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Astro Physics Mach1 GTO Mount


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Astro Physics Mach1 GTO German Equatorial Mount


Astro Physics Mach1 GTO German Equatorial Mount
Reviewed by Jared Willson, January 8, 2009

refractor.jpgBackground

In order to provide some context for this review, I thought I would give you some insight into my own background as an amateur astronomer and, in particular, as an astrophotographer.

I have been an amateur astronomer for more than twenty years and have owned and used a wide variety of telescopes ranging from department store telescopes on spindly mounts to small, high end apochromatic refractors as well as a few mid sized SCT's and reflectors. I currently on an 80mm apochromatic refractor that I use for quick peeks and travel. I also own a 110mm apochromatic refractor that I use for wide field astrophotography and general observing. Finally, I own an 8" modified Dall-Kirkham that I use for faint fuzzies and for longer focal length astrophotography. I live in the San Francisco Bay area, and so am severely "light pollution challenged." I am a member of two local astronomy clubs and am an active participant in astronomy outreach programs in the form of public star parties as well as volunteer work at an astronomy science center run by the public schools in my area.

Over the past twenty years I have made three distinct forays into astrophotography. First, in the early 1990's I tried film-based astrophotography on a 1970's era C8 using manual guiding through an off-axis guider. The results were disastrous-never could get the hang of guiding, and light pollution and sky glow always seemed to ruin my images. I quickly gave up in frustration. Then came the CCD and autoguider revolutions, and I decided to give astrophotography another shot. In 1998 I purchased a Meade Pictor 416 CCD camera and 201XT. Again, I quickly gave up in frustration. The user interface on the 201 was so primitive that

I never once managed to get the scope guiding, and I didn't understand enough about Photoshop and image stacking to make even recognizable images using multiple, short subexposures. Finally, in 2006 I decided to give astrophotography one more chance as part of my astronomy outreach efforts. I purchased a Meade DSI one-shot-color camera, hoping that I would be able to display people real-time images at star parties that showed more depth and detail than the views through the eyepiece. To my amazement, the camera and software actually worked quite well and I was finally hooked. Since 2006 I have moved through a progression of equipment and techniques as I tried to constantly improve the quality of my imaging.

As is the case for most deep sky astrophotographers, the least glamorous but most critical piece of photographic equipment I own is my mount. I have been using an Astro Physics Mach1 GTO for roughly nine months, so the initial excitement has worn off, and this review is based upon a reasonable amount of in-the-field experience-perhaps thirty uses in all.

Just a quick side note on astrophotographic mounts in general... Astrophotographers have a different standard of "light weight" and "portable" than most visual astronomers. The level of stability required in a photographic platform is much higher than that required for visual use, so "portable" to an astrophotographer is anything that can be transported (disassembled, if necessary) in an ordinary sedan, and "light weight" is pretty much anything that can be easily lifted onto a tripod or pier by a single person-say twenty kilos or less. By those admittedly bizarre standards, the Mach1 GTO is both "portable" and "light weight". For reference, the Mach1 head weights 13kg without the counterweight shaft or counterweights. I ordered the optional M1053 counterweight shaft with my mount, which adds an additional 3.5kg to the assembly, bringing the total weight to be lifted onto the tripod to 16.5kg. That's still within my comfort zone, but depending on your age and back strength this could be a stretch. It's definitely not a "grab 'n' go" type of product.

The Mount


Astro-Physics is famous (infamous?) for their long waiting lists. While the wait for an AP mount is much shorter than for an optical tube assembly, depending on demand and where Astro-Physics is in their production runs the wait can still be anywhere from a few weeks to a couple of years. In my case, the time between when I got on the waiting list and when I received my mount was roughly eighteen months. On the plus side, Astro-Physics does not require any sort of deposit to get on the waiting list, so if you have any interest in one of their mounts there is no reason not to sign up.

Over the years Astro-Physics has managed to build arguably the best reputation in the industry for quality and consistency, both with their mounts and with their optical tubes. As a result, I had extremely high expectations when my mount arrived. I had waited a year and a half and paid a substantial sum of money to upgrade from my (not inexpensive) Losmandy GM-8, and I was expecting perfection. While the mount fell short of perfection in a few minor areas, it quickly became clear to me that Astro-Physics reputation was well earned.

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The machining of the mount is absolutely first rate, and there are almost no plastic parts to be found. Literally the only plastic components on the mount are the knobs that attach the head to the pier and that lock down the azimuth adjustment. Everything else is manufactured with top quality materials, and the attention to detail shows everywhere. Take the counterweights as an example. They are machined out of stainless steel and have a bronze sleeve press fit into the center hole to keep the counterweight from marring the finish on the counterweight shaft.

