Principles of Interference

Abstract

Radiotelescopes have a diffraction limited resolution, as contrasted with optical telescopes whose resolution is limited by atmospheric irregularities. Hence, from the early days of radio astronomy methods to overcome the mechanical and economical limits to the size of single dish radio telescopes have been devised. Many ingeneous schemes have been and are still being used. The correlation interferometer, consisting of two separate elements connected by cables and receiving equipment to produce the product of the signals received by the individual elements, is one of the most successful schemes. The resolution of such an interferometer is determined by the separation of the elements, rather than by the size of the individual elements. By varying the distance between the elements (the “baseline”), and its orientation with respect to the sky, the autocorrelation function of the electromagnetic field can be measured over an area with a. diameter up to the longest baseline used. The sky brightness distribution is the Fourier transform of this autocorrelation function. A map of the sky with a field of view equal to the resolution of the individual elements, but with a resolution depending on the size of the synthesized autocorrelation aperture, can thus be obtained.