A Ritchey–Chrétien telescope ( RCT or simply RC ) is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors ( coma ). The RCT has a wider field of view free of optical errors compared to a more traditional reflecting telescope configuration. Since the mid 20th century, a majority of large professional research telescopes have been Ritchey–Chrétien configurations; some well-known examples are the Hubble Space Telescope , the Keck telescopes and the ESO Very Large Telescope .
21-514: The Ritchey–Chrétien telescope was invented in the early 1910s by American astronomer George Willis Ritchey and French astronomer Henri Chrétien . Ritchey constructed the first successful RCT, which had an aperture diameter of 60 cm (24 in) in 1927 (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
42-634: A Ritchey–Chrétien system, the conic constants K 1 {\displaystyle K_{1}} and K 2 {\displaystyle K_{2}} of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is: and Note that K 1 {\displaystyle K_{1}} and K 2 {\displaystyle K_{2}} are less than − 1 {\displaystyle -1} (since M > 1 {\displaystyle M>1} ), so both mirrors are hyperbolic. (The primary mirror
63-409: A larger usable field of view compared to the parabolic designs actually used. However, Ritchey and Hale had a falling-out. With the 100-inch project already late and over budget, Hale refused to adopt the new design, with its hard-to-test curvatures, and Ritchey left the project. Both projects were then built with traditional optics. Since then, advances in optical measurement and fabrication have allowed
84-560: 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 device was built incorrectly (a reflection from an un-intended surface leading to an incorrect measurement of lens position) leading to the error in the Hubble primary mirror. Incorrect null correctors have led to other mirror fabrication errors as well, such as in
105-498: Is most commonly found on high-performance professional telescopes. A telescope with only one curved mirror, such as a Newtonian telescope , will always have aberrations. If the mirror is spherical, it will suffer primarily from spherical aberration . If the mirror is made parabolic, to correct the spherical aberration, then it still suffers from coma and astigmatism , since there are no additional design parameters one can vary to eliminate them. With two non-spherical mirrors, such as
126-563: Is still in operation at the Naval Observatory Flagstaff Station . As with the other Cassegrain-configuration reflectors, the Ritchey–Chrétien telescope (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 its mirrors require sophisticated techniques to manufacture and test. Hence the Ritchey–Chrétien configuration
147-542: Is the eccentricity of the conic section. The equation for a conic section with apex at the origin and tangent to the y axis is y 2 − 2 R x + ( K + 1 ) x 2 = 0 {\displaystyle y^{2}-2Rx+(K+1)x^{2}=0} alternately x = y 2 R + R 2 − ( K + 1 ) y 2 {\displaystyle x={\dfrac {y^{2}}{R+{\sqrt {R^{2}-(K+1)y^{2}}}}}} where R
168-406: Is the radius of curvature at x = 0 . This formulation is used in geometric optics to specify oblate elliptical ( K > 0 ), spherical ( K = 0 ), prolate elliptical ( 0 > K > −1 ), parabolic ( K = −1 ), and hyperbolic ( K < −1 ) lens and mirror surfaces. When the paraxial approximation is valid, the optical surface can be treated as a spherical surface with
189-460: 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
210-560: The New Technology Telescope . In practice, each of these designs may also include any number of flat fold mirrors , used to bend the optical path into more convenient configurations. This article only discusses the mirrors required for forming an image, not those for placing it in a convenient location. Ritchey intended the 100-inch Mount Wilson Hooker telescope (1917) and the 200-inch (5 m) Hale Telescope to be RCTs. His designs would have provided sharper images over
231-481: The Ritchey–Chrétien (R–C) reflecting telescope along with Henri Chrétien . The R-C prescription remains the predominant optical design for telescopes and has since been used for the majority of major ground-based and space-based telescopes. He worked closely with George Ellery Hale , first at Yerkes Observatory and later at Mt. Wilson Observatory . He played a major role in designing the mountings and making