The altitude and azimuth controls work very well, and allow for precise polar alignment. Latitude range is extremely generous, so unless you plan on using this mount within one of the arctic circles you are not going to run into a limitation. The servo system, gearbox, and controls are shared with the 900GTO and 1200GTO and are proven performers that are not likely to be challenged on this relatively small mount.

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One benefit to the comparatively large and heavy motors is that the mount is immune from minor balance issues. As long as your balance is reasonably close, the motors aren't going to strain during high speed slewing, tracking will be smooth without the "chatter" you can get during tracking on some mounts when accidentally balanced slightly west-heavy. It's a good thing, too, because motion in both RA and declination axes is relatively stiff for a mount of this size and capacity, making it hard to tell when you have achieved perfect balance.

For it's weight, the mount is extremely stiff and strong. Astro Physics lists the capacity of the mount for photographic use at 20kg, and that is not an exaggeration. Obviously, the actual amount of weight the mount can carry will depend on tube length, tripod choice, wind speed, and telescope diameter, but I would certainly feel comfortable mounting a C11 or a 150mm refractor on the Mach1.

Cable routing on the mount is quite clean, and while you do need to make a connection from the pier-mounted control box to both drive motors, the declination cable can easily be routed inside the RA axis, thus preventing snags.

So how does it perform in the field? Astro-Physics advertises 7 arc second peak-to-valley periodic error or better for the Mach1. To be honest, I have not measured my mount's actual periodic error since I have never felt that this number was very good indicator of performance. With the advent of modern autoguiders, almost any amount of periodic error can be accommodated as long as the error is slow in its progression. This is where other mounts I have owned always fell short. Even a two or three arc second error can be significant if it is not smooth. Previously, I always felt that I was losing image resolution because my autoguider had to react to errors after the fact, and that the corrections were not happening quickly enough to keep the scope pointed precisely. Here is a concrete example. I use Equinox Image for image capture. One of the features of Equinox Image is that it will report the RMS guide corrections it is making between guide exposures. With my old mount, I would typically get values in the range of 0.4 to 0.6 pixels-that's just under 1.5 arc seconds for my camera and scope. In other words, the centroid of my guide star was moving around by 1.5 arc seconds every second or two, and the autoguider was trying to keep up. Some of that shift is related to seeing, of course, but I always suspected that a better mount would do a better job-that the autoguider wouldn't need to chase the tracking errors so much. Literally the first night I used the Mach1 and turned on the guider I was stunned by the results. Instead of my usual 0.4 to 0.6 pixels, I was seeing numbers from 0.0 to 0.05 pixels-literally a ten-fold increase in guiding accuracy. Those results have been repeated on night after night. Since I have started using the Mach1 I have consistently gotten more detail and resolution in my images than I was ever able to capture with my old mount. The picture below shows two images of the Dumbbell Nebula, each captured with the same 110mm refractor and SBIG STL-11000M camera. The only difference in equipment between the two images is the mount (though the images were taken on different nights).

comparison.jpg

Now that I have extolled the virtues of the mount's tracking, I will point out a couple of niggling little issues I ran into. First, the strain relief brackets on the right ascension/declination cable have a tendency to come unscrewed. Not a huge deal, but you definitely need to watch for it or the strain relief bracket could fall off, resulting in a less durable cable. A dab of lock-tite would probably resolve the problem permanently. Second, on three separate instances my mount failed to turn on and run through its alignment routine. The hand controller would become unresponsive before slewing to the first alignment star. In all three cases I resolved the issue by unplugging the power cable, unplugging the hand controller and right ascension/declination motor cables, and re-plugging everything. After the second occurrence, I suspected that the split pin in the power cable was not making a good connection, so I slightly spread the two halves on the pin. Since that time I have experienced the problem only once in perhaps fifty uses (both in the field and when trying to reproduce the problem at home). I still don't know for certain what the root cause is, but I am not worried about it. The last negative I will mention is something that I know a lot of amateur astronomers hold dear to their hearts-the coffee grinder sound of some mounts when slewing. Yes, the Mach1 can get pretty loud when slewing at top speed, and it is not a particular pleasant sound. Not a coffee grinder, mind you, but it's a long way from a "precision hum." If you don't like it you can simply slow down the max slew speed and the noise diminishes greatly.

Polar Alignment Scope


Astro-Physics sells an optional polar alignment scope for the Mach1 which screws into the right ascension axis. I believe it is the exact same model used by Losmandy. The PAS seems to get mixed reviews generally-even on the Astro-Physics user group I have seen users describe a process for shimming the scope so that it is coaxial with the mount.

