Backlash issue or something else?

Tom Carrico


I have an AP900 that I have had for 15 years. It has worked flawlessly. A couple weeks ago I noticed what I thought was a minor DEC backlash issue, so printed out the instructions and went to work. I could barely detect any movement in DEC with the mount in Park Position 3, but figured I would go through the process.  I verified that the set screw was properly tightened and followed the instructions to 'rock' the dec motor into place. I could not feel any backlash and figured things were going to be back to normal. I apparently made it worse...

This image shows what I now see in PhD 2. At some point it starts making a dec correction and things keep getting worse, almost like the corrections were moving in the wrong direction. I re-did calibration in PhD multiple times to make sure I had not messed anything up. In the PhD graph, each large vertical division is 25 points at 6 seconds per point, or about 150 seconds. The telescope is a Celestron Edge HD 8". The guide camera is off axis and is a ZWO ASI 290.

I changed the min move in dec to 5 to effectively disable dec corrections right after this happened. You can see that the polar alignment is fine, as there were no longer dec corrections. Up until I tried to make the mount better, I was getting rms guide errors consistently < 0.5", typically around 0.4". I have never seen anything like this.

I ran the PhD guiding assistant and it verified that my polar alignment was not too bad, but that there was some serious backlash

This is verified by the backlash graph

I ran this test a year ago, and the backlash graph was a perfect triangle. Clearly I am not doing the adjustment optimally. Do I just need to fiddle with it to get it right? When I try to move the dec axis I can barely feel any movement, am I going for no movement at all?

When I 'rock' the motor into place, I place firm pressure on it while I adjust the bolts per the instructions. I could certainly use more force (or a rubber mallet), but not really sure if that is the way to go.

Any advice would be greatly appreciated.


- Tom C

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