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A Ritchey–Chrétien telescope (RCT or simply RC) is a specialized Cassegrain telescope invented in the early 20th century that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate optical errors (coma). They have large field of view free of optical errors compared to a more conventional reflecting telescope configuration. Since the mid 20th century most large professional research telescopes have been Ritchey–Chrétien configurations. ==History== The Ritchey–Chrétien telescope was invented in the early 1910s by American astronomer George Willis Ritchey and French astronomer Henri Chrétien. Ritchey constructed the first successful RCT, which had a diameter aperture of in 1927 (e.g. Ritchey 24-inch reflector). The second RCT was a instrument constructed by Ritchey for the United States Naval Observatory; that telescope is still in operation at the Naval Observatory Flagstaff Station. The Ritchey–Chrétien design is free of third-order coma and spherical aberration,〔 〕 although it does suffer from fifth-order coma, severe large-angle astigmatism, and comparatively severe field curvature.〔 〕 When focused midway between the sagittal and tangential focusing planes, stars are imaged as circles, making the RCT well suited for wide field and photographic observations. As with the other Cassegrain-configuration reflectors, the RCT has a very short optical tube assembly and compact design for a given focal length. The RCT offers good off-axis optical performance, but examples are relatively rare due to the high cost of hyperbolic primary mirror fabrication; Ritchey–Chrétien configurations are most commonly found on high-performance professional telescopes. A telescope with only one curved mirror, such as a Newtonian telescope, will always have aberrations. If the mirror is spherical, it will suffer from spherical aberration. If the mirror is made parabolic, to correct the spherical aberration, then it must necessarily suffer from coma and astigmatism. With two curved mirrors, such as the Ritchey–Chrétien telescope, coma can be eliminated as well. This allows a larger useful field of view. However, such designs still suffer from astigmatism. This too can be cancelled by including a third curved optical element. When this element is a mirror, the result is a three-mirror anastigmat. In practice, each of these designs may also include any number of flat ''fold mirrors'', used to bend the optical path into more convenient configurations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ritchey–Chrétien telescope」の詳細全文を読む スポンサード リンク
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