Yes, mirror flop was the first thing jumping to my mind when reading this. Like Hy, I think PA should not be affected by cone error. Mirror flop would explain it. As well as play in the optical train. Is it all threaded connections?

With an EC mount you should be able to just track (unguided) through meridian and watch for a systematic shift of the image. How far can you track past meridian? iOptrons aren't particularly good at that

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