and you will enter data the same way whether in Cartes du Ciel, SkyTools, Deep Sky Planner, or any other ASCOM compatible program. The difference is in how you get to the ASCOM Telescope Chooser.

In Cartes, start ASCOM by clicking the little Telescope Control Panel icon. Other programs may require you to choose “scope setup” or something similar from a menu. At any rate, once the Chooser is onscreen as in the picture above, select the desired telescope brand or model . Which that is, brand or model, depends on the telescope driver. Currently, Celestron has a “unified” driver. Pick “Celestron,” and the driver will automatically figure out which particular Celestron scope/mount it's connecting to. Other drivers may require choosing a specific model from the Chooser’s pull-down’s list. “LX200,” for example. Naturally, as mentioned earlier, drivers must be downloaded from the ASCOM website and installed for them to appear in the list. The ASCOM platform only comes with a couple of drivers, "POTH," "Telescope Simulator," and a couple of others.

Once the telescope is selected in the Chooser, click “properties” to enter the specifics of the setup. Here, you’ll give ASCOM the com port number, indicate whether or not the telescope mount is operating in equatorial mode (is a German equatorial mount or a fork mount scope on a wedge), and enter the observing site’s latitude and longitude. You may be asked for different data depending on the particular scope driver, but all will want that all-important com port and also the site’s lat/lon.

Connected and ready for a night of laptop-enabled fun!
When you’ve OKed the settings window and the Chooser window, you’ll connect to the scope much as with built-in drivers. How you do that depends on the program itself, not ASCOM. Cartes has a “connect” button on the scope control panel; other software may have a “connect” or similar choice on a “telescope” menu. When you are successfully connected, a set of crosshairs should appear at the telescope’s current position on the program’s star chart (with some astronomy software, like TheSky, you’ll first have to select “show scope crosshairs” in the setup), and there should be some indication computer and mount are connected and talking, like the green “light” on Cartes’ scope control panel.

Where do you go from here with ASCOM? ASCOM provides useful additional functions, some of which are enabled in the ASCOM set up window and some of which you select in the astronomy program. One feature I like is ASCOM’s “hand control.” If you choose to show that in the driver set up, a little set of HC direction buttons will appear onscreen once the scope is connected. I find that useful when I am imaging. I can sit at the PC and fine-tune my centering with the ASCOM HC instead of having to mess with the real hand control.

Another oft-used ASCOM option, which is accessed from the astronomy program in use, is “sync.” When you go to an object, you may find the cursor is centered on it, but the object is not centered in the eyepiece. Center it in the eyepiece, and it will be then be off onscreen. That can happen for a variety of reasons, but you can cure it with a sync. This is completely different from the sync function in the hand control, and just allows you to center the astro program’s crosshairs on the target when it is centered in the eyepiece.

And you know what? That is all there is to it. Let me say again: the place beginners foul up is usually not with something complicated like entering baud rates or serial data specs. It is almost always in getting that darned USB-serial converter com port correctly entered into the software.

Late Breaking News

Celestron’s most recent hand controls eliminate the need for a USB-serial converter. Well, they don’t really eliminate it, they just make it so that you don’t have to go out and buy one. The newest NexStar HCs have a mini USB receptacle on their bases rather than an RJ-style serial port. You connect a computer to HC with a standard USB-mini USB cable.

Is that a good thing? I’m not sure. You don’t have to worry about finding a USB-serial converter that works properly. BUT…  You are now limited in the length of cable you can run to the scope without using USB boosters. Four or five meters is the max.  The new HCs don’t free you from the need to mess around in control panel to find the com port number, either. This is not really a USB connection. The new hand controls have an internal USB to serial converter, and the PC will see the HC as a serial device. You will still need to enter the proper com port in the software. Me? I think I prefer to just continue using my good, old Keyspan, thank you.

Up next? There’s a big Moon in the sky, and the weather isn’t the best right now, so it may be that we return to the Novice Files for installment 3.

Sunday, February 05, 2017

 

Issue #529: Four Years with a Celestron Advanced VX Mount


I come not to praise Celestron, but neither do I come to bury them. I used to be a Celestron fanboy. For years, I bought only Celestron's gear and praised the company to high heaven. In recent times, things have changed. For one thing, Celestron ain’t the Celestron it used to be. It’s long since been bought out by Chinese telescope giant Synta. I’ve changed too; maybe I’ve grown a little more cynical and skeptical than I was in my salad days.

