coronagraph

coronagraph , 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.

coronagraph 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.