Ritchey-Chrétien telescope

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, 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-Chretien 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. [cite web |url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910003124_1991003124.pdf |title=The Hubble Space Telescope Optical Systems Failure Report |year=1990 |author=Lew Allen (Chairman) |publisher=NASA Technical Report NASA-TM-103443 |format=PDF]

A partial list of large Ritchey-Chrétien telescopes

* The two 10 m telescopes of the Keck Observatory
* The four 8.2 m telescopes comprising the Very Large Telescope in Chile
* The two 8 m telescopes comprising the Gemini Observatory
* The 10.4 m Gran Telescopio Canarias at Roque de los Muchachos Observatory
* The 8.2 m Subaru telescope at Mauna Kea Observatory
* The 3.5 m Calar Alto Observatory telescope at mount Calar Alto (Spain)
* The 3.5 m Herschel Space Observatory orbiting telescope
* 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 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.

Amateur 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, RC Optical Systems and Takahashi.

Meade Instruments, of Irvine, California, introduced the RCX400 series of instruments in 2005, with mirror diameters of 10", 12" , 14" and 16", which they originally claimed are Modified "Advanced Ritchey-Chretien" Cassegrains. But in fact they are a modification of Schmidt-Cassegrain that delivers some advantages of the Ritchey-Chrétien design, such as a coma-free field, without the associated cost. They were soon sued by makers of true Ritchey-Chretien telescopes, because their "Advanced RC" design is actually a variation of SCT designs, and not directly related to the Ritchey-Chretien design. In January 2008, Meade settled the claim and agreed to quit using the initials RC in the name. (As of March 2008, Meade's online literature refers to their now former RCX models as Advanced Coma-Free (ACF) on their LX400 line.)

ee also

* Maksutov telescope
* Newtonian Telescope
* Schmidt-Cassegrain

References

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* http://www.star-instruments.com/lawsuit.html
* http://www.tradingmarkets.com/.site/news/BREAKING%20NEWS/978414/