However, I still like a good bargain if it’s a good bargain, and I’m not afraid to give praise where praise is due. Or criticism when that’s warranted. So, where does one of Celestron’s more popular and inexpensive mount offerings, the Advanced VX German equatorial mount (GEM), fall in the spectrum from damnation to salvation?

About four years ago, I began thinking about a replacement for my then most used rig, a 1995 Celestron Ultima 8 SCT tube riding on the company’s Advanced GT (CG5) telescope mount, a medium-light, computer-equipped GEM. Why? I wanted a new mount mostly because my CG5 was approaching ten years old, and I wasn’t quite sure how much longer the inexpensive rig would go. With early retirement in the offing in 2013, I also wanted to ensure any necessary astronomical gear purchases were taken care of while I was still working full-time, if possible.

I initially considered a wide range of mounts, especially to include the Losmandy G11 and the iOptron CEM60. Eventually, after worrying myself into a tizzy over the choices, I decided the best thing for me would be to get a mount as similar to the CG5 both in weight and capability (and price) as possible. I was also addicted to the Celestron NexStar hand control and didn’t want to give that up. Before spring 2013 was out I ordered the new successor to the CG5, the Advanced VX. Actually I ordered Celestron’s Edge 800 SCT/ VX pairing.

Why did I buy a new telescope to go with the new mount? At the time, I had three freaking C8s, and my most used one, Celeste, the Ultima 8 OTA, who you’ve read about many times if you frequent this blog, had always been a good performer. There were, a couple of reasons. For one, I was attracted by the better field edge offered by Celestron’s corrected Edge SCTs. For another, Celestron always gives you a real good deal if you buy a telescope/mount combo. The Edge 800 with its off-white tube was just so pretty, too. And I just felt like I deserved a retirement telescope, e’en at the somewhat young retirement age of 59.

This story is not the story of the Edge 800, however. I may talk about her, "Mrs. Emma Peel," again someday. My switch (for the most part) to refractors means she doesn’t get used as much as she used to, but she’s still a good telescope and  I still like her and user when I need a long focal length large(er) aperture instrument. What does get used all the time? The Advanced VX mount.

As is sometimes the case in modern amateur astronomy, my experience in obtaining the VX wasn’t overly smooth. While it arrived promptly from my astro-dealer of choice, Bob Black at Skies Unlimited, it had to be shipped to Celestron for replacement shortly thereafter because of two problems. One was that the hole on the declination counterweight bar for the “toe saver” bolt was mis-threaded. The safety bolt, which is intended to prevent the 11-pound counterweight(s) from smashing your toes if the counterweight locking bolt should come undone, would only thread in a few threads.

That was not a huge deal. What was was the other problem, that the hole in the underside of the mount head that the tripod’s threaded rod screws into to secure mount to tripod was also mis-threaded. I screwed the rod into the mount when I set the VX up for the first time, and found out to my dismay that there was no way I would ever be able to unthread it again. I had to destroy the rod and the hole in order to get the mount and tripod apart for return to Celestron.

Celestron replaced the VX promptly with a mount that was perfect (you can see a video of the replacement mount immediately after its unboxing here), but there’s no use denying that receiving a bad mount is not an uncommon experience for buyers at the low end of the astro-market. That knowledge didn’t make me feel a bit better, however. While the VX is considered a bargain mount by some of the folks in our game, to me 900 dollars, which is what it costs, is a not inconsiderable sum.

While most people will not have a problem with their new VX, my experience and the experiences of more than a few other purchasers show its QA can be spotty. Be prepared. Above all, if you receive a mount that is bad out of the box, don’t agree to have Celestron fix it for you. Insist on a replacement, an immediate replacement, from them or your dealer. If you bought a big screen TV at BestBuy or HH Gregg and it was dead out of the box, you wouldn’t agree to ship it to LG or Panasonic and let them keep it for a month or so while they fixed it, would you?

Be that as it may, things began looking up after the second VX arrived. I missed taking the mount to the Spring 2013 Deep South Star Gaze Spring Scrimmage—the replacement arrived just a little too late—and I used good old reliable CG5 with the Edge 800 instead. I was shortly able to get the VX and Edge down to Chiefland, Florida, to the Chiefland Astronomy Village for a Chiefland 4th of July and a good shakedown cruise, though.

