Radio Telescope
Radio waves are a part of the electromagnetic spectrum. Just as an optical telescope gathers visible light and magnifies it, a radio telescope is a large dish-shaped device that gathers and amplifies radio frequencies from space. Radio telescopes do not "see" the way an optical telescope does; the radio signals the telescopes gather are used to create diagrams showing where the strongest radio sources are located.
Cosmic radio waves were discovered accidentally by Karl Jansky (1905-1950) in 1932. He had been using a receiver to search for the source of radio noise that was interfering with long distance radio-telephone conversations. That the source could be out in the cosmos was a big surprise, but there was no follow-up to the discovery because radio astronomy did not exist as a science.
One person, an amateur ham-radio enthusiast named Grote Reber, was electrified by Jansky's discovery. Reber built the world's first radio telescope in 1937, spending $1,300 of his own money to build a receiver in his backyard from lumber and galvanized sheet metal. The dish--the portion of the telescope that collects radio waves--had a diameter of 31 feet (9.5 m). The receiver--the portion of the telescope that amplifies the waves--was mounted at the focus of the dish.
Reber's device was similar in design to the backyard satellite dishes of today, though at a considerably larger scale. Unlike backyard dishes, which are intended only to follow man-made satellites in a narrow band of sky, however, a radio telescope needs to be moved so it can point at different parts of the sky. To keep costs at a minimum, Reber used a meridian-transit mount; the dish could be moved in a north-south line only. The rotation of the earth provided movement from west to east.
The bigger the dish, the more data that can be collected, and the greater the resolution and sensitivity of the telescope. In 1947 Sir Bernard Lovell (1913-) directed the construction of a parabolic radio reflector 216 feet (66 m) in diameter at Jodrell Bank, England. It was also here that a 250-foot (76 m) dish went into operation in August, 1957. A gear-and-rack mechanism once used to turn gun turrets aboard battleships was used to aim the telescope. It was the world's largest steerable telescope, but only for five years.
Although dish diameter determines telescope performance, the size cannot be scaled up indefinitely. Past a certain size, structural issues come into play--how do you support such a massive dish? How do you turn it? Consider, for example, the 300-foot (91 m) diameter dish at Green Bank, West Virginia, built by the National Radio Astronomy Observatory in 1962. It was the largest steerable radio telescope in the world, until it collapsed from metal fatigue in 1989.
The largest individual dish currently in existence is in Arecibo, Puerto Rico. Built by the engineer Thomas Kavanagh (1912-), the radio telescope was created out of a natural depression within a ring of mountains in 1963. With a diameter of 1,000 feet (305 m), the telescope's reflecting surface is too large to be steerable. Instead, a movable feed suspended above it the dish provides access to most of the sky.
Arecibo pushed the limits on the size of a single-dish telescope, but astronomers developed other approaches that circumvented the structural issues involved in building large-aperture radio telescopes. In 1955, Sir Martin Ryle (1918-1984) invented the radio interferometer, a device in which several telescopes are linked to synthesize the performance of a single telescope with an extremely large aperture. As with optical telescopes, the enhanced signal-gathering ability provided by the increased aperture size greatly increased the resolution of these telescopes, permitting the detection of more distant objects.
The Very Large Array (VLA) is one of the best examples of this technique. Built in New Mexico in the 1970s and dedicated in 1980, the VLA consists of an array of radio telescopes that can operate with the resolving power of a single antenna as large as 36 km in diameter. The 27 telescopes are mounted in a "Y" configuration, with nine telescopes on each arm of the "Y". Weighing approximately 360 tons, each of the 82 foot (25 m) dishes can be moved from place via a system of railroad transporters to one of four standard configurations; each configuration yields a different resolving power. The maximum antenna separation ranges from 0.62- 22.3 mi (1-36 km), and changing from one configuration to another requires an average of two weeks.
During observing projects at the VLA, which generally take from half an hour to several days, data from the antennas is acquired in real time in a process called correlation. This technique is feasible for even an array as large as the VLA. Past a certain distance, however, the antennas in a synthesis array are separated widely enough that this level of data communications is no longer possible. To move beyond the size and resolving power of the VLA, then, scientists were forced to develop Very Long Baseline Interferometry (VLBI), in which the signals from widely-spaced antennas are recorded on magnetic tape and correlated later by computer.
In 1986, the National Science Foundation began construction of the VLBA, an array of ten radio telescopes designed for VLBI observation.
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