(ESO), an intergovernmental organization for astronomical research with headquarters in Garching, near Munich, Germany. The ESO began in 1962 as a consortium among Belgium, France, Germany, the Netherlands, and Sweden. Denmark, Switzerland, Italy, Portugal, Great Britain, Finland, Spain, Czech Republic, and Austria subsequently joined. The ESO operates three observatories in the Atacama desert, Chile.
The oldest observatory, inaugurated in 1969, is located on Cerro La Silla at an altitude of about 8,000 ft (2,400 m). The initial instrument was a 142-in. (3.6-m) reflecting telescope, but in 1989 the 141-in. (3.58-m) New Technology Telescope (NTT) was installed. Its primary mirror is three times faster than the original and has only half its weight. It uses a principle called active optics, in which the optics are adjusted by computer to react to the changing seeing conditions of the night sky. Other instruments include a 86.6-in. (2.2-m) reflector, a 20-in. (0.5-m) reflector, a 39.4-in. (1-m) Schmidt camera telescope, twin 15.7-in. (0.4-m) astrographic telescopes, a 60-in. (1.52-m) spectrographic reflector, and a 39.4-in. (1-m) photometric reflector. Also located at Cerro La Silla are a 20-in. (0.5-m) reflector belonging to Denmark, a 23.5-in. (0.6 m) reflector used by the Univ. of Liège, Belgium, and the Univ. of Geneva, Switzerland, to search for exoplanets and comets, and a 24.4-in. (0.62-m) reflector belonging to the Univ. of Bochum, Germany.
The second observatory, initiated in 1988 and inaugurated in 1999, is located atop Cerro Paranal at an altitude of about 8,640 ft (2,635 m). The observatory is the home of the Very Large Telescope (VLT) program. Begun in 1996, it links four 315-in. (8-m) telescopes, usually in groups of two or three, together with several movable 72-in. (1.8-m) telescopes through optical interferometry, a technique in which the signals from each telescope enhance the signals from the others. Completed in 2003, the combination produces a virtual telescope image that is up to 25 times more detailed than an image produced by one of the individual telescopes. Also there is the Visible and Infrared Survey Telescope for Astronomy (VISTA), which was constructed by a British university consortium and officially handed over to the ESO in 2009. VISTA is 13-ft (4-m) wide-field survey telescope and is equipped with a near infrared camera.
ESO's newest site is Chajnantor, located at an altitude of some 16,730 ft (5,100 m), where ESO, the National Radio Astronomy Observatory, the National Astronomical Observatory of Japan, and other partners broke ground for the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2003. It began operations in 2011, and when completed will consist of an array of 66 39-ft (12-m) and 23-ft (7-m) radio telescopes. Also there is the APEX (Atacama Pathfinder Experiment) telescope, a 39-ft (12-m) submilliter radio telescope that is a collaboration among the Max Planck Institute for Radioastronomy, Onsala Space Observatory, and the ESO. It began operations in 2005.
Among the other programs of the observatory is the completion of the photographic Sky Survey for the Southern Hemisphere, in cooperation with the Siding Spring Observatory in Australia. This project is a continuation of the work begun in the Northern Hemisphere with the Schmidt telescope at Palomar Observatory.