device for photographing the surface of the sun
in a single wavelength of light, usually one corresponding to a chief element contained in the sun, e.g., hydrogen or calcium; the resulting photograph is called a spectroheliogram. The spectroheliograph was invented in 1890 independently by G. Hale and by H. Deslandres and modernized (1932) by R. R. McMath to take motion pictures. In operation, the instrument is preset by means of a prism or grating and a narrow slit that passes only one wavelength of light to a photographic plate or digital detector; the image of the sun is then moved slowly or stepwise across the entrance slit until the entire disk of the sun has been photographed. See spectrum
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