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Losmandy GM-8 GoTo Mount |
Review date: May 2007 (last updated: June 2007)
Purchase date: April 2007
Retailer: Ordered directly from HGM/Losmandy
Price: With dual axis motors - US$1495 | With Gemini Go-To - US$2495
The GM-8 is a German equatorial mount manufactured and sold by Losmandy, a division of Hollywood General Machining located in, you guessed it, Hollywood, California. This mount is the little brother of the well-known G-11 and Titan mounts, also by Losmandy. It's available with dual-axis drives (with autoguider compatibility) or as a full-blown GoTo system with the Gemini computer.
I already owned the Gemini as part of my MI-250 set up so I ordered a custom version of the GM-8 that was "Gemini-ready". It came with the Gemini motors but no computer. All I needed to do was install the motors and plug in my Gemini computer. Scott Losmandy priced this version of the mount at US$1995.
- What I Like:
- Availability—This mount is easy to get your hands on. No waiting for months on end (usually). Even with my special order situation, it only took a week and a half to receive the mount. Many dealers have the dual axis drives and Gemini versions in stock. Of course, actual availability can vary due to the timing of production runs.
- Portability—I bought the GM-8 so I could have a relatively lightweight mount for short focal length imaging sessions. I wanted something for those "pick-up-and-go" opportunities when packing up the MI-250 wouldn't be practical. This mount fits the bill perfectly.
- Ease of set up—While there are some awkward things about this mount (see "What I'm Not Thrilled About" section below), overall set up time including scope, camera, and alignment turns out to be roughly 30 minutes for me.
- Polar alignment—The through-the-polar-axis alignment scope (optional) works quite well. On the first night of use the Gemini reported that the mount was within 2' of azimuth and 3' of elevation of the north celestial pole. The altitude and azimuth adjusters are quite smooth. There's a bit of backlash in the azimuth adjuster but this is somewhat normal even on larger, more expensive mounts.
- Performance—The mount exhibits a fair amount of backlash when reversing directions but this is inconsequential when autoguiding (at least in RA). Pointing and tracking are great. Periodic error for my mount is on the order of 12 arcseconds.1 See the image to the right. The error is very periodic (little randomness and hysteresis), quite smooth, and easily guided out.
- What I'm Not Thrilled About:
- Scratches & Finish—Overall, the finish of the mount is great but there are a few scratches—nothing major but annoying on a brand new mount. Also, in bright light, the finish is slightly purple. Black anodized metal does purple over time with exposure to UV radiation but it's never to be expected on a brand new piece of equipment. I'm the first to admit that I'm quite picky about these kinds of things so I'd have to presume that most people wouldn't care or even notice.
- Tripod—The fact that the tripod is included at this price is great but the tripod is somewhat crude and awkward. It's a little difficult to open and close the legs. The feet are raw aluminum, have no rubber footings, and are quite sharp.
- Motor positions—The motors on this mount stick out far to the right side (when standing behind the mount facing north). This is a problem for me when transporting the mount as I must be very careful not to rest the mount on that side. It can also be a problem if you intend to mount any counterweights on the underside of the dovetail plate as it can easily hit the RA motor.
- Additional Notes:
- Adjustments—The only adjustment needed to the mount when I received it was to tighten the worm/worm wheel mesh in the RA axis. It was relatively loose, resulting in a fair amount of backlash and drift after the mount was moved eastward. After the adjustments, this was greatly minimized.
- Modifications—There are no required modifications to speak of. All I've done in the way of modifications is insert plastic washers where the mount connects to the tripod and on the elevation adjuster. I also added a Velcro patch on one of the tripod legs to hold the Gemini hand controller.
- Supplied Voltage—The Gemini system can get rather particular about the voltage supplied to it. It's advertised as a 12-18v system but 12v is the absolute minimum you should use and only if you have to. Much better performance is received if you can provide the mount with 14-16 volts. You will encounter fewer pointing issues and fewer "RA (or DEC) motor lags" messages. My GM-8 seems to be more sensitive to this than my MI-250 but both systems have benefitted from the increased voltage (I currently supply it with 14.5v using a variable voltage DC power supply).
Conclusion:
The Losmandy GM-8 is a top quality mount in its class. There are a couple clunky things about it (the tripod and the motors sticking out) but nothing so dramatic that should prevent one from owning it as a primary, light-duty mount or as a secondary mount used in place of a larger, heavier mount. It's not the least expensive mount out there but you are buying a precision machined instrument that performs better than just about anything else available in this price range. In addition, support and parts are widely available unlike with many other "comparable" mounts.
1 I tested periodic error with an SV80S using a Tele Vue 2x Powermate (for a focal length of 960mm) and an ST-2000XM. I took a series of four 8-minute images with no guiding and with the mount purposely out of polar alignment by a slight amount. Peak-to-valley star travel was 7 to 8 pixels. The image scale was 1.59 arcseconds per pixel, equating to roughly 11-13 arcseconds.
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Click an image below for a larger view.
 GM-8 with SV80S and 20D
 Close up of right side
 GM-8 waiting for darkness
 Motors sticking out on right
 Left side
 Periodic error
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