Radio Astronomy

A major drawback of the radio telescope is that it resolves images much less well than does an optical telescope. The resolving power of a telescope is its ability to separate two objects close to each other in the sky. The resolving power of early radio telescopes was often no better than about a degree of arc compared to a second of arc that is typical for optical telescopes.

Since the resolving power of a telescope is inversely proportional to the wavelengths of radiation it receives, the only way to increase the resolving power of a radio telescope is to increase the diameter of its dish. Fortunately, it is much easier to make a very large dish constructed of metal wire than to make a similar mirror made of glass or plastic. The Arecibo radio telescope was an example of a telescope that was made very large in order to improve its resolving power.

One could, in theory, continue to make radio telescopes larger and larger in order to improve their resolving power. However, another possibility exists. Instead of making just one telescope with a dish that is many miles in diameter, it should be possible to construct a series of telescopes whose diameters can be combined to give the same dimensions.

The radio telescope at the National Radio Astronomy Observatory near Socorro, New Mexico, is an example of such an instrument. The telescope consists of 27 separate dishes, each 85 ft (26 m) in diameter. The dishes are arranged in a Y-shaped pattern that covers an area 17 mi (27 km) in diameter at its greatest width. Each dish is mounted on a railroad car that travels along the Y-shaped track, allowing a large variety of configurations of the total observing system. The system is widely known by its more common name of the Very Large Array, or VLA.