The largest single-dish radio telescope in the world. It came into operation in 1963 and is operated by Cornell University for the National Science Foundation. Occupying a large karst sinkhole in the hills south of Arecibo in Puerto Rico, its area of almost 9 hectares is greater than that of all other such instruments in the world combined. The surface of Arecibo's 305-meter (1,000-foot) fixed, spherical dish is made from almost 40,000 perforated aluminum panels, each measuring 1 meter by 2 meters (3 feet by 6 feet), supported by a network of steel cables strung across the underlying depression. Suspended 150 meters (450 feet) above the reflector is a 900-ton platform which houses the receiving equipment. Although the telescope is not steerable, some directionality is obtained by moving the feed antenna (upgraded in 1996). The immense size and accurate configuration of the dish allows extremely faint signals to be detected. For this reason, it has been used extensively in SETI investigations and in the first attempt at CETI (see Arecibo Message). It also featured in the film Contact. The giant radio telescope dish at the Arecibo Observatory in Puerto Rico is nestled in a natural sinkhole. The Search for Extraterrestrial Intelligence (SETI) has been using the telescope to search for radio signals from space since 1992. The Arecibo radio telescope is located in Puerto Rico about 10 km south of the town of Arecibo, which is located on the north coast of the island. It is operated by Cornell University under cooperative agreement with the National Science Foundation. Arecibo is one of the most famous such telescopes in the world, distinguished by its enormous size; the main collecting dish is 305 meters in diameter, constructed inside the depression left by a karst sinkhole. It is the largest curved focusing dish on Earth, giving it the largest photon-gathering capacity. Arecibo's dish surface is made of 38,778 perforated aluminum panels, each measuring about 3 feet by 6 feet, supported by a mesh of steel cables. It is a spherical reflector (as opposed to a parabolic reflector). This form is due to the method used to aim the telescope; Arecibo's dish is fixed in place, but the receiver at its focal point is repositioned to intercept signals reflected from different directions by the spherical dish surface. The receiver is located on a 900-ton platform which is suspended 450 feet in the air above the dish by 18 cables running from three reinforced concrete towers, one of which is 365 feet high and the other two of which are 265 feet high (the tops of the three towers are at the same elevation). The platform has a 93 meter long rotating bow-shaped track called the azimuth arm on which receiving antennae, secondary and tertiary reflectors are mounted. This allows the telescope to observe any region of the sky within a forty degree cone of visibility about the local zenith (between -1 and 38 degrees of declination). Puerto Rico's location near the equator allows Arecibo to view all of the planets in the solar system. The construction of Arecibo was initiated by Professor William E. Gordon of Cornell University, who originally intended to use it for the study of Earth's ionosphere. Originally, a fixed parabolic reflector was envisioned, pointing in a fixed direction with a 500 foot tower to hold equipment at the focus. This design would have had a very limited use for other potential areas of research, such as planetology and radio astronomy, which require the ability to point at different positions in the sky and to track those positions for an extended period as Earth rotates. Ward Low of ARPA pointed out this flaw, and put Gordon in touch with the Air Force Cambridge Research Laboratory (AFCRL) in Boston, Massachusetts where a group headed by Phil Blacksmith was working on spherical reflectors and another group was studying the propagation of radio waves in and through the upper atmosphere. Cornell University proposed the project to ARPA in the summer of 1958 and a contract was signed between the AFCRL and the University in November of 1959. Construction began in the summer of 1960, with the official opening taking place on November 1, 1963. Arecibo has been instrumental in many significant scientific discoveries. On April 7 1964, shortly after its inauguration, Gordon H. Pettengill's team used Arecibo to determine that the rotation rate of Mercury was not 88 days, as previously thought, but only 59 days. Arecibo also had military intelligence uses, for example locating Soviet radar installations by detecting their signals bouncing back off of the Moon. Arecibo has undergone several significant upgrades over its lifespan.