I cogitated on your reply during my daily jog and came up with ROT_PA is the PA the rotator thinks the (solved) image should have. Open loop it's fingers crossed that they agree (ROT_PA = PA) and closed loop the difference is an error that's fed back to the rotator controller. That's all well and good, but I'm interested in the rotator's internal rotation angle (which should be changing over the course of an imaging run) and not whether it's managing to keep a fixed PA across an imaging run. So...

It also dawned on me to look at the ASCOM iRotatorV3 properties to see what's there (which I should have done before posting). So there's Position and MechanicalPosition. Alas the explanation for those isn't that clear to me, but my guess is that ROT_PA is reporting Position. I'm not even sure MechanicalPosition is what I want since it might be simply be Position + [fixed offset].

Why do I want the rotator's internal angle? Because my understanding is that the flat field rotates with that and if it does, I want to know how the little dust donuts e.g. rotate over the course of an image run.

Thanks,