That’s what I intended, anyway; the weather gods had other ideas. All me and the Edge, Mrs. Peel, saw was the undersides of clouds. Thick rain clouds. That turned out to be the story for much of the summer of 2013, and I didn’t get a chance to see what my new rig could do under dark skies for quite a while.

The Specs

Before I talk more about how the VX performed, I suppose it’s a good idea to summarize the mount’s basic specs. The Celestron Advanced VX is a computerized goto GEM mount that uses Celestron’s NexStar + (plus) computer hand controller. It is powered by two servo motors (with encoders) that are similar to those that were used on the CG5.

While Celestron doesn’t publish a figure for the weight of the mount’s EQ head, it’s about 18-pounds. The 2-inch steel-legged tripod comes in at 18 pounds as well, and is the same basic model (with a slightly different head) that is shipped with the CGEM, the EQ-6, and several other Synta-made mounts. The VX is light enough when separated into components—mount, tripod, and counterweight(s)—that most people will have no trouble transporting it and setting it up.

The VX comes with a single 11-pound counterweight, a DC power cord, a short serial cable, and (usually) a DVD containing a telescope control program—the First Light Edition of TheSky X at the time I bought my mount.

How about payload capacity? Celestron says 30-pounds. That is actually, surprisingly, a downgrade from the figure they published for the CG5, 35-pounds. Is the VX less sturdy than the CG5? No. Either the company decided to be more realistic about the weight this mount can handle, or they just forgot the figure they quoted for the CG5. The capacity of the two mounts is pretty much identical. That is, around 30-pounds for visual, and maybe 15 – 20-pounds for the more demanding task of imaging.

What’s New

Are the CG5 and VX identical save for appearance, then? No. The VX is very much an improvement on the CG5 in several ways. The CG5 was nothing more than a non-goto Vixen Great Polaris mount clone with goto motors pasted on, and it looked it. While the mount worked well, it definitely had rough edges. The CG5’s plastic motor housings could, for example, interfere with movement in R.A. If you lived at 30-degrees latitude or south of that, you’d find you’d have to remove the mount’s forward altitude bolt or the R.A. motor housing would bump into said bolt and prevent you from reaching 30-degrees altitude during polar alignment.

The internals of the VX are much the same as those of the CG5, though the motor control board has supposedly been somewhat improved. However, the mount head has been completely redesigned. The motor housings now look like they are actually part of the GEM and don’t interfere with any of its movements. The R.A. shaft housing is more sleek looking, and the CG5’s pitiful polar scope eyepiece cover, which was always falling off and getting lost, has been replaced with a nice thread-on job.

Perhaps the most important redesign was of the control panel, though it’s taken Celestron a couple of tries to get it exactly right. A bug-a-boo with the CG5 was that the connection for the declination motor’s cable was right there with the rest of the mount’s identical RJ-11 receptacles. Plug the declination cable into the hand control port or vice versa and you could do real damage to the mount. The initial VXes improved on that somewhat, putting the dec receptacle on the top lip of the control panel, which extends out from the R.A. housing. Some folks still managed to plug the declination cable into the wrong receptacle, though. Celestron eventually, in the most recent production runs, gave the dec cable a connector that can’t be plugged into an RJ receptacle.

What else? The mount now features Permanent Periodic Error Correction (PPEC). The CG5 didn’t have PEC at all, permanent or otherwise. Another, more important, improvement is that the altitude and azimuth adjustment knobs are larger and better on the VX and make polar alignment easier. Also helpful is that the VX, like the CGEM, features an internal battery that keeps time and date current when the power is turned off. Finally, the too loose power connector of the CG5 has been replaced by one with a thread on collar that ensures a firm power connection.

The VX mount has shipped with Celestron’s Plus HC from the beginning, and will soon be equipped with the new USB HC, which includes a built-in USB-serial converter for control with a PC without an add-on serial converter.

