Smartt, Raymond N. National Solar Observatory, Sunspot, New Mexico.
Last reviewed:June 2020
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A spectrographic instrument that produces monochromatic images of the Sun. The principle of operation was first suggested by J. Janssen in 1869, and a photographic version was constructed by G. E. Hale at Mount Wilson Observatory in 1892. Around the same time, H. A. Deslandres devised a closely related instrument for operation at l'Obervatoire de Paris, Meudon. In a simple form of the instrument, an image of the Sun from a solar telescope is focused on a plane containing the entrance slit of the spectroheliograph (see illustration). The light passing through the slit is collimated by a concave mirror that is tilted such that the light is incident on a plane diffraction grating. Part of the dispersed light from the grating is focused by a second concave mirror, identical to the first mirror, at an exit slit identical to the entrance slit. By symmetry of the optical system, the portion of the solar disk imaged on the entrance slit is reimaged in the plane of the exit slit with the same image scale but in dispersed wavelength. The light imaged along the exit slit then corresponds to the portion of the solar image falling on the entrance slit, but in the light of only a narrow region of the spectrum, as determined by the spectrographic dispersion. The particular wavelength sampled is set by the grating angle. By uniform transverse motion of the instrument such that the entrance slit is scanned across the solar image, the light passing through the exit slit maps out a corresponding monochromatic image of the Sun, which can be recorded photographically with a stationary camera. Alternatively, the solar image can be scanned across the entrance slit of a stationary spectroheliograph, the camera then synchronously moved in step with the image. See also: Diffraction grating
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