The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a large field of view 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 aperture of 60 cm (24 in) in 1927 (e.g. Ritchey 24-inch Reflector). The second RCT was a 102 cm (40 in) 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,[1] although it does suffer from fifth-order coma, severe large-angle astigmatism, and comparatively severe field curvature.[2] 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.
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Mirror parameters
The radii of curvature of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are
and
where
- F is the effective focal length of the system,
- B is the back focal length (the distance from the secondary to the focus), and
- D is the distance between the two mirrors.
For a Ritchey-Chrétien system, the conic constants K1 and K2 of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is
and
where M = (F − B) / D is the secondary magnification.[3] Note that K1 and K2 are less than − 1 (since M > 1), so both mirrors are hyperbolic. (The primary mirror is typically quite close to being parabolic, however.)
The hyperbolic curvatures are difficult to test, especially with equipment typically 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. A Ritchey-Chrétien then requires minimal additional equipment, typically a small optical device called a null corrector that makes the hyperbolic primary look spherical for the interferometric test. On the Hubble Space Telescope, this gadget was assembled incorrectly, leading to the error in the Hubble primary mirror.[4]
Commercial Instruments
Until very recently, constructing a Ritchey-Chretien Telescope was beyond the requirements of most amateur astronomers and beyond their means. Commercial instrument manufacturers also had little demand. Schmidt-Cassegrain and Maksutov Cassegrain instruments satisfied market needs for good quality optics at moderate prices.
However with better manufacturing technology available, this telescope design is now within budget of many high-end amateurs. And with higher resolution sensors being marketed, the need for better optical performance to fully exploit the capabilities of the imaging chips has grown. Examples of manufacturers catering for the advanced amateur market include Astrosib, Guan Sheng Optical, RC Optical Systems and Takahashi.
Examples of large Ritchey-Chrétien telescopes
- The 10.4 m Gran Telescopio Canarias at Roque de los Muchachos Observatory
- The two 10 m telescopes of the Keck Observatory
- The four 8.2 m telescopes comprising the Very Large Telescope in Chile
- The 8.2 m Subaru telescope at Mauna Kea Observatory
- The two 8 m telescopes comprising the Gemini Observatory
- The 4.1 m Visible and Infrared Survey Telescope for Astronomy at the Paranal Observatory (Chile)
- The 3.5 m Calar Alto Observatory telescope at mount Calar Alto (Spain)
- The 3.5 m Herschel Space Observatory currently operating in orbit at the L2 point 1.5 million km from Earth
- The 3.5 m WIYN Observatory at Kitt Peak National Observatory
- The 2.5 m Sloan Digital Sky Survey telescope (modified design) at Apache Point Observatory, New Mexico, U.S.A.
- The 2.4 m Hubble Space Telescope currently in orbit around the Earth
- The 2.2 m Calar Alto Observatory telescope at mount Calar Alto (Spain)
- The 2 m telescope at Rozhen Observatory
- The 85 cm Spitzer Space Telescope, infrared space telescope currently operating in Earth-trailing orbit
- The 22 inch (56 cm) SDAA telescope at Tierra del Sol Observatory
Ritchey intended the 200-inch (5 m) Hale Telescope to be an RCT. His design would have provided 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 was later vindicated, as the Hale telescope turned out to be the last world-leading telescope to have a parabolic primary mirror.
See also
References
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This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (June 2009) |
- ^ Sacek, Vladimir (July 14, 2006), 8.2.2 Classical and aplanatic two-mirror systems, http://www.telescope-optics.net/classical_and_aplanatic.htm, retrieved 2009-06-22
- ^ Rutten, Harrie; van Venrooij, Martin (2002). Telescope Optics. Willmann-Bell, Inc.. pp. 67. ISBN 0943396182.
- ^ Smith, Warren J. (2008). Modern Optical Engineering, 4th ed. McGraw-Hill Professional. pp. 508–510. ISBN 978-0071476874.
- ^ Lew Allen (Chairman) (1990). "The Hubble Space Telescope Optical Systems Failure Report" (PDF). NASA Technical Report NASA-TM-103443. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910003124_1991003124.pdf.
External links
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