Ritchey-Chrétien telescope

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The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a relatively large field of view as compared to a more conventional configuration. An RCT has a hyperbolic primary and a hyperbolic secondary mirror. It was invented in the early 1910s by American astronomer George Willis Ritchey (1864–1945) and French astronomer 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 third-order coma and spherical aberration, although it does suffer from fifth-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:

C_1 = \frac{(B - F)}{2DF}

C_2 = \frac{(B + D - F)}{2DB}

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).

The hyperbolic curvatures are difficult to test, with equipment available to amateur telescope makers or laboratory-scale fabricators. Thus, older telescope layouts predominate in these applications. However, professional optics fabricators and large research groups test their mirrors with interferometers anyway. A Ritchey-Chretien then requires no additional equipment, for a large gain in "throughput."

[edit] A partial list of large Ritchey-Chrétien telescopes

Ritchey intended for the 200-inch Hale Telescope to be an RCT. His design would provide sharper images over a larger usable field of view. However, he and Hale had a falling out. Hale refused to adopt the new design, with its complex curvatures, and Ritchey left the project. (Given the large delays in construction, Hale could be forgiven for some amount of risk aversion.) Ritchey would later be vindicated, as the Hale telescope turned out to be the last world-leading telescope to have a parabolic primary mirror.

[edit] See also

[edit] References

  • Smith, Warren J. (2000). Modern Optical Engineering. McGraw-Hill. ISBN 0-07-136360-2. 
  • Rutten, Harrie; van Venrooij, Martin (2002). Telescope Optics. Willmann-Bell, Inc.. ISBN 0943396182. 
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