Fred Lawrence Whipple Observatory, astronomical observatory located 35 mi (56 km) S of Tucson, Ariz., at an altitude of 8,500 ft (2,590 m). It is operated jointly by the Smithsonian Astrophysical Observatory and the Univ. of Arizona. Formerly known as the Mount Hopkins Observatory, it was renamed in 1982 for the American astronomer Fred Lawrence Whipple, who was instrumental in establishing the observatory. Until 1998 the observatory was best known for its principal instrument, the highly unusual Multiple-Mirror Telescope (MMT). This consisted of six identical 72-in. (183-cm) reflecting telescopes mounted in a hexagonal array on a common mounting and feeding their images to a single focus. A 30-in. (76-cm) reflector in the center of the mounting served as a guide telescope. The combined light-gathering power of the MMT was equal to that of a conventional 176-in. (447-cm) reflector. The MMT was replaced in 1999 with a conventional 256-in. (6.5-m) single-mirror telescope. Also at the observatory are a 60-in. (152-cm) and a 394-in. (10-m) dish with 248 small mirrors used for gamma-ray astronomy observations.