Jump to content


Photo

GM-8 Imaging Weight Limits

  • Please log in to reply
20 replies to this topic

#1 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 08:06 AM

My main imaging scope weighs right at 16 pounds with everything attached. As the weight increased when I added more things, I could see performance changes with the mount. It worked decently enough like this a week ago after I spent time with the balance and PHD settings but I spent all night on Monday trying to get it to track well enough to take a 5 minute sub with round stars. I tried objects in different parts of the sky and adjusting the balance with no success.

Last night I put a 9 pound package on and it purred right along.

I expected 16 pounds to be doable for it and am very disappointed. :confused:

#2 Raginar

Raginar

    Fly Me to the Moon

  • *****
  • Posts: 6138
  • Joined: 19 Oct 2010
  • Loc: Rapid CIty, SD

Posted 23 October 2013 - 09:13 AM

I would've expected that as well terry. 16 lbs is not that much for a mount rated to 35. How is your backlash? You could see if adjusting your gear mesh smooths it out. And PEC might be an option.

#3 tomcody

tomcody

    Surveyor 1

  • *****
  • Posts: 1776
  • Joined: 06 Jul 2008
  • Loc: Titusville, Florida

Posted 23 October 2013 - 12:33 PM

Also may be flex in the tripod? I assume you are using the stock GM-8 model?
If you suspect tripod flex? perhaps you could borrow a G11 tripod and see if that makes a difference.
Rex
P.S. the fact that you get good tracking at 9 lb and bad a 16lb says to me the system is working correctly but not able to hold position with the extra weight, tripod flex, mount saddle,rings flex, guider flex, or maybe motor stall issues ( you did not say if Gemni or digital drive?) but flex comes to mind first. Also how was the wind that night?

#4 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 01:24 PM

Also may be flex in the tripod? I assume you are using the stock GM-8 model?
If you suspect tripod flex? perhaps you could borrow a G11 tripod and see if that makes a difference.
Rex
P.S. the fact that you get good tracking at 9 lb and bad a 16lb says to me the system is working correctly but not able to hold position with the extra weight, tripod flex, mount saddle,rings flex, guider flex, or maybe motor stall issues ( you did not say if Gemni or digital drive?) but flex comes to mind first. Also how was the wind that night?


Thanks, I'm using Gemini II. No motor stall issues and very calm. I did image with it a week ago ok but only after lots of balancing and adjusting PHD settings. Even then, pointing at different areas required more adjusting. I do wonder about tripod flex but don't know where I'd get a G11 tripod to test with.

I've had it for about 18 months and it has always worked fine with less weight. The RA portion of the PHD graph on Monday looked like a sawblade and the elongation was in RA :(

#5 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 01:28 PM

Thanks Chris. I think Tom Cody is correct about tripod flex.

#6 tomcody

tomcody

    Surveyor 1

  • *****
  • Posts: 1776
  • Joined: 06 Jul 2008
  • Loc: Titusville, Florida

Posted 23 October 2013 - 02:09 PM

Terry,
One last thought, did you make sure the dec lock screws and the azimuth lock screws are tight? Once I left the azimuth screws loose and got bad tracking from the flex on my EM200.
Rex

#7 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 02:52 PM

Terry,
One last thought, did you make sure the dec lock screws and the azimuth lock screws are tight? Once I left the azimuth screws loose and got bad tracking from the flex on my EM200.
Rex


Tom - One of the altitude adjustment bolts was slightly loose. That could have been the issue. I will try it out tonight. Thanks!

#8 RAKing

RAKing

    Fly Me to the Moon

  • *****
  • Posts: 6303
  • Joined: 28 Dec 2007
  • Loc: West of the D.C. Nebula

Posted 23 October 2013 - 03:04 PM

I would've expected that as well terry. 16 lbs is not that much for a mount rated to 35. How is your backlash? You could see if adjusting your gear mesh smooths it out. And PEC might be an option.


My GM-8 is only rated to 30 pounds, but your point is well-taken.

The OP didn't mention what scope he is using but a longer refractor could cause more issues than a short SCT. I spent a lot of time shimming the worm gears on my GM-8 to get better gear mesh and I always had to have the balance "just so" to keep it tracking well. ---- I also used the Losmandy HD tripod.

A friend swapped a 90mm apo for a 110 on his GM-8. His 90mm had always worked great and it surprised both of us that it took him a couple of months to get the slightly heavier rig working satisfactorily. The GM-8 is a small mount with small gears - and it just doesn't have much margin for error.

My .02,

Ron

#9 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 04:02 PM

I would've expected that as well terry. 16 lbs is not that much for a mount rated to 35. How is your backlash? You could see if adjusting your gear mesh smooths it out. And PEC might be an option.


My GM-8 is only rated to 30 pounds, but your point is well-taken.

The OP didn't mention what scope he is using but a longer refractor could cause more issues than a short SCT. I spent a lot of time shimming the worm gears on my GM-8 to get better gear mesh and I always had to have the balance "just so" to keep it tracking well. ---- I also used the Losmandy HD tripod.

