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WHY IS STAR INSTRUMENTS RITCHEY-CHRETIEN DESIGN SUPERIOR TO PRODUCTION-TYPE SCHMIDT-CASSEGRAIN SYSTEMS?

April 26, 2017design instruments schmidt-cassegrain star superior SYSTEMS

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WHY IS STAR INSTRUMENTS RITCHEY-CHRETIEN DESIGN SUPERIOR TO PRODUCTION-TYPE SCHMIDT-CASSEGRAIN SYSTEMS?

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Below are spot diagrams for a 12.5″ f9 Ritchey-Chretien versus an 11″ f10 Schmidt-Cassegrain with an f2 primary and 5x secondary.This spot diagram would be comparable to all production Schmidt-Cassegrains. (1/2 degree off axis) As you can see, the Ritchey-Chretien is several times better than the Schmidt-Cassegrain. There are several reasons for this: • The lower the amplification factor of the secondary mirror, the flatter the field. • The Ritchey-Chretien system has a 2.7x secondary, whereas the Schmidt-Cassegrain has a 5x secondary. • Ritchey-Chretien is coma-free, whereas Schmidt-Cassegrain is not. Production-type Schmidt Cassegrains use a spherical primary and secondary which do not correct for coma. Ritchey-Chretien has a hyperbolic primary and a hyperbolic secondary which correct coma. • Ritchey-Chretien only has two surfaces; Schmidt-Cassegrain has four. Therefore, Ritchey-Chretien has less light loss.