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cassegrain telescope

This is a spot diagram of what an e.g. 8 inch F8 Ritchey Chretien is supposed to perform like off axis. It's from page 68 of Telescope Optics by Rutten and Venrooij. 

I see no coma at all but there is the strongest Field Curvature of any Cassegrain design, (without the ACF coating), plus some astigmatism, but the star images are pretty round a long way off axis (that's why professional astronomers use RC's, since they like to measure the Star images and round images are easier to measure).


 

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