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Jeremy,
The shape of the guide star image...
This should be no worse than the star image at the extreme edge - bearing in mind that the pick off prism is "sampling" the FOV outside your existing camera limits - these will vary with sensor size.
The main issue, I've found, is that the pick-off prism doesn't always allow rotation to bring the guide star image onto the axis of the guide camera.
(Think about it: the prism is just the "end" of a diagonal mirror, the centre of this "diagonal" would sit on the optical axis of the telescope BUT the guide tube is not, it's sitting rearward to suit the body design of the OAG. I have a diagram somewhere which shows this OAG mis-alignment, I'll find it and upload.)
Last edited by Merlin66; 16-09-2017 at 04:45 PM.
Reason: diagram added
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