An Introduction to Radio Astronomy

The Michelson interferometer, whose basic properties were reviewed in the preceding chapter, was originally designed to measure the angular diameters of stars. It was first used in the radio domain by Ryle and Vonberg (1946) to find the angular diameter of sunspot radiation, and by McCready et al.(1947), who showed that interferometry could be used to make a map of the radio emission from the whole Sun. At radio wavelengths of the order of 1 m, no single aperture could map the Sun with enough angular resolution to be interesting, because of diffraction. The use of Michelson interferometry turned out to be an effective tool to obtain the necessary angular resolution. The technique was soon applied to study discrete radio sources, and, from these modest beginnings, large and complex interferometer systems have been built to map the distribution of brightness across small diameter radio sources, overcoming the limitations of diffraction that are inherent in single aperture telescopes. The resulting angular resolution now exceeds the resolving power of the largest optical telescopes.

The essential link between interferometer observations and the brightness distribution of a source is the Fourier transform, as the analysis in Section 5.4 has demonstrated: the amplitude and phase of the fringe visibility, defined by Equation (5.18), give one complex Fourier component of the brightness distribution. An array of radio telescopes, their outputs separately amplified and combined pairwise to form all possible interferometric combinations, is called an aperture-synthesis array.