A friend swapped a 90mm apo for a 110 on his GM-8. His 90mm had always worked great and it surprised both of us that it took him a couple of months to get the slightly heavier rig working satisfactorily. The GM-8 is a small mount with small gears - and it just doesn't have much margin for error.

My .02,

Ron


Hi Ron, I have an Orion 80ED with Moonlite focuser, Starlight Xpress FW, Moonlite motor controller, Atik 314L+ camera, laser pointer and Orion 50mm/SSAG. All attached to an ADM Losmandy dovetail bar.

Edit: :fingerscrossed: that the loose altitude lock bolt was the issue. I'll know this evening

#10 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 23 October 2013 - 08:49 PM

Terry,
One last thought, did you make sure the dec lock screws and the azimuth lock screws are tight? Once I left the azimuth screws loose and got bad tracking from the flex on my EM200.
Rex


Tom - One of the altitude adjustment bolts was slightly loose. That could have been the issue. I will try it out tonight. Thanks!


That appears to have been the problem. Tracking/guiding is tight and a 5 minute sub looks great. Thanks again!

#11 Jeff B

Jeff B

    Vanguard

  • *****
  • Posts: 2008
  • Joined: 30 Dec 2006

Posted 24 October 2013 - 09:02 AM

I think the HD tripod would solve your issues. I've used an HD tripod with my GM and the difference is not subtle. The GM 8 acts like a different, and better, mount.

The weak link of the GM8 system with the LW tripod is the joint flexure between the MAL and the tripod legs.

Jeff

#12 Raginar

Raginar

    Fly Me to the Moon

  • *****
  • Posts: 6138
  • Joined: 19 Oct 2010
  • Loc: Rapid CIty, SD

Posted 24 October 2013 - 09:45 AM

Awesome :) glad Tom figured it out :)

#13 RAKing

RAKing

    Fly Me to the Moon

  • *****
  • Posts: 6303
  • Joined: 28 Dec 2007
  • Loc: West of the D.C. Nebula

Posted 24 October 2013 - 09:52 AM

Interesting. There are two altitude bolts and only one was loose - yet it caused such an effect. I guess the added weight was just enough to mess things up. :thinking:

I am very happy to hear you solved your issue. :)

Cheers,

Ron

#14 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 24 October 2013 - 10:09 AM

Interesting. There are two altitude bolts and only one was loose - yet it caused such an effect. I guess the added weight was just enough to mess things up. :thinking:

I am very happy to hear you solved your issue. :)

Cheers,

Ron


Thanks Ron. I checked them both and only one was loose. Think I'll get the altitude adjustment knobs though.

#15 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 24 October 2013 - 10:11 AM

I think the HD tripod would solve your issues. I've used an HD tripod with my GM and the difference is not subtle. The GM 8 acts like a different, and better, mount.

The weak link of the GM8 system with the LW tripod is the joint flexure between the MAL and the tripod legs.

Jeff


Thanks Jeff. I've been giving that a lot of thought even before this happened.

#16 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 24 October 2013 - 10:12 AM

Thanks Chris. I'm sure happy :D

#17 RAKing

RAKing

    Fly Me to the Moon

  • *****
  • Posts: 6303
  • Joined: 28 Dec 2007
  • Loc: West of the D.C. Nebula

Posted 25 October 2013 - 07:57 AM

Interesting. There are two altitude bolts and only one was loose - yet it caused such an effect. I guess the added weight was just enough to mess things up. :thinking:

I am very happy to hear you solved your issue. :)

Cheers,

Ron


Thanks Ron. I checked them both and only one was loose. Think I'll get the altitude adjustment knobs though.


:waytogo: With my arthritic hands, those are mandatory -- as are the clutch knobs with spokes. :lol:

I second the HD tripod idea. For visual use, the LW tripod works fine; the HD tripod can be a little bulky to use. For imaging, the HD tripod is a much better choice.

Cheers,

Ron

#18 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 25 October 2013 - 08:18 AM

I found the knobs and since I'm 100% imaging I think the tripod is a good idea as well.

#19 nemo129

nemo129

    Gemini

  • *****
  • Posts: 3115
  • Joined: 03 Jan 2010
  • Loc: WMass

Posted 25 October 2013 - 07:40 PM

Terry,
For some reason, I always thought you had the altitude adjustment knobs for your GM-8. I love them on my G11, and they are not expensive at all...like $21!

An HD tripod would be nice as well, keep an eye on that other site, they come up used every now and then.

#20 ErikB

ErikB

    Vostok 1

  • -----
  • Posts: 198
  • Joined: 05 Feb 2006
  • Loc: Central Arizona

Posted 29 October 2013 - 02:21 AM

Terry, I recall that I once forgot to tighten the tripod leg hinges, and that sure caused trouble. Also, it helps to not extend the legs very far.

#21 terry59

terry59

    Fly Me to the Moon

  • *****
  • Posts: 5063
  • Joined: 18 Jul 2011
  • Loc: Colorado, USA

Posted 29 October 2013 - 05:54 AM

Thanks Erik. I extend the legs only enough to be level.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics