astronomical observatory founded in 1965 in Chile as the southern branch of the Kitt Peak National Observatory. It is located on top of two mountains, Cerro Tololo, which is 7,200 feet (2,200 metres) high, and Cerro Pachon, which is 8,900 feet (2,700 metres) high; both mountains are about 285 miles (460 km) north of Santiago and 50 miles (80 km) inland from the coastal city of La Serena. It is operated by the Association of Universities for Research in Astronomy and funded by the U.S. National Science Foundation.

The CTIO houses several telescopes and auxiliary instruments, the most significant of which are the 26-foot (8-metre) Gemini South, the 13-foot (4-metre) Southern Observatory for Astrophysical Research Telescope, and the 13-foot Victor M. Blanco Telescope. The observatory is best noted for its research on the central region of the Milky Way Galaxy, the Magellanic Clouds, and high-energy cosmic radio and X-ray sources.