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traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of
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refractor(ri-frak -ter) Short for refracting telescope.
a telescope fitted with an objective lens. The refractor was first used for astronomical observations by Galileo in 1609. Refractors are used for visual, photographic, and, less often, spectral or photoelectric observations. A visual refractor consists of an objective and an ocular. A photographic refractor, often called an astrograph or astrographic camera, is a large camera; a cassette containing a photographic plate is mounted in the focal plane.
The objectives of refractors contain at least two lenses, of which one (a convex lens) is made of light and optically less dense glass (crown glass) and the other (a concave lens) is made of heavy glass (flint glass); the refractor’s spherical and chromatic aberrations are thus corrected simultaneously. Coma also can be corrected in a doublet objective. Astigmatism and curvature of field cannot be corrected in a simple doublet objective, and the refractor’s field of view therefore does not exceed the angle (in degrees) , where D is the diameter of the objective (in millimeters).
The dependence of residual spherical aberration on wavelength (spherochromatism) accounts for the occurrence of a violet aureole approximately 40” in radius around the images of stars (with the aperture ratio of 1:15 usually used in a refractor). A thin cemented objective is practically free of lateral chromatic aberration, but in an uncemented objective such aberration is appreciable and causes the images of stars at the edge of the field of view to be stretched into a short spectrum, and a purple aureole appears around the images of planets. A doublet objective also has a secondary spectrum, which causes colored aureoles to appear around the images of stars. The linear diameter of such an aureole in the focal plane of an ordinary doublet objective is about 0.0005D, and the angular diameter (in seconds of arc) is h = 50D/f, where f is the focal length (in millimeters) of the objective. Therefore, in order to ensure good image quality, aperture ratios must be limited to 1: 14–1:18. The secondary spectrum can be reduced only by using special types of glass and by increasing the number of lenses in the objective. Cementing lenses in the objectives of small refractors decreases ghost images and light losses. Light losses caused by reflection from lens surfaces can also be reduced by using antireflection coatings on the optics. Large objectives cannot be cemented because of the difference in the coefficients of linear expansion of crown glass and flint glass.
Small refractors used by amateurs are mounted on altazimuth or equatorial mountings. Large refractors are mounted only on equatorial mountings, chiefly German mountings; English mountings are less often used.
The diameter of objectives for refractors is restricted by the difficulties in casting large homogeneous blocks of optical glass and by flexures and light absorption in the glass. The largest refractor in the world (D= 1.02 m) is located at Yerkes Observatory (USA); the largest refractor in the USSR (D = 0.65 m) is located at the Pulkovo Observatory. Refractors are used extensively in small visual instruments designed for various purposes, particularly in astrometric instruments.
REFERENCESMaksutov, D. D. Astronomicheskaia optika. Moscow-Leningrad, 1946.
Kurs astrofiziki i zvezdnoi astronomii, vol. 1. Moscow-Leningrad, 1951. Chapters 2–3.
Sovremennyi teleskop. Moscow, 1968.
N. N. MIKHEL’SON