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This can happen if the RA direction is reversed and there is some backlash in the RA axis. It happens when you slew the mount toward the east. What happens then is that it takes a few seconds for the teeth to re-engage in the west direction. Until the worm teeth re-engage the RA axis will not be tracking at the sidereal rate and the stars will appear to drift westward. You may want to set a small backlash compensation setting for the RA axis. This will re-engage the teeth quickly after an eastward slew.
All mounts have backlash except for encoder mounts. Normally RA backlash is not noticed during tracking and guiding because those actions never reverse the axis. Therefore in RA the gear teeth always maintain contact in one direction and during guiding the axis simply slows down or speeds up to move the star back and forth. Not so during slewing where the gear teeth must reverse direction in order to slew the scope eastward. When the mount stops slewing, the teeth must then traverse the backlash distance at the very slow sidereal rate before tracking resumes. During this time the stars will be drifting westward.
Normal backlash time of reversal is usually 1 second or less fort he Mach1, but if the teeth are out of mesh it could be as much as 3 - 4 seconds. So the thing to do is to check the mesh once in a while and re-mesh the gears if they are too far out.
There is one other thing that can cause a large amount of reversal delay, other than mesh. And that would be the end-play nut that holds the worm in place. If that is loose, the worm wont turn the worm wheel in a timely manner when reversal of direction takes place.
Once you understand how the mount works, you won't be chasing red herrings down a blind alley. So it's important to do a bit of digging as to how the mechanics of mounts are constructed and operate.
From: Leo Shatz <leonid.shatz@...>
Sent: Mon, Jun 27, 2022 9:30 am
Subject: Re: [ap-gto] Mach1 tracking issue after meridian flip #ASCOM_V2_Driver #APCC
A screen shot of failed plate solve image