Spectrohelioscope

spectrohelioscope [¦spek·trō′hē·lē·ə‚skōp]

(optics)

An instrument based on the principle of the spectroheliograph but used for visual observation, and not for photography.

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Spectrohelioscope 

an astronomical spectroscopic instrument used for visual observation of the sun in monochromatic light.

The spectrohelioscope is a monochromator in which an image of the sun is projected on the plane of the entrance slit by means of a long-focus lens or mirror. The exit slit isolates a particular spectral line, usually the red hydrogen line H_a, of the solar spectrum. In the monochromatic light of the selected spectral line the observer sees the part of the solar image that is transmitted by the entrance slit of the instrument. When the two slits are vibrated synchronously at a sufficiently high frequency, persistence of vision permits observation of an area of the solar disk or of a region beyond the edge of the disk. Solar formations that emit or absorb in the given spectral line can be identified—for example, filaments and bright and dark flocculi on the solar disk, prominences at the sun’s limb, and chromospheric phenomena beyond the edge of the solar disk.

The first attempts at designing spectrohelioscopes were made in 1891 by H. Deslandres and G. E. Hale.