![]() 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. However, professional optics fabricators and large research groups test their mirrors with interferometers. 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. (The primary mirror is typically quite close to being parabolic, however.) R 1 = − 2 D F F − B = − 2 F M ), so both mirrors are hyperbolic. The radii of curvature of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are: In addition, the support for the secondary (the spider) may introduce diffraction spikes in images.ĭiagram of a Ritchey–Chrétien reflector telescope This MTF notch has the effect of lowering image contrast when imaging broad features. 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. In a Ritchey–Chrétien design, as in most Cassegrain systems, the secondary mirror blocks a central portion of the aperture. 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. However, 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. The Schmidt camera can deliver even wider fields up to about 7°. 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. When this element is a mirror, the result is a three-mirror anastigmat. Īstigmatism can be cancelled by including a third curved optical element. The remaining aberrations of the two-element basic design may be improved with the addition of smaller optical elements near the focal plane. When focused midway between the sagittal and tangential focusing planes, stars appear as circles, making the Ritchey–Chrétien well suited for wide field and photographic observations. However, the two-surface design does suffer from fifth-order coma, severe large-angle astigmatism, and comparatively severe field curvature. The basic Ritchey–Chrétien two-surface design is free of third-order coma and spherical aberration. However, such designs still suffer from astigmatism. This allows a larger useful field of view. With two non-spherical mirrors, such as the Ritchey–Chrétien telescope, coma can be eliminated as well, by making the two mirrors' contribution to total coma cancel. 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. If the mirror is spherical, it will suffer primarily from spherical aberration. Hence the Ritchey–Chrétien configuration is most commonly found on high-performance professional telescopes.Ī telescope with only one curved mirror, such as a Newtonian telescope, will always have aberrations. The RCT offers good off-axis optical performance, but its mirrors require sophisticated techniques to manufacture and test. 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.Īs 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. Ritchey constructed the first successful RCT, which had an aperture diameter of 60 cm (24 in) in 1927 (e.g. The Ritchey–Chrétien telescope was invented in the early 1910s by American astronomer George Willis Ritchey and French astronomer Henri Chrétien. ![]() The 40-inch (1.0 m) Ritchey at United States Naval Observatory Flagstaff Station.
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