I continue to receive a lot of hits on my website from people reading this review. I would like to point out this review is now quite old (see dates below) and there are many new alternatives to the Losmandy GM-8 out there (iOptron, various Celstron and Meade mounts, Skywatcher & Guan Sheng just to name a few). I strongly suggest you look around and investigate newer options.
In June 2005 I received my Losmandy GM-8. I had ordered it a few months prior for a cost very close to AU$4000 including shipping. In the time since then (now February 2006) I have had the chance to get to know the mount, and feel it’s worth me noting down a few thoughts for those owners and prospective buyers of the Losmandy GM-8.Over all I have been very happy with the mount to date. It definitely is built strong, easily handling any payload I have put on it. The Gemini system is quite easy to use once you get to know it’s menu system. The whole package is all easy to use, always ready to go, and quick to set up.
First I’ll address a few pre-purchase concerns I had.
Pre-purchase concern 1: Cables
One of the things I wasn’t looking forward to was having external cables between motors and control box, then to the hand controller and battery. This is as compared to a mount like the Sphinx where most cables are internal. As it turns out, the cables haven’t been of a problem. Ocassionally I find the DEC cable is not where I want it, so re-locate it such that it can’t catch on anything when the scope is slewing, but I haven’t actually had a problem of it catching yet. The hand controller is a nice size and shape to hold, and the one cable coming from it is a very nice soft flexible one, with screw-in ends such that it can’t fall out.
Pre-purchase concern 2: Gemini Usability
With only a single line display I was concerned about the Gemini’s usability. To start of with it certainly did take some getting used to, and I still wish they’d give it a 4 line display or something of that size. However like anything, you soon get used to it. I now know by memory the entire menu structure and am used to how it operates and what menu option it will choose when. It is quite smart with respect to showing you what menu options you are most likely going to use, and in an appropriate sequence. It varies what it shows to you depending upon it’s current state and while it varies so you may think it’d be hard to get used to and predict, it’s actually very intuitive. So, I don’t have a problem with it, but I really wish they would just give it a bigger screen and a number pad, just to make life a little easier.
The buttons on the hand controller are very nice, and the light emitted is of a good level. I ocassionally have problems pressing buttons, needing to re-press them harder for it to accept the input.
I have been impressed with the Gemini’s capabilities, particularly with respect to advanced features such as it’s pointing algorithm, PEC and Backlash Correction.
Pre-purchase concern 3: Payload Capacity
When deciding which mount to purchase I had people say to me “don’t get the GM-8 Roger, get the G-11, it will be much more sturdy and capable for your astro photography”. Well, while I’m sure they are correct that it’s more sturdy, it would definitely be overkill for my equipment.
The GM-8 has easily handled any payload I have put on it. The standard payload I have mounted on it is: Megrez 80 + SkyWatchcher 70×700 + Camera (or 2) + Lens + counter weight + eyepieces. This totals about 9kg. The mount laughs at this weight, it really does. There is no degredation in performance compared to a bare minimum payload.
I have also used the GM-8 with a William Optics FLT 110 refractor. That was also easily handled while doing astro photography. If using the FLT110 with a significant train of large CCD equipment I’d consider if a larger mount was required, but the ST7 or webcam is no problem.
Pre-purchase concern 4: Guiding Accuracy
I feel I have yet to really sort this one out. I have used PEC a few times with good success, however have not reduced the PE to an un-noticable amount. To be fair, I have not given it a concerted solid attempt, I tend to try a few updates and then leave it be and use it as-is without trying to perfect the PEC.
Roughly speaking (I never can work out how to accurately measure these things) the PE has been comparable to that of my LX200 with it’s PE correction enabled.
I have been impressed with the Gemini’s capabilities regarding PE correction and Backlash Correction. The PE correction has an ‘offset’ feature that allows it to correct for the PE as it happens, not after it happens. This is something I wish the LX200 had.
I have had a recurring niggling problem in DEC guiding, where it will skip ahead some times. I’ll mention this later.
When I started using the Losmandy GM-8 I ran it from 12v direct from a sealed lead acid battery. I now run it from that battery but through a converter that converts the 12v to one of 13v, 15v, 17v, 19v, 20v.
When running the GM-8 at 12v I found I ocassionally had odd guiding, where it wouldn’t behave as accurately as I expected. Similarly with slewing, it wouldn’t sound as healthy as expected. Running it in the 15v – 18v range has dramatically change that, with huge improvements in reliabliity, accuracy and ‘healthy sound’, with the motors having a nice solid high-torque sound to them.
DEC ‘skip’ problem
I have had an anoying situation when guilding a long exposure photograph that the DEC will ‘skip’ a small amount after several small DEC corrections (correcting for polar alignment error). What would happen is I’d make about 4 corrections over 10 or so minutes, then on the 5th (or there abouts) the guide star would ‘jump’ or ‘skip’ on more than expected. It is like tension builds up in the worm/gears/motor and is then suddenly released. The amount of movement is tiny, but noticable for accurate guiding.
I have reduced the occurance of this problem by using the mount at 15v or 17v instead of 12v. That definiately makes a difference. I have been recommended to re-adjust the worm gears and have done this once, with some imprevement, but then with the problem re-occuring a couple of months later. I’m not an expert at adjusting these worm gears, etc, I expect someone else could do a much better job. I have managed to more consistently remove the problem by precisely balancing the mount in DEC. I have discovered that the DEC balance is critical, absolutely critical. With an imbalanced DEC this problem occurs. I had the ituation where the cameras and position of the OTA’s on the mount were such that it was heavily unbalanced towards the back. Correcting this by taping some heavy spanners to the front of the OTA removes the problem immediately.
