At some point it starts making a dec correction and things keep getting worse, almost like the corrections were moving in the wrong direction.
The wrong direction is caused by very high friction somewhere in the geartrain. It's called retrograde motion and is caused by static friction which builds up and finally breaks loose. I suspect that the center spur gear that you tightened is way too tight and you will need to back it off slightly. Spur gears must have a slight clearance between the teeth, otherwise they will cause this reverse motion.
From: Tom Carrico <tom@...>
Sent: Sat, Aug 28, 2021 2:28 am
Subject: [ap-gto] Backlash issue or something else?
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