Reviews > Mounts > Losmandy GM8
After struggling to get good results with my old Helios EQ-5 mount for several years I decided enough was enough and it was time to invest in something more capable for astrophotography. I started off as primarily a visual astronomer and for that purpose the Helios mount was satisfactory. Astrophotography demands a stable mount capable of good tracking over extended periods of time with support for autoguiding. The Helios EQ-5 was not capable of either of these, even after a fairly thorough overhaul and re-lubrication.
I chose the Losmandy GM-8 after reading many complimentary reviews written by their owners. I had considered choosing the G11 instead, but after seeing one in person I realised it was just too large for a portable setup. I don't have a permanent setup and for me it is important that I can move the mount around without too much difficulty. The Losmandy GM8 is portable and I'm able to move it around with the tripod and mount connected together. It's a fairly heavy lump but doesn't present too much of a problem if only being moved from the house to the garden.
I bought my Losmandy GM-8 from Telescope House and decided to choose the Gemini option. When I tested the Gemini unit for the first time it quickly developed a fault which caused the system not to start, but rather emit a constant beep. According to the manual this meant the CPU had failed. Telescope House were very understanding and replaced the Gemini system with another unit the next day. Unfortunately the replacement unit also developed a fault within the first half an hour of use. The clock would not set and every second one month would increment on the display. This problem was almost certainly a failure of the RTC chip which is a fairly common problem on Gemini units shipped over the past year or so. Telescope House replaced this unit for me with another that they had soak tested on the bench. This replacement Gemini system has worked fine since. This was the first time I had bought anything from Telescope House and I'm happy to say that they are now top of my list for astronomical purchases given the fantastic support I received.
The Losmandy GM-8 is a very well engineered mount and everything fits together perfectly without any excess play. I did at first think I had excess play on the counterweight shaft but it turns out that 1-2mm is desirable to prevent too much friction with the worm gear. It certainly pays to join the Losmandy Yahoo group as there is a wealth of information available in their archives, and many knowledgeable users happy to help out where they can.
The first few nights I went out with the GM-8 were spent testing the tracking. Using my 6" Maksutov Cassegrain with an AST SC3 modified webcam I carried out a number of exposures, increasing the exposure length each time to determine the point at which stars begin to show evidence of drift. You can find the results of these tests at the end of this review. I never level my mount with the bubbles as I've read this is not required. I do align as best I can with the polar scope and once I'm happy I'm as close as possible to polar alignment I use the Gemini to carry out a three star alignment using well separated stars (in both DEC and RA). To summarise, I find that the GM-8 will track well over 60s and I'm able to retain 90% of the exposures I take. At 120s I'm able to keep about 60% of the exposures. Going beyond 2 minutes unguided is unpractical as I end up throwing away too many exposures.
I'll discuss autoguiding elsewhere on this site, but to guide I decided to use a side by side mounting system. I had read that there is generally less differential flexure in side by side systems so I opted for the ADM side by side mounting plate with dual Losmandy/Vixen saddles. That gives me plenty of flexibility in terms of dovetail rails etc. What I hadn't realised is just how heavy this would be, about 3.5Kg! The GM8 is able to carry payloads up to 30lbs though for astrophotography most people don't recommend going above 20lbs on this mount. The adapter alone is over 7lbs and I still need to add the weight of the scope, guidescope, camera and autoguider. My results so far have confirmed that my current setup with the following equipment doesn't seem to be a problem:
- Intes Micro Alter M603 (6" Maksutov Cassegrain)
- Canon 350D DSLR
- Orion 80ST (80mm achromatic refractor)
- Orion Starshoot Autoguider
Another problem I discovered was balancing the side by side setup. This involves balancing in 3 axis and takes a little more time and effort than balancing a piggyback setup. I find it's easiest to balance along the counterweight shaft first, followed by balancing the side by side mounting bar in the GM-8 saddle. Finally the scopes themselves need to be balanced in the saddles in the side by side bar. It's generally necessary to test the balance in the other axis after a change is made in any one. I discovered that if the mount was not well balanced the Gemini alignment would not work. This turned out to be caused by the clutches slipping when the mount slewed. I tried solving the problem by tightening the clutches as much as possible but this is not that easy with the stock clutch handles. I upgraded to the clutch handles with knobs which meant I could tighten the clutches better but the clutches still slipped. I eventually took the clutches apart and discovered grease had found its way onto the nylon clutch pads. I should point out that taking the clutches apart is literally a 2 minute job. Working on the GM-8 is a pleasure as it's so well engineered. After cleaning the clutch pads I tested again only to find that the clutches no longer slipped, but the motors stalled. It was only then that I realised the problem was all down to balance. When I balanced my setup I didn't have the cables attached to the cameras. I didn't think it would make much difference but I can confirm that it really does. After balancing with the cables attached I no longer suffer from the motor lags or clutch slips. One other change I made was to reduce the Gemini slew and GoTo rates from 800x sidereal to 400x sidereal. I just felt this would reduce the stress on the motors and decrease the likelihood of the clutch slipping. It does mean that I have to wait twice as long for the scope to move to the correct location, but an extra 15s is a small price to pay if it means the motors will last longer.
So to conclude this review I can say that aside from the hardware issues with the first two Gemini units I received I have not had any problems that couldn't be resolved. I've very happy with the Losmandy GM-8 and wouldn't hesitate to buy another. I do have a feeling that one day I may wish to mount larger scopes and the GM-8 may not be large enough, but as a portable imaging platform I think it's a very good choice. Some people say the Gemini system is a little outdated and the menu structure is difficult to get to grips with. It's fair to say that the menu structure is not intuitive at first, but given some time it becomes second nature. As for it being outdated, I find it has all the functionality I need but I am concerned that the availability of replacement parts in the future may become an issue. While we'd all like things just to work without any frustration, sometimes we learn more by finding, understanding and resolving problems. Personally I think that half the fun of amateur astronomy is finding out for ourselves what works and what doesn't. As for astrophotography, it demands even greater levels of perseverance and patience but nobody ever said it was easy!
10s unguided exposure at F10 on 6" Maksutov Cassegrain.
30s unguided exposure at F10 on 6" Maksutov Cassegrain.
60s unguided exposure at F10 on 6" Maksutov Cassegrain.
120s unguided exposure at F10 on 6" Maksutov Cassegrain.