DrWho,
The description of your problem matches exactly mine.
In my case it was the longitude that was wrong. The hemisphere obviously also needs to be the correct, and lastly the hour.
I suggest you have it gathering the hour from the system to troubleshoot, but ideally you would get coordinates and time from the GPS.
The key aspect is that if the time or longitude are wrong the calculated Zenith is pointing on the wrong direction.

One more critical aspect.
My DEC always starts with a full rotation Eastwards, regardless where the target is. I don’t know why, but it does once at the begging of a session IF I didn’t park at the end of the previous session. If I park it, it never does that full rotation. If I don’t park it, it always starts with a full rotation (once I start slewing)

Give it a try. Apart from these little tricks, I super pleased with the portability of the RPi.

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