Ritchey-Chrétien telescope
Encyclopedia
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...
, 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
George Willis Ritchey
George Willis Ritchey was an American optician and telescope maker and astronomer born at Tuppers Plains, Ohio....
and French astronomer Henri Chrétien
Henri Chrétien
Henri Jacques Chrétien was a French astronomer and an inventor.Born in Paris, France, his most famous invention is the anamorphic widescreen process, that resulted in the CinemaScope, and the co-invention of the Ritchey-Chrétien telescope , which was anadvanced type of astronomical telescope, now...
. Ritchey constructed the first successful RCT, which had a diameter aperture of 60 cm (23.6 in) in 1927 (e.g. Ritchey 24-inch reflector). The second RCT was a 102 cm (40.2 in) instrument constructed by Ritchey for the United States Naval Observatory
United States Naval Observatory
The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...
; that telescope is still in operation at the Naval Observatory Flagstaff Station
United States Naval Observatory Flagstaff Station
The United States Naval Observatory Flagstaff Station , is a scientific astronomical observatory operated as a Navy Echelon V command and the national dark-sky observing Facility/observatory subordinate to the United States Naval Observatory . USNO and NOFS are commands within the CNMOC claimancy,...
.
The Ritchey–Chrétien design is free of third-order coma
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...
and spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
, although it does suffer from fifth-order coma, severe large-angle astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
, 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
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
. 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.
Mirror parameters
The radii of curvatureRadius of curvature (optics)
Radius of curvature has specific meaning and sign convention in optical design. A spherical lens or mirror surface has a center of curvature located in either along or decentered from the system local optical axis. The vertex of the lens surface is located on the local optical axis...
of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are
and
where
-
is the effective focal lengthFocal lengthThe focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
of the system, -
is the back focal length (the distance from the secondary to the focus), and -
is the distance between the two mirrors.
If, instead of
and 
, the known quantities are the focal length of the primary mirror, 
, and the distance to the focus behind the primary mirror, 
, then 
and 
.For a Ritchey–Chrétien system, the conic constant
Conic constant
In geometry, the conic constant is a quantity describing conic sections, and is represented by the letter K...
s
and 
of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is
and
where
is the secondary magnification. Note that 
and 
are less than 
(since 
), 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
Null corrector
A null corrector is an optical device used in the testing of large aspheric mirrors. A spherical mirror of any size can be tested relatively easily using standard optical components such as laser, mirrors, beamsplitters, and converging lenses. One method of doing this using a Shack cube is shown...
that makes the hyperbolic primary look spherical for the interferometric test. On the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, this device was positioned incorrectly (due to an unnoticed paint flake being lodged in the test mounting) leading to the error in the Hubble primary mirror.
Commercial instruments
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 Alluna Optics, Astrosib, Deep Sky Instruments, Guan Sheng Optical, Officina Stellare, Optical Guidance Systems, RC Optical Systems
RC Optical Systems
RC Optical Systems is a high-end American telescope and optics manufacturer that specializes in Ritchey-Chrétien telescopes with hyperbolic mirrors. RC also makes related mounts and systems for the telescopes, with a focus on open and closed carbon fiber trusses for low expansion...
and Takahashi
Takahashi Seisakusho
Takahashi Seisakusho is a Japanese manufacturer of telescopes and related equipment, such as eyepieces and mounts.Often known simply as 'Tak', the brand is especially noted amongst amateur astronomers for its range of apochromatic refractors, but also produces various types of reflectors, and...
. In about 2009, Astro-Tech corporation introduced a line of significantly less expensive Ritchey–Chrétien telescopes Astro-Tech.
Examples of large Ritchey–Chrétien telescopes
- The 10.4 m Gran Telescopio CanariasGran Telescopio CanariasThe Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
at Roque de los Muchachos ObservatoryRoque de los Muchachos ObservatoryRoque de los Muchachos Observatory is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands... - The two 10 m telescopes of the Keck Observatory
- The four 8.2 m telescopes comprising the Very Large TelescopeVery Large TelescopeThe Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
in ChileChileChile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far... - The 8.2 m Subaru telescopeSubaru (telescope)Subaru Telescope is the 8.2 metre flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades...
at Mauna Kea ObservatoryMauna Kea ObservatoryThe Observatories at Mauna Kea, , are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawai'i, USA. The facilities are located in a special land use zone known as the "Astronomy Precinct," which is located in the Mauna Kea... - The two 8 m telescopes comprising the Gemini ObservatoryGemini ObservatoryThe Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...
- The 4.1 m Visible and Infrared Survey Telescope for AstronomyVISTA (telescope)The VISTA is a reflecting telescope with a 4.1 metre mirror, located at the Paranal Observatory in Chile. It is operated by the European Southern Observatory and saw first light in December 2009...
at the Paranal ObservatoryParanal ObservatoryParanal Observatory is an astronomical observatory located on Cerro Paranal at 2,635 m altitude and operated by the European Southern Observatory. The Very Large Telescope is the largest telescope on Paranal, actually composed of four separate 8.2 m telescopes...
(Chile) - The 3.9 m Anglo-Australian TelescopeAnglo-Australian TelescopeThe Anglo-Australian Telescope is a 3.9 m equatorially mounted telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory, Australia at an altitude of a little over 1100 m...
at Siding Spring ObservatorySiding Spring ObservatorySiding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics at the Australian National University , incorporates the Anglo-Australian Telescope along with a collection of other telescopes owned by the Australian National...
(AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
) - The 3.5 m Calar Alto ObservatoryCalar Alto ObservatoryThe Calar Alto Observatory is an astronomical observatory owned and operated jointly by the Max-Planck-Institut für Astronomie in Heidelberg, Germany, and the Instituto...
telescope at mount Calar Alto (Spain) - The 3.5 m Herschel Space ObservatoryHerschel Space ObservatoryThe Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....
currently operating in orbit at the L2 point 1.5 million km from Earth - The 3.5 m WIYN ObservatoryWIYN ObservatoryThe WIYN Observatory is owned and operated by the WIYN Consortium. Its telescope, a 3.5-meter instrument, is the newest and second largest telescope on Kitt Peak in Arizona...
at Kitt Peak National ObservatoryKitt Peak National ObservatoryThe Kitt Peak National Observatory is a United States astronomical observatory located on 2,096 m Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O'odham Nation, southwest of Tucson... - The 2.5 m Sloan Digital Sky SurveySloan Digital Sky SurveyThe Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
telescope (modified design) at Apache Point ObservatoryApache Point ObservatoryThe Apache Point Observatory is located in the Sacramento Mountains in Sunspot, New Mexico 18 miles south of Cloudcroft. The observatory consists of the Astrophysical Research Consortium's 3.5-meter telescope, the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope,...
, New MexicoNew MexicoNew Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
, U.S.A. - The 2.4 m Hubble Space TelescopeHubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
currently in orbit around the Earth - The 2.2 m Calar Alto ObservatoryCalar Alto ObservatoryThe Calar Alto Observatory is an astronomical observatory owned and operated jointly by the Max-Planck-Institut für Astronomie in Heidelberg, Germany, and the Instituto...
telescope at mount Calar AltoCalar AltoCalar Alto is the highest peak in the Sierra de Los Filabres, a mountain chain in Andalusia, Spain. It is also the highest peak in the province of Almería.It hosts an important astronomical observatory.-External links:**...
(Spain) - The 2.1 m San Pedro Martir Observatory telescope at Baja California (Mexico)
- The 2 m telescope at Rozhen ObservatoryRozhen ObservatoryRozhen Observatory is a Bulgarian astronomical observatory, located 90 km south of the city of Plovdiv, Bulgaria. The nearest town, Chepelare, is 15 km away. It is owned and operated by the Institute of Astronomy of the Bulgarian Academy of Sciences. The Observatory is one of the largest in Europe...
- The 2 m Himalayan Chandra Telescope of the Indian Astronomical ObservatoryIndian Astronomical ObservatoryThe Indian Astronomical Observatory , located near Leh in Ladakh, India, has one of the world's highest sites for optical, infrared and gamma-ray telescopes. It is operated by the Indian Institute of Astrophysics, Bengaluru. It is currently the second highest optical telescope in the...
, Hanle, India - The 1.8 m Pan-STARRSPan-STARRSThe Panoramic Survey Telescope and Rapid Response System is a planned array of astronomical cameras and telescopes and computing facility that will survey the sky on a continual basis, including accurate astrometry and photometry of detected objects...
telescopes at HaleakalaHaleakalaHaleakalā , or the East Maui Volcano, is a massive shield volcano that forms more than 75% of the Hawaiian Island of Maui. The western 25% of the island is formed by the West Maui Mountains.- History :...
on MauiMauiThe island of Maui is the second-largest of the Hawaiian Islands at and is the 17th largest island in the United States. Maui is part of the state of Hawaii and is the largest of Maui County's four islands, bigger than Lānai, Kahoolawe, and Molokai. In 2010, Maui had a population of 144,444,...
, HawaiiHawaiiHawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of... - The 1.3 m telescope at Skinakas Observatory, Crete, Greece
- The 1.0 m Ritchey Telescope at the United States Naval Observatory Flagstaff StationUnited States Naval Observatory Flagstaff StationThe United States Naval Observatory Flagstaff Station , is a scientific astronomical observatory operated as a Navy Echelon V command and the national dark-sky observing Facility/observatory subordinate to the United States Naval Observatory . USNO and NOFS are commands within the CNMOC claimancy,...
(the final telecope made by G. Ritchey before his death). - The 85 cm Spitzer Space TelescopeSpitzer Space TelescopeThe Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, infrared space telescope currently operating in Earth-trailing orbit - The 22 inch (56 cm) SDAA telescope at Tierra del Sol Observatory
- The 2.56 m effective f/11 Nordic Optical TelescopeNordic Optical TelescopeThe Nordic Optical Telescope is an astronomical telescope located at Roque de los Muchachos Observatory, La Palma in the Canary Islands. First light came in 1988, with regular observing beginning in 1989. It is funded by Denmark, Sweden, Iceland, Norway and Finland...
on La Palma, Canary Islands.
Ritchey intended the 200-inch (5 m) Hale Telescope
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
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. 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
- List of largest optical reflecting telescopes
- List of telescope types
- Reflecting telescopeReflecting telescopeA reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
- Schmidt–Cassegrain telescope
- Maksutov telescopeMaksutov telescopeThe Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the...