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An instrument that enables the Sun's corona to be observed outside the times of total eclipses, invented by B. Lyot in 1930. It is a form of refracting telescope with an objective lens kept free of dust and static charge, sometimes by means of a thin layer of oil. An occulting disk is placed at the prime focus to form an artificial eclipse. A lens directly behind the occulting disk forms an image of the objective on a diaphragm, thereby removing most of the stray light from the objective. A third lens behind the diaphragm actually forms the image of the corona on film or a detector. Only observatories at high altitude with exceptionally clear atmospheric conditions are suitable sites for coronagraphs. Even then, only the inner part of the E corona can be observed, although the K corona can be imaged using polarization analysers. Spacecraft coronagraphs can observe the corona out to several solar radii, using electronic imaging instead of photography. The LASCO (Large Angle Spectroscopic Coronagraph) on the SOHO spacecraft observes the corona out to 30 solar radii.

Subjects: Astronomy and Astrophysics.

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