My personal experience is much better than that. Simply by screwing in the PAS and using its markings I am able to obtain a polar alignment that is accurate enough to show no sign of field rotation in ten minute subexposures across a two degree field of view. Everyone seems to have their favorite method of polar alignment, whether it be drift alignment, computerized routines using the go-to functionality of the mount, or using a polar alignment scope. I happen to prefer the polar alignment scope because, at least in my case, it gives adequate results with very little effort. Would a drift alignment be more accurate? Almost certainly, but when your time under the stars is extremely limited and every minute counts it's nice to be able to get an adequate alignment for even fairly long subexposures in less than five minutes of time. In my case the PAS does a great job.

Hand Controller


When you first look at the keypad, the first thing that crosses your mind is, "Wow,

M13.jpg that is so last century." At least that's the first thing that crossed my mind. It is large and clunky and doesn't seem very elegant. However, that's before you start using it. In the field, the hand controller proves to be nearly as well thought out as the rest of the mount. Unlike with some go-to systems, you can type in catalog numbers directly, so you spend less time scrolling up and down. The buttons are large enough to easily be used with winter gloves on, and the display is easy to ready at night without being blinding. The display is just as responsive in cold weather as it is in warm, and it comes with a rubber sleeve that protects it from minor bumps and drops.

Menu structures have a few quirks, but the UI is generally pretty good. One example of a "quirk" is figuring out how to navigate to a star using its Bayer designation. You navigate to the "Objects" menu (as you might expect), but then, instead of choosing "Stars" you actually choose "Tour" followed by "Constellation." Nothing really silly, though. As with most go-to controllers, you will quickly learn the ins and outs.

I was surprised to find that at least one feature I was used to with my previous Gemini system is absent in the Mach1. Specifically, the Mach1 has no ability to build up a pointing model to provide super-accurate go-to's. The Gemini system used by Losmandy and Mountain Instruments will calculate and account for both polar misalignment and cone error-where the declination axis and RA axis are not at a perfect 90 degrees. The Mach1 appears to have no such capability. As a result, if you haven't done an accurate polar alignment, slews across a large swath of sky will not necessarily result in perfectly centered objects. I think this lack of functionality is an artifact of Astro-Physics emphasis on astrophotography and mechanical precision. Why would you need a sophisticated pointing model if your polar alignment is spot on (as is required for astrophotography) and your cone error is virtually zero (as it should be if your mount and tube rings are well made)? In any event, this is a noticeable limitation if you happen to be using the mount for casual visual observing since you will need to re-sync the mount in different parts of the sky to get the best pointing accuracy. Not a big deal, but definitely worth noting. By the way, there are software products available that will give you high precision pointing if you happen to be controlling your mount via computer. This would be critical in the case of, for example, remote observatory operation.

Pier


I had a tough time deciding between the Eagle portable pier that Astro Physics offers for the Mach1 and the wooden travel tripod. In the end, I chose the Eagle pier because I was using the mount as an astrophotographic platform and needed all the stability I could get while still maintaining portability. Duh. In hindsight it should have been an easy choice. Why spend $6,000 purchasing a mount that can track ridiculously accurately and then ruin it all with a tripod that will bounce around in a light breeze? I'm glad I went with the pier.
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The Eagle is not without its flaws-I don't like the giant "AP" machined into each of the three legs, and the height is difficult to adjust in the dark, especially when it's cold. However, it is extremely stable, provides a surprising range of possible heights, is quite light for the amount of stability it provides, requires no disassembly to fold up and be put in a car, and does not require level ground to operate. It is as well machined as the rest of the mount, and is a great choice for either imaging or visual use. It also looks cool.

Final Thoughs


Do you need this mount? If you aren't an astrophotographer looking for a portable, light weight mount then I think the answer is probably no. A mount such as the Orion Atlas/Skywatcher EQ-6 is every bit as capable for visual use as the Mach1 GTO. You'll give up a lot in terms of quality of construction and elegance, but in terms of pure functionality you can get the exact same results out of a dramatically less expensive mount if you so choose. If you are looking for something with a better fit and finish than the Atlas, then either a Celestron CGE or Losmandy G-11 will do just as good a job visually as the Mach1 at only half the cost, and you still get a really nice level of fit and finish.

The only real justification for the Mach1 is its astrophotographic performance. Right out of the box this mount is capable of providing a level of tracking that, in my experience, is unmatched by less expensive mounts. If you need a portable astrophotography mount and you want to make sure the resolution of your pictures is limited only by the seeing conditions and not by the accuracy of your guiding, then this is the right mount for you. In fact, I was so encouraged by the tracking accuracy that I decided to purchase a longer focal length scope for those tiny little galaxies and planetary nebulae! As a platform for smaller aperture imaging scopes the Mach1 leaves little to be desired. A new mount may not be as exciting as a high-end optical tube or flashy new CCD camera, but it may well make a larger impact on the quality of your images.

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