RADIO TELESCOPESThe first radio
TELESCOPE, built by Karl Jansky in 1929, was originally intended for the purpose of studying
the causes of short-wave interference. It was an ensemble of simple dipole antennas that could be rotated on a track. Most modern instruments are large parabolic reflector radio antennas that focus the radio emission from a small region of the sky to a focal point, where a small antenna captures the radio energy and delivers it to a very sensitive radio receiver. Other types of radio
telescopes include the Mills Cross and the Very Large Array (see National Radio Astronomy Observatory).
Resolution SensitivityThe resolution of a radio telescope is the size of the region in the sky from which the telescope collects radiation. Sometimes called the beamwidth, the area is usually a circular region whose angular size is approximately 57¡ times the ratio of the observed wavelength to the diameter of the telescope. The largest single paraboloidal telescopes have a beamwidth, or resolution, of about one arc-minuteÑabout the same as that of the human eye. Much greater resolution can be obtained with interferometers in which two or more radio telescopes are connected together to simulate the performance of a much larger telescope. In this case the resolution is 57¡ times the ratio of the wavelength to the maximum separation of the telescopes used in the interferometer. This separation may be thousands of meters or even an intercontinental distance, leading to resolutions as small as a few ten-thousandths of an arc-second.The sensitivity of a radio telescope refers to the faintness of the signals that can be detected. It depends on both the energy-collecting area of the telescope and on the radio noise added to the incoming radio signals, primarily by the radio receiver but also to a lesser extent by the antenna itself. Special circuits have been constructed to minimize the effects of this noise, but physical laws prevent its complete elimination. In a high-quality receiver the minimum detectable signal becomes fainter in proportion to the reciprocal of the square root of the bandwidth used and in proportion to the reciprocal of the square root of the time over which the received signal is averaged. Both these qualities are utilized in modern radio telescopes to improve sensitivity. In some cases bandwidths of hundreds of Megahertz are used. In such cases, signal averaging times may be as much as 30 hours.Astronomers prefer to build their radio telescopes on as large an area as possible, in order to capture the maximum amount of energy, and with energy-collecting elements that are constructed to precise geometric configurations that allow operation at higher radio frequencies and provide better resolution. They have found that it is particularly important to design structures that preserve their precise geometry in the presence of the changing force of gravity as the structure moves, the force of the wind, and the thermal deflections due to uneven heating of the telescope structure.