What’s Not New

That’s a pretty impressive line-up of improvements. What didn’t get fixed, though? Mainly, the declination axis. Unlike the right ascension axis, which features ball bearings, the declination axis uses a thrust bearing. The axis rides on plastic. Some people have expressed concerns about that, and it’s true the declination axis doesn’t move as freely as the R.A. axis, but I’ve never had a problem balancing even lighter scopes in declination. My mount also auto-guides reasonably well in dec, so I’ve pronounced this a non-issue. How about the mount’s sound? The CG5 is a notoriously noisy mount when it is slewing at high speed. That is caused in part by the motor housings resonating. The VX is noticeably and substantially quieter than the CG5 and at least slightly less noisy than my CGEM.

In Use

The VX is identical to all the other Celestron GEMs in most respects when it comes to alignment. The only exception is the more expensive mounts’ homing/limit switches. The VX doesn’t have them. Instead, as with the CGEM and CG5, you set the mount to a home position manually using marks on the R.A. and declination axes. The marks are improved over the CG5’s stick on labels, at least. The VX has engraved R.A. and declination home position marks that are easy to see with a dim red light.

Once you are in home position, it’s the same old story as with other Celestron GEMs. You do a 2+4 alignment for best goto accuracy. You align on two stars the hand control chooses for you. When they are centered in finder and eyepiece, you go on to add as many as four “calibration” stars. These stars allow the mount’s computer to take cone error—misalignment between the telescope and the VX’s R.A. axis—into account, and are what is mostly responsible for the mount’s excellent goto accuracy.

And the VX’s goto accuracy is outstanding. I’ve never worried about getting objects in the field of view of a medium power eyepiece, even with the f/10 SCT, or in the frame of fairly small camera chips. Any object you request, from horizon to horizon is just there assuming you’ve been careful in your goto alignment—used a medium power reticle eyepiece and done final star centering with the mount’s up and right keys only.

Polar alignment? A GEM mount must be accurately polar aligned for good tracking. Like the CG5 and CGEM, the VX is amazingly immune to goto accuracy problems caused by polar misalignment. If you are just observing visually, it’s usually enough to merely point the mount north and raise the R.A. axis’ altitude to a value equal to your latitude. If you are taking pictures, however, you need a good polar alignment.

The VX doesn’t come with a polar alignment borescope (one is available as an option), but you don’t really need one. Like the other NexStars, you can employ the hand control’s built-in polar alignment routine, AllStar. Once you’ve done a good 2+4 alignment, AllStar will have you center a star using the mount’s altitude and azimuth adjusters. Allstar is more than adequate for most imaging purposes.

Making Alignments Simpler

The alignment process, centering up to six stars, can be something of a hassle, but Celestron’s optional StarSense alignment camera takes all the pain out of that. The StarSense easily and accurately performs a goto alignment with the VX without user intervention—other than to set the mount to home position and start the procedure. Since you don’t have to do a second 2+4 alignment following the ASPA—StarSense does it for you—it’s painless to do two ASPA iterations and really dial in polar alignment. In my opinion, StarSense is almost a must-buy for Celestron GEM owners.

In the Field

Since my mount came with the Edge 800 OTA, that was the scope I used with it initially and for about a year and a half thereafter. I was mostly imaging with deep sky video cameras like the Mallincam, and, for video, the setup was a dream. While there was some backlash in the VX’s declination axis, there was little on the R.A., pointing was excellent, and my results were everything I expected and more. While the tracking quality of the VX was not worlds better than that of the CG5, it was somewhat better and more than good enough for unguided video imaging with short (usually around 15-seconds) exposures.

However, my purchase of the VX came at a time when changes were in the offing for me. One of those changes was that after using nothing but video for picture taking for the previous several years, I was turning away from that and back to using CCD and DSLR cams. Oh, I’d had a ball with video, but I suddenly wanted prettier, more finished looking pictures than what my Xtreme or Stellacam could deliver. How would the VX cope with the longer, guided exposures demanded by my DSLRs?

The answer was “fairly well,” though more than a few of my images with the Edge 800 (reduced to f/7) weren’t quite perfect. The stars might be a little off-round if you were zoomed-in far enough on the picture. Mostly, I think that was my fault. For one thing, I was using a fast 50mm guide scope. Most people will tell you these can work OK up to about 1500mm of focal length, but I was almost there at 1400mm and was pushing it. Also, I often forgot to lock the telescope’s primary mirror down. Finally, me being me, I sometimes (usually) wasn’t as exacting with the AllStar polar alignment as I could have been. When you get up around 1400 - 1500mm, everything becomes critical.

