refractor


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refractor:

see telescopetelescope,
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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Refractor

 

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) Refractor, 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.

REFERENCES

Maksutov, 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

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Through the refractor at 48x, M55 is a large, ghostly glow with a dash of very faint stars overlaying its outer reaches.
A refractor is what everyone thinks a telescope is supposed to look like: a skinny tube with a lens at one end that brings light to a focus in the eyepiece at the other.
By this time, Hale also shared the view of many astronomers that refractor telescopes had reached their practical limit, and that only reflectors could provide the apertures required for better spectroscopy.
Head of the BHVI, Professor Brien Holden, said: "The Virtual Refractor provides a realistic experience that allows optometry students to learn refraction faster, and in their own time."
This is a pretty refractor. Although similar in appearance to many current APOs, it has the distinctive "Meade" blue lens cap and trim.
The Hypochromat serves well the need for school science projects, where a low cost reasonably large aperture refractor can be constructed from cheap available recycled components, such as damaged binoculars where the OG can be rescued and used for the reduction lens.
High-quality refractors offer the best optical performance that can be achieved with any given aperture, as long as false color can be kept to negligible levels.
Refractor lens elements need to be precisely spaced in order to work together properly.
A refractor is generally more rugged than other types of scopes since its lenses are not likely to become misaligned due to rough handling.
It has a concave shape to the cluster, as if it's gobbling stars ahead of it." Also observing with a small refractor, Ireland's Kevin Berwick says that M67 "is very rich and bright, and to the east there's a very bright star.
I have been in contact with a Mr David Wallis who, as an eighteen year old apprentice to the scientific instruments trade in 1949, assisted in dismantling Hay's Hendon-based refractor, following his death in April of that year.