astronomical observatory located on the Papago Indian Reservation 50 miles (80 km) southwest of Tucson, Ariz., U.S., at an elevation of 6,888 feet (2,100 metres). It was established in 1958 by the National Science Foundation (NSF) in response to a long-felt need by astronomers in the eastern half of the United States for access to excellent optical observing facilities in a favourable climate. Operated by the Association of Universities for Research in Astronomy (Association of Universities for Research in Astronomy) under contract with the NSF, the observatory houses a collection of two radio and 21 optical telescopes, the largest of which is the 13-foot (4-metre) Mayall Telescope. The observatory also features the largest telescope ever designed for solar observation and a smaller vacuum tower telescope, which is also used for studying the Sun.