Arecibo radio telescopeThe 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.
The Arecibo telescope 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. The Arecibo telescope's dish surface is made of 38,778 perforated aluminium 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; the telescope'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 the Arecibo telescope 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.
The Arecibo telescope has been instrumental in many significant scientific discoveries. On April 7 1964, shortly after its inauguration, Gordon H. Pettengill's team used it to determine that the rotation rate of Mercury was not 88 days, as previously thought, but only 59 days. The telescope also had military intelligence uses, for example locating Soviet radar installations by detecting their signals bouncing back off of the Moon. Arecibo was also the source of data for the SETI@home distributed-computing project put forward by the SETI Institute. The telescope has undergone several significant upgrades over its lifespan.