The cornerstone for exploration of the Sun, the science of astrophysics, was laid by George Hale in 1890 when, for his senior thesis at the Massachusetts Institute of Technology, he built the first successful spectroheliograph.
The spectroheliograph makes a photographic image of the Sun's visible surface in light of a single, selected spectral line. A telescope's image of the Sun is focused onto the entrance slit of a spectrograph where a collimator forms the light into a parallel beam and directs the beam at a diffraction grating that separates the beam into a spectrum; a second slit isolates a selected wavelength from this spectrum. Coordinating the movement of both slits produces, on the photographic plate detector, a monochromatic image of the Sun's surface.
Hale's later invention of a spectrohelioscope replaced the photographic detector of the spectroheliograph with the human eye. Now direct visual examination of the Sun's surface is made possible by vibrating the slits in unison in a small amplification range. The viewer sees an image that appears steady because of persistence of visionthe perception that rapidly changing images are constantly steady.
NRL spectroheliograph image of the Sun, taken aboard Skylab in 1973, using the extreme ultraviolet radiation from ionized helium, 304 Angstom wavelength. (537k)
Letter to George Ellery Hale, Requesting a portrait photograph for The Franklin Institute archives, 6/4/1927 (694k)