Losmandy GM8

[Carl]

Anyone out there with a Losmandy GM8 I need advise. Will it handle a CST8" with piggyback 80ed and track accurately enough for DSLR imaging up to 10min exposures at prime focus thru the CST8"?.
If not what other mount for under $5000 will help me achieve the above

Regards
Carl

[gbeal]

Ponders will have the answer as he has one.
My gut feeling is yes it will, they are an excellent mount.
If not I have an EM200 goto in the For Sales.
Gary

[JohnH]

Sounds like a job for the Vixen Sphinx SXD...I have the SXW and have an 8" VC200L on it - though I fought shy of the ED80 due to its weight. The SXD should be able to handle it fine and is a lot less $$$ than the Losmandy/Gemini. If I had the money though I would have a G11....

[JohnG]

See attached.

GM-8, C-8 and carried a William Optics ZS-80 as a guidescope without any problems.

You will need at least the 11lb counterweight.

Cheers

JohnG

[Carl]

Hi John
are you imaging thru the CST or refractor?
I'm currently having no probs imaging thru the 80ed, The accuaracy in the mount is needed for shooting thru the CST.
Comments please

Regards
Carl

[JohnH]

Carl,

Nearly all of my imaging is through the VC200L at prime and the SXW manages this well. I have had some issues getting my guiding accurate enough at the longest focal length (1800mm) however this is normal and I have found solutions to the issues I have encountered.

In my case my early attempts at autoguided long exposures were frustrating, I discovered I have to pay carefull attention to balance (need to be heavy in the E), DEC settings (guide in one sense only) and polar alignment. Having done all that I found my issue was mostly down to software centroiding accuracy - a nasty surprise and not one you think of first as the cause of tracking errors.

I have threads eleswhere on this if you want to take look...my most recent image is this one:

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=28133&d=1181826917

[[1ponders]]

As JohnG says, it should handle it np, though I prefer to use a side by side arrangement to keep the center of gravity closer to the mount head. But then I'm using the digital drive with stepper motors and not the Gemini with servo and gearbox as John is. This will make a difference, especially for imaging through the longer focal length.

[Garyh]

Hi Carl,
I have a 6" newt with a 70mm guidescope side by side etc on the Gm-8 and after I fixed most of the flex probs I can usually manage up to 8 min subs maybe longer..Total weight would be about 11kgs..with a extra counterweight I have no probs with the mount at all. Could easily go to the max 15kg I reckon..imaging at 850mm by the way...
cheers

[JohnG]

Hi Carl

I don't use this setup anymore, I use a G-11 with a Takahashi refractor, the GM-8 is now my field setup with an 80mm refractor and a 66mm guidescope in a piggyback configuation on a G-11 tripod. That said, the GM-8 was designed to handle the C-8 and so long as you bear in mind that the weight limit with the standard tripod is around 15 Kgs all up and remember the 2/3rds weight rule for imaging, you shouldn't have any problems.

If I were you and you want to image at the native f/10 of a C-8 for the time periods you mention, I would go for a more substantial mount, G-11 or similar. I doubt very much that you will get 10 minutes of unguided tracking from a GM-8, that is 1 and a bit worm rotations from memory, I think PE will get you.

the other thing I would consider is that you are in for a lot of frustration shooting at f/10, I would be considering using a Focal Reducer.

Just my thoughts.

Cheers

JohnG