It is an optical device that serves to modify both the quality and quantity of the light radiations emitted by a star.
A filter, in general, consists of a sheet of colored flat and parallel faces. Put in the optical path of a telescope, it has the function of letting only a few wavelengths of light pass.
A yellow filter, for example, which is one of the most commonly used in astronomy, absorbs the wavelengths between ultraviolet and green, and lets yellow, orange and red lac pass. Its effect on the image of a planet or the Moon, is therefore to reduce reverberation due to excess brightness and increase contrast.
In case you want to reduce the luminosity of a star in all the wavelengths it emits. the so-called neutral filters are used. On the other hand, if you want to isolate a narrow range of wavelengths for the study of phenomena visible only in monochromatic light (for example, some details of the solar surface that are made very visible at the wavelengths emitted by hydrogen), filters called interferentials are used, because they usufruct the phenomenon of light interference to let only the radiation from a narrow band of the spectrum pass.
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