Ahh... alt and az... didn't think of that.
It was meant to be a theoretical question rather than a practical question.
Having a CGEM mount there is a polar alignment mode where you can align off any star - so I was wondering how close the initial alignment would need to be for a set exposure length. For example, if the initial align was say wtihin 5 arc minutes from a polar alignment then you would be able to expose for X minutes with Y focal length.
Thinking it through now it would depend on sky position, as you say, since the target would affect any trailing.
Oh well - it was a theoretical question, so back to theories!
