Radio telescopes

A radio telescope is used in radio astronomy to measure the intensity of the radiation received from various parts of the sky. Such a telescope must be able both to detect and to locate faint radio sources of small angular size, and also to measure the brightness distribution across extended radio sources or over large sky areas. Ideally the telescope should be capable of making such measurements over a wide frequency range and for different types of polarization of the incoming waves. The noise powers available in radio astronomy are very small, and some of the radio sources have angular sizes or angular structure of, perhaps, only one second of arc, so that a radio telescope needs both high gain and good resolving power. The paper describes various types of radio telescopes which have been built and tested, and outlines the astronomical needs which they fulfill. The parabolic reflector antenna is first described, with particular reference to the fully steerable 210-foot telescope at the Australian National Radio Astronomy Observatory and to the 300-foot transit telescope at the U. S. National Radio Astronomy Observatory. Of the telescopes which use fixed or partly fixed reflector surfaces, those at the University of Illinois, at the Nançay station of the Paris Observatory, and at the Arecibo Ionospheric Observatory in Puerto Rico are described in some detail. Instruments in which the resolution is improved without a corresponding increase of collecting area, such as the cross-type antennas, are briefly described. The future progress of radio telescope design is certain to follow the development of parabolic dishes to still greater sizes, and the exploitation of synthetic antenna systems; the article concludes with a survey of both developments.