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Astronomy In California 1850 – 1950:
Telescope Makers, Telescopes, and Artifacts
Aluminized Astronomical Mirror
By John Donovan Strong, California Institute of Technology
An important development in telescope construction in the 1930s was the introduction of the aluminizing process for forming reflecting films on optical surfaces. Prior to this development, astronomical mirrors were coated with a silver film. Aluminum coatings are, in many ways, superior to silver coatings. Aluminum reflects 89 percent of visible light, and an aluminum film is more durable than a silver one. Also, tests done at the Lick Observatory showed that for stellar photography, an aluminized mirror reflects about 50 percent more light than does one of silver. Aluminum is by no means the ideal metal for coating astronomical mirrors, but it is durable and had the highest reflectivity of all the metals tried through 1950. (After 1932, all of California's giant telescope mirrors were aluminized rather than silvered.)
John Donavan Strong, a young physicist at the California Institute of Technology, was one of the first to coat a mirror with aluminum. He did it by thermal vacuum evaporation. The mirror in this exhibition was his first, and it is the earliest known example of a telescope mirror coated by this technique. The process was quickly adopted by both professional and amateur astronomers.
The display is the original astronomical mirror in a wooden box. The mirror is plate glass, parabolic, of 6 5/32 inches aperture, and aluminized on one side.
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