Last night my GM811G gave really poor autoguiding with a setup that had previously worked very well all the time.? C8 EdgeHD, iGuider guidescope bolted to the side of the mount saddle, PHD2, NINA.? Normally this setup gives me approximately 0.7" RMS guiding at my site.

Last night, though, there were frequent large irregularities and lurches (2" to 12"), mostly in RA, somewhat in declination too (enough to deflect suspicion away from the RA drive but not totally exonerate it).? They were at very irregular intervals, 30 to 90 seconds apart, and a few were bad enough to produce multiple images of stars.

My first thought was that a cable was dragging, but rearranging the cables a couple of times didn't fix it.? Nor did the guidescope or guide camera appear to be loose in any way, though I'm going to take them off and put them back on carefully the next time I have the mount out.

I had just upgraded the Gemini firmware, and I made sure that its PEC was turned off.

I made sure the RA clutch was tight enough but not too tight.

I had PHD2 recalibrate, just in case it had been mistaken about the guidescope, and tried a couple of different guiding algorithms.

The air didn't *look* unsteady, but could this be an atmospheric problem after all?? An effect of the slight east breeze (4 mph)?

Or does my RA drive require cleaning and lubrication or something?? The mount is 3 years old and has been used lightly.

Thanks!

--
Michael A. Covington, Ph.D.
Consultant, Covington Innovations, Athens, Georgia, USA

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Right, those are basically all the things to check!? It will be a week before we have another clear night for testing.? I was wondering if any specific cause jumped out at anyone, and in particular if it was associated with the firmware upgrade (although I can't see how a firmware problem would cause random lurches).

Could it be too little DC voltage (12.6 V)?? My main power supply has to stay under 13.0 because that's the limit for my Altair camera.? I do have a 19-volt power supply I can use with the GM811G.

It may still have been cable management -- something dragging that I didn't catch.? In that case it won't recur next time.

Hi Michael,

?

Difficult to diagnose from a distance. A 12" jump sounds like something beyond seeing conditions, unless you had clouds passing through. Here are the things I'd check, in this order:

?

1. Check that everything is tightened down, nothing is loose in the imaging/autoguider system

2. Check motor cables, make sure they are clean and making solid contact

3. Reset Gemini to HGM default settings, to clear out any odd settings, then reenter all the correct settings

4. Check for backlash in RA worm/gear, adjust if necessary to reduce (adjustment depends on whether you have spring-loaded worm or not). Large temperature swings can also result in some issues with both, binding, and increased backlash.

5. Try moving to a different part of the worm-wheel: start in CWD. Loosen the clutches, move the RA axis 90 degrees by hand, tighten the clutches. Then, using Gemini HC arrow buttons, rotate the axis back to CWD, or -90 degrees. Restart Gemini using cold start from CWD position. This should move the mount to use a different portion of the worm wheel, just in case there were some problems or dirt on the part where you had trouble.

6. Check that your gearboxes are attached properly, possibly swap gearboxes to see if the RA problem goes away

7. Check the oldham coupler and tighten the screws on both sides if anything is loose

8. Check/clean/relube the worm -- there may be dirt or particles that are in the way that are causing sudden jumps.?

9. Try reverting to a previous version of firmware to see if that would make a difference

?

Regards,

?

? -Paul

?

?

On Tue, Apr 29, 2025 at 10:26 PM, Michael A. Covington wrote:

Last night my GM811G gave really poor autoguiding with a setup that had
previously worked very well all the time.? C8 EdgeHD, iGuider guidescope
bolted to the side of the mount saddle, PHD2, NINA.? Normally this setup
gives me approximately 0.7" RMS guiding at my site.

Last night, though, there were frequent large irregularities and lurches
(2" to 12"), mostly in RA, somewhat in declination too (enough to
deflect suspicion away from the RA drive but not totally exonerate it).?
They were at very irregular intervals, 30 to 90 seconds apart, and a few
were bad enough to produce multiple images of stars.

My first thought was that a cable was dragging, but rearranging the
cables a couple of times didn't fix it.? Nor did the guidescope or guide
camera appear to be loose in any way, though I'm going to take them off
and put them back on carefully the next time I have the mount out.

I had just upgraded the Gemini firmware, and I made sure that its PEC
was turned off.

I made sure the RA clutch was tight enough but not too tight.

I had PHD2 recalibrate, just in case it had been mistaken about the
guidescope, and tried a couple of different guiding algorithms.

The air didn't *look* unsteady, but could this be an atmospheric problem
after all?? An effect of the slight east breeze (4 mph)?

Or does my RA drive require cleaning and lubrication or something?? The
mount is 3 years old and has been used lightly.

Thanks!

--
Michael A. Covington, Ph.D.
Consultant, Covington Innovations, Athens, Georgia, USA

-- 
Michael A. Covington, Ph.D.
Consultant, Covington Innovations, Athens, Georgia, USA