Losmandy gm8

mitch (Mitch) · #1 01-02-2013, 12:08 PM

Hi all,

I've been dabbling in imaging for the past two years with a cgem coupled with an orion ed102t cf. However with a 30" periodic error (after pec

) I feel it's time to upgrade to a higher quality mount.
I don't plan on upgrading to larger aperture anytime soon so am really just after a quality mount to support my current scope. All together with cameras and guiders etc.. the current mass is about 7-8 kg.

I've narrowed it down to a Losmandy gm8. Just wanted to get peoples opinions on whether they would suggest any other mount? I know the g11 has twice the capacity but it's also twice as heavy (portability is important as I live in the city) and pushing the budget.

Cheers in advance

John0z (John) · #2 01-02-2013, 12:28 PM

Does your CGEM allow autoguiding? If so, are you using an autoguider? It seems to me that an autoguider would give you better results than just using PEC correction.

Notwithstanding this, I have a GM8 (non-Gemini) which I have not used very much in the past 10 years, and recently got a HEQ5Pro to play around with Goto etc, since the Gemini upgrade for the GM8 costs more than the HEQ5Pro.

Also I now have an autoguider so that I can start to do some proper imaging.

-John

tlgerdes (Trevor) · #3 02-02-2013, 10:12 AM

Hi Mitch,

I run a G11 that I trapes out to the sticks easily once a month. I think the big difference if you are imaging is the G11s tripod. I honestly think that is its best feature. You could stand your car on it and wont flex

.

Poita (Peter) · #4 02-02-2013, 12:20 PM

I have a CGEM and a G11, and the G11 in stock form the reworked CGEM gets better PE than the G11.

I'd suggest getting your CGEM stripped down and cleaned up and adjusted. I have imaged with a C11 on a CGEM with no dramas. I don't think you will see any improvement with a GM8 and they need regular maintenance as well.

Have you had your CGEM apart or had it redone along the lines of the 'hypertuned' service? They perform radically better after a rebuild and you should have no trouble getting perfectly round stars, especially if autoguiding.

mitch (Mitch) · #5 02-02-2013, 05:30 PM

Thanks for the replies.
I ordered the hypertune kit and pulled it apart and did the service, after that the pe was improved but still hovering at the 30pe mark. I think the problem lies in the ra gear. The housing doesn't seem completely uniform. From the pe curve, it behaves normally and then has a large spike at a particular point.

By getting the gm-8 as opposed to the g11 I thought I would get the ovision worms with the extra cash. In the meantime I'll continue to work on the mount to try and squeeze some better performance out of it. I would prefer to get the mount working instead of buying a new one. But at the moment I don't see much choice.

Logieberra (Logan) · #6 03-02-2013, 10:55 PM

Mitch, are you intersted in an update that will last for years? I was in a similar position to you a few years back:
- HEQ5 Pro, to
- NEQ6 Pro (did consider the GM8 at that time), to
- G11, to
- MX. Current mount.

I looked very closely at mounts under the 10k mark. If you could stretch to an Astro Physics Mach1, perhaps with a bit of patience and saving, you'll be well pleased. Or perhaps an MX? They are on special at the moment.

Reason I didn't go with the GM8 - you don't see good images take with them, the RA gear is approx 1/2 the size of the G11 which equate to approx 2 x the periodic error. Do a google image or flickr with the terms 'GM8 / G8' and look at the images out there. Compare with the Mach1!

If I was you, after owning the mount's you've owned, I'd go for a longer-term upgrade. Just my two cents. Good luck. Logan.

Logieberra (Logan) · #7 03-02-2013, 11:09 PM

P.S. Mitch, I actually paid a deposit on the GM8 with Michael @ Bintel. He was good enough to refund. Glad I never went down that path, personally.

Forgot to mention the new Skywatcher coming out, not sure when. EQ7/8, whatever it's call. Will be amazing value, by all accounts.

Poita (Peter) · #8 05-02-2013, 02:18 PM

In all honesty I just don't think a G8 will be a step up from what you have.
Jump on the CGEM yahoo group and see if you can sort your existing mount, you would be taking the same lucky dip with the GM8.

Improve the one you have or save up for a Mach1 or similar, anything lower and you may end up with about the same level of performance that your CGEM is displaying now.

Logieberra (Logan) · #9 05-02-2013, 02:41 PM

Sound advice Peter, good luck to you Mitch.

Geoff45 (Geoff) · #10 06-02-2013, 07:55 AM

Good advice from Logan. I went from a G8 to an AP900. The G8 is best classed as an " expensive mistake". Fiddled with it endlessly, bought the OPW block, but could never get reliable tracking until I bought an AO unit (more money). Save up and go upmarket. You won't regret it.
Geoff

Poita (Peter) · #11 06-02-2013, 11:14 AM

Mitch, if you post some guide logs and some graphs, I can take a look and see if we can pinpoint the problem.

