He coronograph It is an instrument used to study the solar corona, even when there is no solar eclipse.
Under normal conditions, the light emanating from the solar disk is such as to exceed the dim luminosity of the crown and to prevent its observation both with the naked eye and with optical instruments.
It consists of a small disk called "concealment disk", located inside the telescope, which intercepts the image of the Sun by hiding it. In practice, the instrument does nothing but produce an artificial eclipse, making the crown visible. Therefore, the astronomer can observe it directly, or photograph it.
Before the French astronomer Bernard Lyot invented the coronograph in 1931, the crown could only be observed and photographed during total eclipses, since the luminosity of the crown is only one millionth of that of the Sun as a whole.
Typically, the coronograph is installed at high altitude to prevent light scattering due to particles and dust suspended in the atmosphere and includes a narrow-band polarizing filter to correct chromatic aberration.
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