RCTs with the same primary mirror diameters have smaller overall tube lengths than classical Cassegrains and SCTs. Typically you don’t need to use a field flattener with an RCT unless using a large format camera. With smaller secondary mirror magnifications, RCTs also have less field curvature than SCT. Image 2Ĭomparing RCTs to Schmidt-Cassegrains telescopes (SCTs), which usually have secondary mirror magnifications of 5x and overall focal ratios around f/10, the RCTs are also much faster. The overall focal ratios are f/7 to f/9, much faster than classical Cassegrains. RCT usually have f/2 to f/3 primary mirrors with secondary magnifications factors from 2.5x to 3.5x. Both have hyperbolic secondary mirrors.Ĭlassical Cassegrains usually have focal ratio (f-number) f/4 primary mirrors with secondary mirrors that yield 3x-5x magnification factors. Unlike classical Cassegrains, RCTs do not have parabolic primary mirrors, but hyperbolic mirrors. Ritchey-Chrétien telescopes are a type of Cassegrain telescopes designed to eliminate coma. The second RCT was a 40-inch built in 1934 for the Naval Observatory and is currently at their Flagstaff Station. Ritchey actual built the first RCT in 1927. Image 1Īmerican astronomer George Willis Ritchey and French astronomer Henri Chrétien invented the Ritchey–Chrétien telescope (RCT) in the early 20 th Century. The classical Cassegrain would be the better choice for visual use, while the Ritchey–Chrétien is the better instrument for astronomical imaging. In this article, I review the GSO 8-inch GSO Ritchey–Chrétien. I recently reviewed a Guan Sheng Optical (GSO) 8-inch classical Cassegrain telescope for Astronomy Technology Today.
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