Schmidt Telescope

Schmidt telescope: see telescope.

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Schmidt Telescope 

an astronomical mirror-lens telescope designed for photographing large areas of the sky. The Schmidt telescope was invented in 1929 by the German opticist B. Schmidt. Its main components are a spherical mirror and a Schmidt correction plate, which is mounted at the center of curvature of the mirror (see Figure 1). As a result of the placement of the correction plate, all beams of rays passing through it from different parts of the sky prove to be equally corrected with respect to the mirror. Consequently, the Schmidt telescope lacks such aberrations of optical systems as coma, astigmatism, and distortion.

Figure 1. Optical scheme of a Schmidt telescope: (1) correction plate, (2) concave spherical mirror, (3) convex focal surface, (C) common center of the mirror and focal plane

The spherical aberration of the mirror is corrected by the correction plate, the central part of which acts as a weak positive lens, and the outer part, as a weak negative lens. The focal surface, on which the image of a segment of the sky is formed, has a spherical shape, whose radius of curvature is equal to the focal distance of the telescope. The focal surface can be converted into a flat surface by means of a Piazzi-Smyth lens. In small Schmidt telescopes, the field of view is limited by a cassette that screens the beam of rays incident on the mirror; in large Schmidt telescopes, it is limited by the aberrations of oblique rays, which are introduced by the correction plate. This plate, in correcting spherical aberrations, gives rise to small residual aberrations (coma and astigmatism), which spoil the image. The chromatism of the correction plate limits the relative aperture of Schmidt telescopes and the operating region of the spectrum. A Schmidt telescope with a primary mirror of a small diameter at a relative aperture A = 1:1 provides a field of view 20° in diameter; at larger diameters of the primary mirror and at A from 1:2 to 1:3, the diameter of the field of view is about 5°. To eliminate vignetting of the rays at the edge of the field, the diameter of the primary mirror must exceed the diameter of the correction plate by 4l, where 2l is the diameter of the field of view in linear measurement.

A shortcoming of Schmidt telescopes is the considerable length of the tube, which is twice the focal length of the telescope. A number of modifications, including the use of a second (supplementary) convex mirror and the placement of the correction plate closer to the primary mirror, have been proposed to eliminate this shortcoming.

The largest Schmidt telescopes are mounted at the astronomical observatory in Tautenberg in the German Democratic Republic (D = 1.37 m, A = 1:3), the Mount Palomar Observatory in the United States (D = 1.22 m, A = 1:2.5), and the Biurakan Astrophysical Observatory of the Academy of Sciences of the Armenian SSR (D = 1.00 m, A = 1:2,1:3).

REFERENCES

Dimitroff, G., and J. Baker. Teleskopy i prinadlezhnosti k nim. Moscow-Leningrad, 1947. (Translated from English.)
Teleskopy. Edited by G. Kuiper and B. Middlehurst. Moscow, 1963. (Translated from English.)
Mikhel’son, N. N. Opticheskie teleskopy: Teoriia i konstruktsiia. Moscow, 1976.
Linfoot, E. H. Recent Advances in Optics. Oxford, 1955.

N. N. MIKHEL’SON