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A Radio telescope consists of a radio receiver and an antenna system that is used to detect radio-frequency radiation emitted by extraterrestrial sources. Because radio wavelengths are much longer than those of visible light, radio telescopes must be very large in order to attain the resolution of optical telescopes.

The radio telescope at Jodrell Bank, Cheshire, Eng., has a steerable paraboloid antenna 76 m (250 feet) in diameter.

Radio waves are electromagnetic radiation, so they are reflected and refracted like ordinary light waves. In radio astronomy, however, mostly reflecting telescopes are used.

A radio telescope collects radiation in an aperture or antenna, from which it is transformed to an electric signal by a receiver, called a radiometer. This signal is then amplified, detected and integrated, and the output is registered on some recording device, e.g. a strip chart recorder, or in a modern radio telescope, on a magnetic tape or a computer disk.

Radio telescopes are used to measure broad-bandwidth continuum radiation as well as spectroscopic features due to atomic and molecular lines found in the radio spectrum of astronomical objects. In early radio telescopes, spectroscopic observations were made by tuning a receiver across a sufficiently large frequency range to cover the various frequencies of interest. This procedure, however, was extremely time-consuming and greatly restricted observations. Modern radio telescopes observe simultaneously at a large number of frequencies by dividing the signals up into as many as several thousand separate frequency channels that may range over a total bandwidth of tens to hundreds of megahertz.

Dish and Antenna

The most common antenna type is a parabolic reflector, or "dish", which works the same way as an optical mirror telescope, by reflecting the waves to focus the incoming radiation onto a small antenna referred to as the "feed". The feed is typically a waveguide horn and is connected to a sensitive radio receiver.

At long wavelengths, the reflecting surface does not need to be solid, because the long wavelength photons are larger than the holes in the reflector and, is therefore usually made in the form of a metal mesh. At high frequencies, the surface has to be smooth, and in the millimetre-submillimetre range, radio astronomers equip large optical telescopes with radiometers.

At low frequencies, the antennas are usually dipoles (similar to those used for radio or TV), but in order to increase the collecting area and improve the resolution, one uses whole fields of dipoles, dipole arrays, where all dipole elements are connected to each other.

Sensitivity

One has to use sensitive receivers, because the received signal is very weak. Cryogenically cooled solid-state amplifiers with very low internal noise are used to obtain the best possible sensitivity and minimize the noise, which could otherwise mask the signal from the source.

The sensitivity of a radio telescope--i.e., the ability to measure weak sources of radio emission--depends on the area and efficiency of the antenna, the sensitivity of the radio receiver used to amplify and detect the signals, and the duration of the observation.

For broadband continuum emission the sensitivity also depends on the receiver bandwidth. Because some astronomical radio sources are extremely weak, radio telescopes are usually very large and only the most sensitive radio receivers are used. Moreover, weak cosmic signals can be easily masked by terrestrial radio interference, and great effort is taken to protect radio telescopes from man-made interference.

Observing times up to many hours are expended and sophisticated signal-processing techniques are used to detect astronomical radio signals that are as much as one million times weaker than the noise generated in the receiver.

Signal-processing and analysis are usually done in a digital computer. Although some of the computations may be carried out by microcomputers (i.e., those of the personal-computer class), other tasks require large, high-speed machines to translate the raw data into a form useful to the astronomer.

Interferometers

Interference is the net effect of the combination of two or more wave trains moving on intersecting or coincident paths. The effect is that of the addition of the amplitudes of the individual waves at each point affected by more than one wave.

If two of the components are of the same frequency and phase (i.e., they vibrate at the same rate and are maximum at the same time), the wave amplitudes are reinforced, producing constructive interference; but, if the two waves are out of phase by 1/2 period (i.e., one is minimum when the other is maximum), the result is destructive interference, producing complete annulment if they are of equal amplitude.

Radio interferometers consist of two or more widely separated antennas connected by transmission lines. With their greatly increased resolving power, they can be used to determine the position or diameter of a radio source or to separate two closely spaced sources.

Aperture Synthesis

It is possible to combine the data from two or more telescopes in such a way as to produce an image whose detail is equivalent to a telescope whose diameter was equal to the separation of the telescopes.

The ability to "see" fine detail in sources depends on the ratio of the size of the telescope to the radio wavelength, and in order to make this as good as possible a method known as aperture synthesis was developed at Cambridge. Many antennas are linked electronically, and the signals are recorded as the antennae are moved relative to each other by moving them along a rail track and by the rotation of the Earth.

The computer then takes all of the data and synthesises a map with as high resolution as we would obtain if we were able to build a much larger dish. This method has been adopted at observatories around the world, and extended to include telescopes operating on different continents or even on satellites.

Phase-array telescopes consist of large numbers of relatively small antenna elements arranged in any of various configurations over a relatively large area, yielding the effective sensitivity and resolution of an antenna much larger than could practicably be built. An example of such a system is the 27-antenna Very Large Array near Socorro, N.M., which is one of the world's largest and most sensitive radio telescopes (see photo above).