It’s anoying that this problem occurs at all, but at least it can be fixed, and worked-around.
WD Reset Error
I once had the situation where when I turned on my Losmandy/Gemini it beeped and showed an error character, then froze. No meaningful message was displayed even though it appeared there was enough power to power the processor & display.
Turns out this was because of the battery supply to the unit being dead (my sealed lead acid had been draining while in storage without me realising – one LED had been left on.
So I switched the sealed lead acid for another and booted it up expecting all problems to be gone! Not so!
Upon bootup on the new battery it would proceed thorugh the GPS timeout and instead of displaying the options of Cold Start, Warm Restart and Warm Start it would display “WD Reset 0800”.
Now the manual mentions absolutely nothing about “WD” “or “WD Reset” or “0800” helpful, really helpful! The only reset message I could find reference to was “CMOS Reset” which apparetly suggests a dead CMOS batter – unsure, never had this message show.
After much digging on the news groups I was suggested to remove the CMOS battery, wait at least 5 minutes and put it back in. I did this, this fixed the problem such that the unit would now start properly again with no “WD Reset” error displayed.
Also another few symptons to note: When the WD Reset error displayed a dull soft beep would be made from the Gemini control box. Also, when slewing in RA/DEC, the RA would halt and beep continuously and loudly at some point usually after about 2 seconds of slewing from the park position. I’m unsure why it would do that. The continuous beep made was perhaps the same as what the manual says would be done when a slew lmiit is reached. I tried resetting the slew limits and restoring factory defaults, none of that solved the beeping or the “WD Reset” error. Only removing the CMOS battery solved the problem.
Interstingly it is worth nothing that in the newer units like mine a rare panasonic CR 2354 battery is used, not the CR 2032 that is stated in the manual. Good move guy’s! List a different replacement battery in your manual to the one required – is that covered by warranty when the wrong one is put in?? Anyway, if you have a newer unit, ensure you get a spare 2354, as they are hard to come by and you won’t get one quick when you need it.
Interestingly, it has been said the CR2032 can still be used, just you need to hold it there, perhaps with a washer or the like, as the CR 2032 is smaller but the same voltage. (Thanks Rainer for that info). I haven’t tried this but am sure it’d work. I’m taking the option of having a spare instead.
One final note on the WD Reset error: It was mentioned on the group (after I solved the problem) that a hex error code could have been viewed by selecting the “Info” menu option. This would allow you to know the exact error code that the WD Reset message is referring to. What you could do with that error code I have no idea, as the Gemini manual is not detailed enough to contain such error codes from what I can find. For me it was too late anyway, the WD Reset error had gone, but for you it might be useful if you have not yet pulled the battery out.
To counter the problems of the Losmandy GM-8 I will mention the good points of the mount which keep me liking this piece of machinery.
Stability & Stength
The mount is strong, it’s built solid and has little play in moving parts. You tap the mount and vibrations dampen quickly. You use the mount in a strong wind and it handles it better than could be expected. You adjust the polar alignment and there’s little play in the adjustments. This really is a solid and precision machined piece of equipment.
The mount is very portable. There’s a few things that help with this: Having the optional tripod knobs, having boxes neatly containing the various parts and what else would be of great assistance is a polar scope. You can be setup and viewing relatively quickly, probably only a few minutes if you’re working from the back of your car and doing visual observing only. Longer if you’re doing photography of course.
The gemini is a nice GoTo system. It’s not my favourite by a long shot, but it is functional and does the job. It’s nice to have simplicity when out in the field especially if dealing with spectators/visitors and the simple interface and menu structure of the Gemini provides this. More buttons would often be nice.
The GM-8 handles what weight I put on it easily, more easily than I had previously expected. Sure it has a limit, but I feel the ratings provided are sound and if you’re not wanting to carry around something larger and are on the borderline of capacity this will probably get you by. People kept telling me to get the G-11 instead, I strongly disagree and am glad I only purchase the GM-8.
Suggestions/Improvements for GM-8 owners
I’ve found a few little things have helped along the way. One was putting velcro down the tripod legs and on the back of the hand controller. It means I can put the hand controller anywhere within reach on the tripod legs, on any of the legs. Easy and convenient. The other is using the GM-8 on 15v or 18v instead of 12v. This has solved some motor problems and provides much stronger torque to the mount, you can litterally hear the difference when slewing the telescope.
As with any telescope, balance is important with the GM-8. I know when it is out of balance, it doesn’t behave as nicely. It is very easily balanced, by loosenning the clutches and swining it around, making adjustments, etc. DEC balance is absolutely critical to remove guiding problems. I have found DEC balance is more of an issue than RA balance.
I’ve marked refernce points on the mount’s rotation at the base such that when adjusting the azmith it is easy to see a slight movement. I sued reflective tape for this with sharply cut edges as the guide.
Get a spare battery for your CMOS – you might suddenly need it some day. The CR 2032 batteries apparently last about 12 months, the CR 2354 I am not sure how long they last but they are hard to come by, so if your unit uses one, get it before you need it.