Precise measurement of the angular positions of stars and other cosmic objects is the concern of astrometry Radio interferometry enables such studies to be made. It is also very important to be able to measure parameters such as intensity, polarization, and frequency spectrum with similar angular resolution in both the radio and optical domains. For progress in astronomy, it is particularly important to measure the positions of radio sources with sufficient accuracy to allow identification with objects detected in the optical and other parts of the electromagnetic spectrum. In the optical range, the diffraction limit of large telescopes (diameter ∼ 8 m) is about 0.015 ′ ′, but the angular resolution achievable from the ground by conventional techniques (i.e., without adaptive optics) is limited to about 0.5 ′ ′by turbulence in the troposphere. For example, the beamwidth of a 100-m-diameter antenna at 7-mm wavelength is approximately 17 ′ ′. Practical considerations limit the resolution to a few tens of arcseconds. Of a single radio antenna is insufficient for many astronomical purposes. Radio interferometers and synthesis arrays, which are basically ensembles of two-element interferometers, are used to make measurements of the fine angular detail in the radio emission from the sky. This process is experimental and the keywords may be updated as the learning algorithm improves. These keywords were added by machine and not by the authors. Many of the principles of interferometry have counterparts in the field of X-ray crystallography (seeBeevers and Lipson 1985). The techniques of radio interferometry developed from those of the Michelson stellar interferometer without specific knowledge of the van Cittert–Zernike theorem. 15), derived in the 1930s in the context of optics but not widely appreciated by radio astronomers until the publication of the well-known textbook Principles of Opticsby Born and Wolf ( 1959). The basic formulation of this principle is called the van Cittert–Zernike theorem This Fourier transform is normally called the fringe visibility function, which in general is a complex quantity. As an introduction, we consider in this chapter the applications of the technique, some basic terms and concepts, and the historical development of the instruments and their uses.The fundamental concept of this book is that the image, or intensity distribution, of a source has a Fourier transform that is the two-point correlation function of the electric field, whose components can be directly measured by an interferometer. The uses of such measurements lie mainly within the domains of astrophysics, astrometry, and geodesy. The subject of this book can be broadly described as the principles of radio interferometry applied to the measurement of natural radio signals from cosmic sources.
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