Quote:
Originally Posted by Gama
RC's have curvature. You need a flattner to get rid of curvature.
Theo.
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RCOS, and similar high end RC's keep field curvature to a minimum by adjusting the focus with both the secondary mirror position and the back focus. (The secondary mirror on RCOS scopes moves to change focus.)
Generally the RC design has a field that remains flat(ish) when the relationship between the primary to secondary remains proportional to the secondary to back focus position.
While I have not seen the GSO in the flesh, all focussing seems to be at the back plate. This does not matter much for visual observing, but may be the reason why the image showed curvature.