Atlas Pro - odd declination behavior

[dhickey]

I have an Atlas Pro mount that is 4 years old, the last two of which it has lived on a pier in the backyard covered by a Telegizmos 365 cover.  Last week I put my AT130EDT back onto it and started seeing some odd declination behavior.  See the attached screenshot from the PHD2 Guiding Assistant.  Has anyone seen (and solved) this problem before?

 

I saw a thread on Cloudy Nights that was inconclusive about a similar issue and most of the responses centered around backlash.  The Dec backlash has been adjusted and is fairly small (~150ms) and as the graph shows, the south motion goes along fine until it suddenly doesn't, which does not seem to indicate backlash as a cause.  I have checked the Dec balance and tried both perfectly balanced and slightly out of balance with very similar results each time.  The attached graph was done with the scope pointed nearly at the zenith although I have tried other orientations as well.  I had one run where it moved fine all the way to completion, but the next looked similar to this.  I haven't been able to correlate it to anything reliably yet.

The AT130EDT with the filter wheel, OAG, and camera comes in at 26lbs.  Although that's 59% of the advertised 44lb capacity, I have not seen this behavior in the past two years of using this scope and mount.  For power, it is being driven by a Powerwerx AC/DC supply at 14.1v, which is what I've used also for about two years.  Slews are fine, everything sounds fine.  I tried a short imaging run to see how it would manifest itself and after a few dithers the Dec will go off into never-neverland and take several minutes to be brought back.  I took the side panel off to take a look at the belts to make sure they weren't loose or dry-rotting - the tension on both RA and Dec felt good and they appeared to be in good condition.

Since the weather has halted any further troubleshooting for the next several days... any ideas?

Thanks,

Dave

Edited October 10, 2021 by dhickey

[michael8554]

Hi Dave

This seems to be the reverse of a graph with Dec Backlash.

Usually the red dots head off away from the origin instead of following the white line.

Then when the backlash has been taken up, the red line starts to run parallel to the white line.

Beats me.

I would post your image, and a PHD2 GuideLog and PHD2 DebugLog on the PHD2 Help Forum.

Make sure the dates in the filenames match.

https://openphdguiding.org/getting-help/

Michael

 

Edited October 10, 2021 by michael8554

[dhickey]

Michael,

I will do that, thanks.

Dave

[newbie alert]

It could be mechanical/ stiction  or balance/ cable snag rather than backlash.  Backlash would show at the peak when it reversed from north pulse to south

Are you pulse or st4 guiding? As you should be calibrating or using the guide assistant on  the equator DEC 0/ meridian

[dhickey]

Agree, I don't think it's backlash as that should present at the peak when the direction is changed from north to south.  Pulse guiding.  Calibrating and using the GA near the equator/meridian (as close as I can due to tree obstructions anyway).

It seems most likely it is stiction.  I opened the mount up over the weekend and the Dec worm-end was way too tight, which has been corrected.  No ground up bugs, metal shavings, or other debris was seen.  The grease had very thick globs on one side of the Dec worm.  There was some dried grease on the RA bearings.  After reassembly and testing I still see the problem show up.  The interesting part is that guiding works very well (~0.5" rms), plate solves work fine, etc.  I tried swapping to a lighter OTA (70mm SV70T) and saw the problem there as well, so it doesn't seem to be related to weight.

Dave

Edited October 21, 2021 by dhickey