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The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope with a hyperbolic primary and secondary mirror. It was invented in the early 1910s by American astronomer George Willis Ritchey (1864–1945) and French optician Henri Chrétien (1879–1956). Ritchey constructed the first successful RCT, which had a diameter of 0.5 metres, in 1927. The second RCT was a 1-metre instrument constructed by Ritchey for the United States Naval Observatory.
The Ritchey-Chrétien design is free of first-order coma and spherical aberration, although it does suffer from third-order coma, severe large-angle astigmatism, and comparatively severe field curvature (Rutten, 67). 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.
The curvature of the two mirrors in the Ritchey-Chrétien design are described by the following relationships:
<math>C_1 = \frac{(B - F)}{2DF}<math>
<math>C_2 = \frac{(B + D - F)}{2DB}<math>
where:
- C1 and C2 are the Schwarzschild deformation coefficients for the primary and secondary mirrors, respectively,
- F is the effective focal length of the entire system,
- B is the back focal length, or the distance from the secondary to the focus, and
- D is the distance between the two mirrors.
Appropriate selection of B, D, and F allow for any mechanical RCT configuration (Smith, 479).
A partial list of large Ritchey-Chrétien telescopes
- The two 10m components of the Keck Observatory
- The four 8.2m components of the Very Large Telescope in Chile
- The 4m Mayall telescope at Kitt Peak National Observatory
- The 3.5m WIYN telescope at Kitt Peak National Observatory
- The 2.4m Hubble Space Telescope