Radio Interferometer
A radio interferometer was invented by merging radio astronomy with computer technology.
Radio astronomy began in the 1930s when Karl Jansky (1905-1950) built a radio receiver designed to locate interference that was plaguing long distance telephone communications. His discovery of radio signals coming from space created a sensation in 1933.
The first actual radio telescope was invented when Grote Reber built a 29.5-ft (9-m) parabolic dish receiver in 1937. With it he was able to create a map of numerous radio sources in the sky.
In the case of optical telescopes, the greater the aperture, the more light can be gathered. Likewise, the greater the area of the radio telescope dish, the more radio waves detected. Unfortunately, radio waves are at the low end of the electromagnetic spectrum. These wavelengths are long and stretched out. A really gigantic dish is needed to produce sharp resolution of radio objects, and that does not come inexpensively.
English astronomer Sir Martin Ryle (1918-) invented an ingenious alternative. Instead of building a massive radio dish with a huge aperture, he planned to link up smaller radio telescopes to synthesize an aperture that would achieve the same result.
In 1955, Ryle used a combination of twelve radio telescopes to simultaneously observe the same object. The data was recorded and then transmitted to a single receiver. A computer was used to synchronize and analyze the information to produce a single image that was much more detailed than any single radio telescope could have created.
This process became known as radio interferometry with aperture synthesis. In 1964 Ryle used three radio telescopes that were 1 mi (1.6 km) apart, creating a combined resolution that was essentially equivalent to a single dish of the same diameter.
The radio interferometer greatly expanded the potential of radio astronomy. Interferometers have been established at radio observatories throughout the world. A three-telescope interferometer was built by the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia, with each dish having a diameter of 85 ft (26 m).
In the late 1970s, NRAO built the Very Large Array (VLA) radio interferometer in New Mexico. It includes 27 telescopes, each with a diameter of 82 ft (25 m), mounted on rails in a "Y" shape. Each arm of the "Y" has a maximum length of about 12 mi (19 km). This is currently the world's largest single unit, but even bigger interferometers have been synthesized.
For instance, Very Long Baseline Interferometry (VLBI) creates a radio telescope with an effective diameter up to several thousand kilometers. The Merlin system of the United Kingdom and the United States VLBA are examples. When finished, VLBA will be able to resolve an arc in the sky the thickness of a pea in San Francisco, California, as seen from New York City. Supernova 1987A, in the Large Magellanic Cloud, was studied with radio telescopes in Australia, South America, and South Africa in a VLBI array. Two telescopes on opposite sides of the earth could produce the resolution of a single dish nearly 8,000 mi(12,756 km) across. Not even the sky's the limit, since plans for orbiting radio interferometers would create an effective diameter larger than the earth.
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