We should be able to get your mount working a lot better with a bit of investigative work.

Logieberra (Logan) · #12 06-02-2013, 12:00 PM

From memory, Grant aka 'GEM' uses the CGEM. He purchased his new fron Andrews. Lovely round stars in his shots, and not from a permanently mounted site! Setup each time. That mount can perform.

mitch (Mitch) · #13 06-02-2013, 01:48 PM

Thanks for all the advice.
The problem I have with the cgem is that it will require a lot more money to be spent on it to get it to an acceptable state. Which is something i'm not really prepared to do as I have no guarantee it will work. There is bad casting in the dec housing which results in a poor fit for the ring gear and when hypertuning it for the second time the RA ring gear (stupid me) rolled off the bench and was damaged.

An AP mount is well out of my budget for the foreseeable future along with other mounts over the 5k mark.

My only goal really is to be able to get some nice images from the current setup I have which is just under 7kg. Portability is a big concern for me as I don't have a permanent setup and lifting any one thing over 15kg is out of the question after a spinal fusion (cgem was uncomfortable to lift). Which is why the G8 had such an appeal to me.

The furthest I could push my budget is on a G11 but it just seems overkill for a such a light setup with no desire to upgrade to larger aperture in the future. It's also getting very heavy/bulky for me.

At the end of the day, I know a mount under 5k will never be in the Mach1 or paramount class but for the time being i'm happy to accept that. As said, I'm really just after a mount which is very portable and can handle 7kg and 700mm focal length to a level equal to or better than my cgem.

Thanks again for all the advice and thoughts. I think i'm just going to continue looking around, see what pops into the classifieds and continue tinkering with the mount.

Poita (Peter) · #14 06-02-2013, 02:56 PM

I don't think it will cost much to fix really.
Next time I'm in Sydney I could came take a look.

The GM8 won't do what you want, the EQ6 could if you get a good one, but it may be too heavy for you. Not sure how a HEQ5Pro would go with that weight, maybe an owner could chime in.

Geoff45 (Geoff) · #15 06-02-2013, 03:17 PM

Quote:

Originally Posted by mitch

The furthest I could push my budget is on a G11 but it just seems overkill for a such a light setup with no desire to upgrade to larger aperture in the future. It's also getting very heavy/bulky for me.

Overkill is good.

rogerg (Roger) · #16 06-02-2013, 04:04 PM

I own a GM-8. Used it extensively for many years. Edit: but only twice in the last 2 or so years.

I'm not sure if I would recommend one or not. I know the situation you are in, I was there when I bought mine (must be 7-8 years ago now). Back then there was no HEQ5 no EQ6 etc. My option was G-11 or GM-8 or Tak Temma or Vixen Polaris or the like.

I don't use it enough these days to comment with a fresh perspective. I also don't have experience with HEQ5 or such. But you may be interested in reading my review from all those years ago:

http://rogergroom.com/astronomy-reso...lescope-mount/

Regards,
Roger.

tlgerdes (Trevor) · #17 07-02-2013, 03:35 AM

This reminds me of a Top Gear segment where they had a Toyota Prius go up against a BMW M3, they were measuring fuel economy. The Prius went around the track as fast as it could and the M3 followed it.

The result was that 1) the M3 didnt have any issues keeping up with the Prius (not suprising) 2) the M3 used about 30% less fuel than the Prius in doing so.

Upshot, a G-11 at 40% load will perform way better than a GM-8 at 80% load. The G-11 isnt much more time consuming to setup than a GM-8, and I doubt you are carrying it on the back of a push bike so weight shouldnt be too much of a consideration.

mitch (Mitch) · #18 10-02-2013, 09:44 AM

Apologies for the delay, I've been working extensively on the mount for the past few days. I stripped down everything, re-sanded and polished the ring gears and worm gear. Resat the gear housing and cleaned the ball bearings. The mount now (finally) is running very smooth and the go-to accuracy is spot on.
However it is now having problems guiding. What i've found is that there is quite a lot of play in the gear housing on the motor. At guiding speeds, the motor has to be engaged for about 2 seconds before the gears will turn. Is this normal? How can it be corrected in the phd guide settings? as the star is not moving at all yet I can here the motor's engaging with each pulse.

Secondly, after a pretty unanimous discussion i've decided i'll upgrade to the G11 mount, it's a little heavier but much more versatile and stable. However if I can get the mount guiding nicely i'll hold off on the G11 for the time being and spend some of the extra cash on a moonlite focuser.