Quote:
Originally Posted by marki
RC's do not have a flat field it is curved. They have no coma and produce tight stars.
Mark
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Without a field flattener RC's produce astigmatism off -axis , which given that star images ,as you go off axis, are on one side of focus this means increasingly elliptical star images as you go to the corner of the frame. The 8" GSO's seem ok for an APS size chip but clearly need a field flattener for larger chips. See Peter Wards full ( 35mm ) frame shot of Omega Centauri and APS size crop and that pretty much explains it all.
I would imagine before too long there will be a number of after market flatteners available for them.