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The Arecibo Observatory is a very sensitive radio telescope located approximately 9 miles (14 km) south-southwest from the town of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation.
The observatory works as the National Astronomy and Ionosphere Center (NAIC) although both names are officially used to refer to it. NAIC more properly refers to the organization that runs both the observatory and associated offices at Cornell University. The observatory's 305 m radio telescope is the largest single-aperture telescope (cf. multiple aperture telescope) ever constructed. It carries out three major areas of research: radio astronomy, aeronomy (using both the 305 m telescope and the observatory's lidar facility), and radar astronomy observations of solar system objects. Usage of the telescope is gained by submitting proposals to the observatory, which are evaluated by an independent board of referees. The telescope is visually distinctive and has been used in the filming of notable motion picture and television productions: as the villain's antenna in the James Bond movie GoldenEye, as itself in the film Contact and in the "X-Files" episode "Little Green Men". The telescope received additional international recognition in 1999 when it began to collect data for the SETI@home project. General information The Arecibo telescope is distinguished by its enormous size: the main collecting dish is 305 m in diameter, constructed inside the depression left by a karst sinkhole The dish is the largest curved focusing dish on Earth, giving Arecibo the largest electromagnetic-wave gathering capacity. The Arecibo telescope's dish surface is made of 38,778 perforated aluminum panels, each measuring about 1 m by 2m (3 ft by 6 ft, 1 yd by 2 yd), supported by a mesh of steel cables. The telescope has three radar transmitters, with effective isotropic radiated powers of 20TW at 2380 MHz, 2.5TW (pulse peak) at 430 MHz, and 300MW at 47 MHz. The telescope 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, and the receiver is repositioned to intercept signals reflected from different directions by the spherical dish surface. A parabolical mirror would induce a varying astigmatism when the receiver is in different positions off the focal point, but the error of a spherical mirror is the same in every direction. The receiver is located on a 900-ton platform which is suspended 150 m (500ft) in the air above the dish by 18 cables running from three reinforced concrete towers, one of which is 110m (365ft) high and the other two of which are 80 m (265 ft) high (the tops of the three towers are at the same elevation). The platform has a 93 m long rotating bow-shaped track called the azimuth arm on which receiving antennas, 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, though the round trip light time to objects beyond Saturn is longer than the time the telescope can track it, preventing radar observations of more distant objects. Design and architecture 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 150m (500ft) tower to hold equipment at the focus. This design would have had a very limited use for other potential areas of research, such as planetary science 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 the Advanced Research Projects Agency (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 1959. Cornell University published a request for proposals (RFP) asking for a design to support a feed moving along a spherical surface 435 feet (133 m) above the stationary reflector. The RFP suggested a tripod or a tower in the center to support the feed. George Doundoulakis, director of research for the antenna design company General Bronze Corp in Garden City, N.Y. received the RFP from Cornell and studied it with his brother, Helias Doundoulakis, a civil engineer. The brothers devised a more efficient way to suspend the feed, and finally designed the cable suspension system that was used in final construction. The U.S. Patent office granted Helias Doundoulakis a patent on this approach. Construction began in the summer of 1960, with the official opening on November 1, 1963. As the primary dish is spherical, its focus is along a line rather than at a single point (as would be the case for a parabolic reflector), thus complicated 'line feeds' had to be used to carry out observations. Each line feed covered a narrow frequency band (2-5% of the center frequency of the band) and a limited number of line feeds could be used at any one time, limiting the flexibility of the telescope. The telescope has undergone significant upgrades, the first one in 1974 when a high precision surface was added for the current reflector. A Gregorian reflector system was installed in 1997, incorporating secondary and tertiary reflectors to focus radio waves at a single point. This allowed the installation of a suite of receivers, covering the whole 1-10GHz range, that could be easily moved onto the focal point, giving Arecibo a new flexibility. At the same time, a ground screen was installed around the perimeter to prevent receivers from sensing the ground (which, due to its temperature, would make observations less sensitive) and a more powerful transmitter was installed.
Research and discoveries Many significant scientific discoveries have been made using the Arecibo telescope. On 7 April 1964, shortly after its inauguration, Gordon Pettengill's team used it to determine that the rotation rate of Mercury was not 88 days, as previously thought, but only 59 days. In 1968, the discovery of the periodicity of the Crab Pulsar (33 milliseconds) by Lovelace and others provided the first solid evidence that neutron stars exist in the Universe In 1974 Hulse and Taylor discovered the first binary pulsar PSR B1913+16, for which they were later awarded the Nobel Prize in Physics. In 1982, the first millisecond pulsar, PSR J1937+21, was discovered by Don Backer, Shri Kulkarni and others. This object spins 642 times per second, and it was until 2005 the fastest-spinning pulsar known. In August 1989, the observatory directly imaged an asteroid for the first time in history: 4769 Castalia. The following year, Polish astronomer Aleksander Wolszczan made the discovery of pulsar PSR B1257+12, which later led him to discover its three orbiting planets and a possible comet. These were the first extra-solar planets ever discovered. In 1994, John Harmon used the Arecibo radio telescope to map the distribution of ice in the poles of Mercury. In January 2008, detection of prebiotic molecules methanimine and hydrogen cyanide were reported from Arecibo Observatory radio spectroscopy measurements of the distant starburst galaxy Arp 220.
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