I could have tightened things up with a better guide scope or an off-axis guider and a better polar alignment, but as 2014 wound down, one of the things that began to change for me was my choice in telescopes. One day, I began wondering how the combination of my 80mm Megrez II Fluorite refractor and my Canon 60D DSLR would do with the VX. I just happened to be heading out to my club dark site that evening, and decided to take the William Optics APO rather than the SCT. I was bowled over by the wide field shots I got.

And it wasn’t just that the smaller scope’s wider field was cool. It was eye-opening how darned easy it was to get perfectly guided shots at 550mm. The 50mm guide scope was more than adequate at this image scale. And sometimes it wasn’t even needed. One night I was shooting with the Megrez and VX in the backyard, where I really need to keep exposures down to two minutes or so because of the bright sky background. Watching the subs coming in, I thought to myself, “Man, PHD 2 (my auto-guiding program) sure is guiding well tonight.” Then, I realized I’d forgotten to start PHD 2. While I do usually guide for exposures of a minute or more, you can get away with a lot at 550mm, that you can’t at 1400mm.

There were more changes ahead for me as 2015 began, including changes in my approach to astronomy. I sold my 12-inch Dobsonian (the fabled Old Betsy), three C8s, an RV6, and some other gear (finally to include my old CG5) I wasn’t using, and applied part of the proceeds to a 120mm APO refractor. As I’d expected, it was easier to manage for imaging than my SCTs had been, even with its fairly substantial focal length of 900mm. If nothing else, there was none of the SCTs’ dratted focus shift to annoy me. And, yeah, I gotta admit I found myself becoming addicted to the refractor visual experience, that certain-special look of images in a lens scopes.

How did the VX do with the 120mm? Until recently, I didn’t know. I only used it on that mount for visual, moving it to the CGEM for imaging. The other day, however, as I was preparing to test a new camera, I got lazy. I like the CGEM a lot. In some ways, the mount gets a bum rap on Cloudy Nights (ya think?), but one thing I don’t like about it is its weight. Lifting over 40-pounds onto a tripod just ain’t my bag these days. I was not convinced the VX would handle the longer focal length for imaging, though. But, on the night in question I was, yeah, feeling lazy and also more than a little sore from working in the yard, and thought I’d give the VX a try with the 120mm.

I wasn’t in the mood to set up for guiding, either. I just wanted to figure out how to operate the camera and its software. So, what I did was limit my exposures to 30-seconds. If I had to throw out every other sub-frame at 900mm I would just do that. Surprise! I didn’t have to toss a single sub all night. I did take pains with balancing the scope, and I did do two iterations of the ASPA polar alignment, but other than that I just let the VX do its thing and it performed admirably.

In retrospect, I suppose I shouldn’t have been surprised. The VX handles large payloads well for visual use. My 6-inch f/8 achromatic refractor, which approaches 30-pounds with a 2-inch diagonal and a heavy 100-degree AFOV eyepiece onboard, is good on the VX. It would actually be very good for visual with the VX if Celestron just sold a half-pier extension for the tripod. As is, the 6-incher’s tube is a little too long and can crash into the tripod if you are not careful. With that experience in mind, the idea that it can handle the 11-pound 120mm refractor, including for imaging, shouldn’t be a surprise.

One of the great advantages of the VX for me is that I am willing to set it up even on iffy nights. Lately, the sky has to look darned near perfect before I’m going to wrestle with the CGEM or my old EQ-6. The mount is also robust. I’ve never worried about leaving the VX set up in my backyard under a Desert Storm Cover for three or four days.

That’s good, but how has the mount held-up over nearly four years of fairly frequent use? No complaints. It’s never done anything crazy. It is working as well today as the day I got it out of its box. Heck, I haven’t even had to replace the little button cell battery that keeps time and date current yet.

The greatest complement I can give Celestron’s "bargain" goto mount? I’ve often speculated as to what I would replace the VX with if it went up in smoke one night (not that I expect that). It would be a similar size mount, and I’ve been attracted to some of the newcomers in this class like the iOptron CEM25 and the Exos PMC-8 from Explore Scientific. But I would probably just get another VX. It’s never been a hassle, has never irritated me, and has never failed to do what I want done on the observing field. What greater praise can you give a mount than that?

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