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Alvan Clark (March 8, 1804 – August 19, 1887), born in Ashfield, Massachusetts, was an American astronomer and telescope maker. He was a portrait painter and engraver, but at the age of 40 become involved in telescope making. Using glass blanks made by Mantois of Paris his firm, Alvan Clark & Sons, ground lenses for refracting telescopes, including the largest in the world at the time: the 18.5-inch at Dearborn Observatory at the Old University of Chicago (the lens was originally intended for Ole Miss), the 26-inch at the United States Naval Observatory, the 30-inch at Pulkovo Observatory (destroyed in the Siege of Leningrad; only the lens survives), the 36-inch telescope at Lick Observatory (still third-largest) and later the 40-inch at Yerkes Observatory, which remains the largest successful refracting telescope in the world. One of Clark's sons, Alvan Graham Clark, discovered the dim companion of Sirius. His other son was George Bassett Clark; both sons were partners in the firm.
Craters on the Moon and on Mars are named in his honor.
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