Radio Interferometer
Astronomers have searched endlessly for better resolving power. It can be achieved by building bigger telescopes or by observing at shorter wavelengths for a chosen aperture or by using the phenomenon of interference. Interferometry is a technique involving two or more radio telescopes. Radio radiation from an astronomical source received at the individual telescopes is combined to obtain data that have a spatial resolving power equal to that of a single telescope as large as the distance between the individual receivers. With interferometry an astronomer can obtain details about the spatial structure of a given celestial object that a single radio telescope could never reveal.
For many years the separation between the individual telescopes of the interferometer was limited by the lengths of the cables connecting the antennas because the technique depends upon combining, at the same instant, the signals received by the separate telescopes. With the advent of the atomic clock (a clock governed by the vibrations of certain atoms) it became possible to record the signals received by the different telescopes, along with the precise time, and to compare them later. This allowed the individual telescopes to be greatly separated, even on opposite sides of the earth. The technique is called Very Long Baseline Interferometry (VLBI). It has been used with a geosynchronous or geostationary satellite as the link in the communications channel between the telescopes.
In the Very Large Array (VLA) radio interferometer recently put into operation in New Mexico signals from each of 27 individual radio telescopes are combi ned by a computer. Each dish is 25 meters in diameter, and the 27 individual telescopes are moved along railroad tracks arranged in the shape of an enormous 21-kilometer Y. Nine dishes will be located on each branch of the Y, and the system can provide a total of 351 interferometer pairs of antennas. The energy-collecting power of the VLA is roughly equivalent to a single 122-meter telescope. The VLA will be able to achieve spatial resolution of about 1 second of arc in 10 hours of observing, or about that of the 5.1-meter Hale optical telescope. This ability makes it comparable to large optical telescopes in ferreting out the structure of various cosmic bodies.
