Strange RA tracking issue with Mach2


I've been using the Mach2 in the backyard for a few months now.
I also took it to a dark sky site in (irrc) May 2021, and also had no problems there.

On Sept 5 I took it out to a different dark sky site and had very strange and intermittent RA tracking issues: Trailed stars in what looks like an oscillation pattern.
I've attached a few screenshots to hopefully shed some light on what I'm seeing.

I'm using a 12" SCT-ACF with ~0.5 x reducer for about 1500mm focal length.
Afaict the Mach2 is just about "fully loaded".   
I've been using this setup in the backyard with APCC and APPM without any problems.

This is also after recently updating to APCC Pro, but I'm pretty sure I used 1.9.05 in the backyard also, and without any problems.
I'm using SGPro for imaging with APPM and using ASTAP directly from APPM.

One difference at this new site, is the tripod was not perfectly level in the north-south direction, but it was close.
The other difference is the new site has much better horizons than my backyard -- in the backyard there's about 30% of the sky to the zenith blocked in the south and west.

After setting up, and balancing, I used pole-align-max + sharpcap to get polar alignment.

I then went to focus the camera at zenith and start APPM.
When I took a short 10 second image near zenith, I got star trails in the RA direction.

I then pointed near Polaris and did the same exposure... no star trails.  So I focused the camera there.

Here's some things I tried to identify/fix RA star-trail the problem:
- Re-balanced the OTA, 
- Looked for any conflicting software, etc.   I did exit sharpcap, but it wasn't connected to the AP driver.
- Looked for anything loose, e.g. altitude axis clamp, or cable tugging.
- There was zero wind at this site.
- The ground was, I believe, stable... It was mostly dirt and some gravel, but it did not feel spongy under foot.
- At one point I did a complete reboot of the computer and CP5 controller.

One thing I wasn't clear about was some of the APCC/APPM settings:
- For example, the APPM model in use -- e.g. whether the previous model (from the backyard) could be conflicting.   However, I am pretty sure I had the previous APPM model "disabled".
- The horizon and horizon-limits -- I'm pretty sure I had horizon-limits turned off, but it may have been on initially.   I did get a "slew warning" in APPM and I'm not sure exactly why.
- I was thinking the APPM "horizon limit" (ie. the circle defining the edge of the points) was conflicting with the horizon I had defined, but I reduced the APPM circle just a bit and this didn't seem to help overall.
- I also tried stopping tracking to see if something like the horizon limit was turning off tracking and causing the trail -- but with tracking off the same image duration generated a much longer star trail.

Also, I had defined a new site, and new horizon, and was using the new site and horizon.    
The location of the new site was defined by the GPS box.

I wasn't able to find anything and couldn't fix the odd star-trailing, so I decided to attempt APPM anyhow.
I used a "small" model with the new horizon, which was about 50 points.

The model mostly worked, but some points had the "trailed" stars, and did not plate-solve.
Oddly it seemed some of these points were in the "outside" areas of the "old" horizon; I was thinking the "old" horizon model was getting used, but this theory simply was not consistent.

Anyhow, I did create the model, and went on to image the bubble near zenith for most of the night (no guiding) and it seemed fine.

I've not had a chance to setup the mount since 9/5, but will hopefully have some time to "debug" this mystery later this week (assuming it is reproducible) at home in the backyard.
(I also do not have access to the PC I use with the Mach2 for a few days, but can perhaps pull some archived data from it later this week.)

Has anyone seen something like this and know the root cause?




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