Coronagraph

coronagraph (kərō`nəgrăf'), device invented by the French astronomer B. Lyot (1931) for the purpose of observing the corona of the sun and solar prominences occurring in the chromosphere. Because of the intense light of the sun, the corona and chromosphere can ordinarily be seen only during a total eclipse. The coronagraph consists of two refracting telescopes in tandem. A solid disk placed in front of the prime focus of the first telescope plays the part of the moon and eclipses the sun's image in the telescope so that only the outer layers of the sun's atmosphere are focused by the second telescope onto photographic film. A monochromatic filter is also used to improve optical clarity and remove chromatic aberration.

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coronagraph [kə′rō·nə‚graf]

(astronomy)

An instrument for photographing the corona and prominences of the sun at times other than at solar eclipse.

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Coronagraph 

a telescope used for observation of the corona of the sun. Since the light scattered from the sun’s photosphere in the earth’s atmosphere and in the telescope is hundreds of times brighter than the light of the sun’s corona, the latter was observed until the 1930’s only during total solar eclipses, when the photosphere is covered by the moon. The coronagraph used for such observations is a long-focus photo-graphic camera, usually mounted horizontally; the light to it is supplied by a coelostat. The standard coronagraph used in the USSR has an objective of 5-cm diameter and 500-cm focal length.

Observations of the corona of the uneclipsed sun became possible after the invention of a coronagraph by the French astronomer B. Lyot, by means of which he first observed the corona at the observatory at Pic-du-Midi (France) in 1931. The corona-graph used to observe the uneclipsed sun consists of a primary objective that forms an image of the sun on a metallic disk, which occults the light of the photosphere and thus produces an artificial eclipse. In order to eliminate the scattered light that appears around the edge of the primary objective as a result of diffraction, a lens is placed behind the metallic screen. This lens forms the image of the primary objective on a diaphragm, which has an opening too small to pass the image of the edges of the primary objective. A second objective forms the final image of the corona or prominences on a spectrograph slit or on a photographic plate. In the last case, the light passes through a monochromatic interference-polarization filter, which removes all rays except the spectral line of the corona or prominence being studied. The use during observations of special filters with a density that decreases from the center to the edges makes it possible to obtain images of the bright inner part and of the fainter outer part of the corona on the same photograph.

Coronagraphs provide best results when set up on mountains, where the atmospherically scattered light is significantly less. In the USSR the first observations of the corona of the uneclipsed sun were made at the Kislovodsk Mountain Astronomical Station in 1950, using a coronagraph with an objective of 20-cm diameter. The largest coronagraphs in the world, with objectives of 53-cm diameter, are located at this station and at the observatories near Irkutsk, in Abastumani, and in Alma-Ata.

REFERENCES

Gnevyshev, M. N. Kislovodskaia gornaia astronomicheskaia stantsiia. Moscow-Leningrad, 1965.
Martynov, D. la. Kurs prakticheskoi astrofiziki, 2nd ed. Moscow, 1967.

M. N. GNEVYSHEV