The Cerro Tololo Inter-American Observatory (CTIO) is a complex of astronomical telescopes and instruments located 70 kilometers east of La Serena, Chile, at an altitude of about 2,200 meters on Cerro Tololo mountain. Founded in 1963 and part of the U.S. National Optical Astronomy Observatories, its main instrument is the 4-meter Blanco Telescope, opened in 1976, that is the southern twin of the 4-meter instrument at Kitt Peak National Observatory. On a peak 10 kilometers to the southeast is the 8-meter Gemini South telescope.