Radio Telescope
RF waves that can penetrate Earth’s atmosphere range from wavelengths of a few millimeters to nearly 100 meters. Although these wavelengths have no discernable effect on the human eye or photographic plates, they do induce a very weak electric current in a conductor such as an antenna. Most radio telescope antennas are parabolic (dish-shaped) reflectors that can be pointed toward any part of the sky. They gather up the radiation and reflect it to a central focus, where the radiation is concentrated. The weak current at the focus can then be amplified by a radio receiver so it is strong enough to measure and record.
Electronic filters in the receiver can be tuned to amplify one range (or “band”) of frequencies at a time. Or, using sophisticated data processing techniques, thousands of separate narrow frequency bands can be detected. Thus, we can find out what frequencies are present in the RF radiation and what their relative strengths are. As we will see later, the frequencies and their relative powers and polarization give us many clues about the RF sources we are studying.
The intensity (or strength) of RF energy reaching Earth is small compared with the radiation received in the visible range. Thus, a radio telescope must have a large “collecting area,” or antenna, in order to be useful. Using two or more radio telescopes together (called arraying) and combining the signals they simultaneously receive from the same source allows astronomers to discern more detail and thus more accurately pinpoint the source of the radiation. This ability depends on a technique called radio interferometry. When signals from two or more telescopes are properly combined, the telescopes can effectively act as small pieces of a single huge telescope.
A large array of telescopes designed specifically to operate as an array is the Very Large Array (VLA) near Socorro, New Mexico. Other radio observatories in geographically distant locations are designed as Very Long Baseline Interferometric (VLBI) stations and are arrayed in varying configurations to create very long baseline arrays (VLBA). NASA now has four VLBI tracking stations to support orbiting satellites that will extend the interferometry baselines beyond the diameter of Earth.
Since the GAVRT currently operates as a single aperture radio telescope, we will not further discuss interferometry here.