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#1732787968742252-881: The U.S. Naval Observatory Flagstaff Station in Flagstaff, Arizona. In 1924, he received the Prix Jules Janssen , the highest award of the Société astronomique de France , the French astronomical society. Craters on Mars and the Moon were named in his honor. A very readable biography of Ritchey and Hale is in Don Osterbrock's book "Pauper and Prince - Ritchey, Hale and the Big American Telescopes" (The university of Arizona Press, 1993) where
273-515: The RCT design to take over – the Hale telescope, dedicated in 1948, turned out to be the last world-leading telescope to have a parabolic primary mirror. George Willis Ritchey George Willis Ritchey (December 31, 1864 – November 4, 1945) was an American optician and telescope maker and astronomer born at Tuppers Plains, Ohio . Ritchey was educated as a furniture maker. He coinvented
294-502: The Ritchey–Chrétien telescope, coma can be eliminated as well, by making the two mirrors' contribution to total coma cancel. This allows a larger useful field of view. However, such designs still suffer from astigmatism. The basic Ritchey–Chrétien two-surface design is free of third-order coma and spherical aberration . However, the two-surface design does suffer from fifth-order coma, severe large-angle astigmatism , and comparatively severe field curvature . When focused midway between
315-514: The Schmidt requires a full-aperture corrector plate, which restricts it to apertures below 1.2 meters, while a Ritchey–Chrétien can be much larger. Other telescope designs with front-correcting elements are not limited by the practical problems of making a multiply-curved Schmidt corrector plate, such as the Lurie–Houghton design . In a Ritchey–Chrétien design, as in most Cassegrain systems,
336-508: The idiosyncratic personalities of both Ritchey and Hale are exposed. This United States astronomer article is a stub . You can help Misplaced Pages by expanding it . Conic constant In geometry , the conic constant (or Schwarzschild constant , after Karl Schwarzschild ) is a quantity describing conic sections , and is represented by the letter K . The constant is given by K = − e 2 , {\displaystyle K=-e^{2},} where e
357-469: The mirrors of the Mt. Wilson 60-inch (1.5 m) and 100-inch (2.5 m) telescopes. Hale and Ritchey had a falling-out in 1919, and Ritchey eventually went to Paris where he promoted the construction of very large telescopes. He returned to America in 1930 and obtained a contract to build a Ritchey-Chrétien telescope for the U.S. Naval Observatory. This last telescope produced by Ritchey remains in operation at
378-711: The primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are: and where If, instead of B {\displaystyle B} and D {\displaystyle D} , the known quantities are the focal length of the primary mirror, f 1 {\displaystyle f_{1}} , and the distance to the focus behind the primary mirror, b {\displaystyle b} , then D = f 1 ( F − b ) / ( F + f 1 ) {\displaystyle D=f_{1}(F-b)/(F+f_{1})} and B = D + b {\displaystyle B=D+b} . For
399-506: The result is a three-mirror anastigmat . Alternatively, a RCT may use one or several low-power lenses in front of the focal plane as a field-corrector to correct astigmatism and flatten the focal surface, as for example the SDSS telescope and the VISTA telescope ; this can allow a field-of-view up to around 3° diameter. The Schmidt camera can deliver even wider fields up to about 7°. However,
420-405: The sagittal and tangential focusing planes, stars appear as circles, making the Ritchey–Chrétien well suited for wide field and photographic observations. The remaining aberrations of the two-element basic design may be improved with the addition of smaller optical elements near the focal plane. Astigmatism can be cancelled by including a third curved optical element. When this element is a mirror,
441-486: The secondary mirror blocks a central portion of the aperture. This ring-shaped entrance aperture significantly reduces a portion of the modulation transfer function (MTF) over a range of low spatial frequencies, compared to a full-aperture design such as a refractor. This MTF notch has the effect of lowering image contrast when imaging broad features. In addition, the support for the secondary (the spider) may introduce diffraction spikes in images. The radii of curvature of
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