Vixen GP-DX or Losmandy GM-8

I would like to get some opinions on my situation. I have been using an 8" Celestron SCT on a CG-5 mount for almost three years and have wanted to upgrade for a long time. I recently saw the clearance prices on Orion and impulse purchased the GP-DX for $999, and am now at the point of deciding whether I did the right thing or should return and get something else instead.

After a LOT of reading on the web, I believe I have made the right choice, but would like to hear from those who have used both the GP-DX and GM-8 (not the smaller standard GP vs the huge G-11). Once I add the motors, the price of the two will be identical.

My needs:
- mostly for C8 but also for TV85
- high power (up to 500x) tracking for lunar/planetary
- light weight so I don't think twice about 4am lifting
- easily transportable to dark site in car

GP-DX pros:
- I have confirmed it meets my needs (stable at 500x)
- much lighter (18.7# vs 29# w/o counterweight)
- less bulky shape and size for transport

Losmandy pros:
- porter slip ring clutch system (pro and con...see below)
- "advertised" as supporting more weight
- anything else I'm missing?

As I don't have a GM8 to play with, I'm not even sure of it's pros. For example, a lot of people seem to like the clutch system, but I have found that my use of the C8 with binoviewer requires frequent rebalancing when moving across the sky (turning the tall binoviewer shifts weight center on BOTH axes), and the GP-DX flip-clutches are fast to release for this purpose. I have the feeling this frequent rebalancing would take longer with the Losmandy clutches but just can't experiment.

Also I can't tell if the GM-8 can indeed handle more weight than the GP-DX. Everything I've read says both handle a C8 for photo/visual, a C9.25 visual-only, and not a C11. Is this correct? If so, then they must handle the same weight, right?

The Tak EM2 also looks really nice and meets my needs but would set me back a few hundred dollars more... I would also like to hear from anyone that could think of a good reason I should get this over the GP-DX. (it seems the design goals for GPDX & EM2 were the same, the only difference being that Takahashi relaxed the cost constraint)

Thanks for any help,
Bruce.

Hi Bruce,

I own a GM8 (for my C9.25 and Pentax SDHF76), and I like it, but I still think you made the right choice. Here are the strengths and weaknesses of the two, as I see it:

GM-8 advantages:
- Carries more weight; the specs I've seen claim 22 # for the GP-DX vs. 30 # for the GM8, and that sounds about right to me. It's a more solid mount.
- Astrophotography: the included drive system has features like PEC, TVC, and autoguider input that I dont think you get on the GP-DX without upgrading to their Goto system. Also, my impression is that the gears and motors on the GM8 are a little smoother (not enough to notice visually, but a factor in astrophotography; you see a LOT more good astrophotos taken on GM8s than on GP-DXs).
- I think it's better looking, but that's just me.
- The slip clutches are nice, in that you don't have to worry about stripping the motors or unlocking them before slewing to a new object.

GP-DX advantages:
- Substantially lighter (though not exactly light)
- Probably is easier to rebalance quickly, thanks to the clutch levers instead of the slip clutches
- The annoying part about the GM8 is that there's no manual slow motion; you either have to push it around by hand (making it hard to make fine changes) or use the motors (which, even at top speed, are painfully slow when you're acquiring an object using your finder or a wide-field eyepiece). The slow-motion knobs on the GP-DX offer a good middle speed.
- While it's not quite as strong or as smooth as the GM8, it's plenty strong enough to handle your C8, and plenty smooth enough to track at high power; the difference probably only matters if you're trying prime-focus astrophotography.
- You can get the SS2k goto system a lot cheaper than the Losmandy Gemini Goto, if you're interested in upgrading.

There's a good comparison of the GM8 vs. a super polaris on Cloudynights, which you've probably already read; you can extrapolate his comments on the SP to the GP-DX. Anyway, I think the only reason to go for the GM8 is for astrophotography (that goes double for the Tak; it would really be a waste to spend that much extra on a mount where the main improvements were intended for photographers, if you're not doing photography).

Hope that helps.

PB

I recently made a simlar upgrade but wound up going from a CG-5 to a GM-8. While I think that the GP-DX is a fine mount, I do a lot of public astronomy and the clutch arrangement on the Vixen mounts was a big negative for me. I lost a motor on the CG-5 when somebody tried to "help" me by centering an object I'd intentionally offset in the field. The need to disengage the slip clutches on the GP was going to eventually cost me motors, either from another helper or my own inattention (I hadn't done it yet, but it seems like a setup for a fatigued user, and I pull some all-nighters.)

I thought the GM-8 clutches would be a pain but the dovetail can be slid fore and aft for quick balance adjustments on the C-8 - keep the safety knobs installed on the dovetail and you have a fairly idiot proof arrangement. The stability is extremely good, and I found that setting up clutch tension is much easier than I anticipated. Tracking is very smooth and accurate, and I really appreciate the four rates available since I do a lot of lunar and solar work.

I don't miss the slo-mos at all, which really surprised me. I can set the tension so that the scope can be slewed easily, yet stops where I leave it and picks up tracking. I do have large hands, which probably makes tension setting on the GM-8 easier. The optional fancy clutch knobs Losmandy sells have clearance issues on the GM-8, and I have mine only installed on the ascension axis. The dec knob extensions will rub on the ascension motor cover. I have no problem with the ascension knob, but I'm at latitude 34 and I understand that much above 45 is a problem for the fancy ascension knob.

It's heavier, but I never carry it far. I bought a big new Pelican case (1660, I think)with wheels, and it holds the mount head, the counterweights and shaft, the electronics boxes, my solar filters, and a Stellarvue 80mm I use for solar.

Just my 2 cents - not sure I'd switch if I already had a GP-DX and didn't do a bunch of public astronomy.

Clear Skies!

Jim Mahon

Paul and Jim:

Well, after reading even moooooorrre all over the web, considering what the two of you have said, and thinking really good about what I want, I just returned the GPDX and ordered a GM8. The reasons for my change of mind are:

- I have read many regrets over getting a GPDX instead of GM8, but never the opposite
- your scope can never be too stable, can it?
- it doesn't seem the GM-8 will be that much heavier (same as GPDX plus 1 counterweight)
- I'm annoyed I still haven't been able to get drives for the GPDX
- I just can't get the clutches off my mind

I'm really happy about my decision, which says a lot in itself (as you can tell, I've been doubting my previous decision even after getting the GPDX). The main lesson learned is "no more impulse purchases"... maybe I have to put my credit card in a timed-release safe. ;^)

Thanks again,
Bruce.