The interferometer in radio astronomy

A theory is developed for the response of a two-element radio interferometer to a partially coherent field, without restriction as to bandwidth or antenna properties. It is shown that for a completely incoherent source the narrow-band interferometer output is a component of the Fourier transform of the source brightness, which can therefore be mapped by repeated interferometer observations. A partially or completely coherent extended source, on the other hand, cannot be mapped by this technique. The history of radio interferometry in astronomy is discussed and examples are given of the current uses of the technique in the precise measurement of source positions, in the measurement of the diameters of extremely small sources, and in the detailed mapping of complex sources.