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George Ellery Hale: Solar Physics and Related Inventions, 1927

Collecting Beams

Hale's spectroheliograph set-up occupied an area over 30 feet long consisting of an exterior light collection apparatus and a small building. The collected beam was directed into the building through a hole in the wall and inside the building were the mirror, the apparatus holding the two slits, the diffraction grating, the photographic plate arrangement, and connectors to control the exterior apparatus.

The sunlight was collected by a heliostat, a clockwork-driven apparatus that keeps a mirror tracking the Sun by compensating for the Earth's rotation and reflecting the Sun's rays to a fixed spot. The light rays passed through a lens, into the building, and through the first slit. Light of the selected wavelength then traveled 13 feet to reflect off a concave mirror onto a diffraction grating which further refined the wavelength to reflect off another mirror and back to the viewer or photographic plate.

The new instrument was put to work each sunny day, recording the Sun's surface activity to yield photographic evidence of violent magnetic storms, and changes in erupting solar prominences and faculae around the edge of the solar disk.

Laboratory experiments comparing the spectral lines of know hot gas mixtures with those of sunlight components showed that certain chemical elements, especially hydrogen, were common to the Earth and the Sun.