American family of telescope makers and astronomers who supplied unexcelled lenses to many observatories in the United States and Europe during the heyday of the refracting telescope.

Alvan Clark (b. March 8, 1804, Ashfield, Mass., U.S.—d. Aug. 19, 1887, Cambridge, Mass.) built a career as a portrait painter and engraver, but at the age of 40 he became interested in optics. With his son George Bassett Clark (b. Feb. 14, 1827, Lowell, Mass.—d. Dec. 20, 1891, Cambridge, Mass.), he opened the optics firm Alvan Clark & Sons in Cambridge, Mass., in 1846. Alvan Graham Clark (b. July 10, 1832, Fall River, Mass.—d. June 9, 1897, Cambridge, Mass.), joined his father and brother in the business in the early 1850s. Recognition of the family's superb lenses was slow to come. The discovery of two double stars by the elder Alvan Clark in the late 1850s, however, attracted attention abroad, and the firm began to flourish. They made the 36-inch (91-centimetre) lens for the Lick Observatory, Mt. Hamilton, Calif. (1888); the 30-inch lens for the Pulkovo Observatory, near St. Petersburg in Russia (1878); the 28-inch for the University of Virginia, Charlottesville (1883); and 24-inch lenses for the U.S. Naval Observatory, Washington, D.C. (1873), and the Lowell Observatory, Flagstaff, Ariz. (1896). All these telescopes remain in operation except that at Pulkovo, which was destroyed during World War II.

The younger Alvan Clark directed the fabrication of the 40-inch lens of the Yerkes Observatory, Williams Bay, Wis., the largest refractor lens in the world. Using telescopes of his own construction, he discovered the companion of the star Sirius as well as 16 double stars.