Hi Jerry,
I think I understand what you are getting at, and I agree. However for the best accuracy the subsystem needs to transform all coordinates into a common reference frame which is fixed in relation to the centre of mass of the earth. This is because in general the main sources of alignment errors (gravity, fabrication, refraction, etc) are fixed in relation to that as well. I need to think more about this, but I think it should be possible for the subsystem to just throw back the adjusted coordinate in the same reference frame provided it is fixed in the relation to the centre of mass.
I was about to start incorporating the position of the mount relative to the centre of mass into the calculations. This would probably be the time to do it. Any volunteers -).

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