Quote:
Originally Posted by Atmos
Just to reiterate in this, technically the RC design gives a large flat field... flat in the sense that it doesn't suffer from coma (field curvature) BUT only suffers from astigmatism. For science this is fine because without field curvature stars remain in their correct astrometric positions and it doesn't matter if their size (FWHM) changes. A corrector is needed to correct for astigmatism to give what us astrophotographers would consider a flat field in that stars remain uniform in size across the inaging field.
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Well... flat field means EXACTLY that, flat. That is, Petzval curvature is zero. RC's like any Cassegrain follow a very simple rule - remaining field curvature is directly proportional to a difference in radii between primary and secondary mirror. Now, nearly all commercial RC's have different radii of curvature, hence focal plane is not flat, but curved (concave to the sky). Without field flattener, both astigmatism AND field curvature remain.
BTW, coma has nothing to do with field curvature. Typical exmaple is a standard Gauss camera lens, which covers enormous field in comparison to telescopes, and is FLAT yet has heaps of coma in corners.
For further reading I suggest look up Seidel aberrations.