An Introduction to Radio Astronomy

Radio wavelengths are hundreds to millions of times longer than optical wavelengths. Consequently, all single-aperture radio telescopes are hindered by severe diffraction effects, and their angular resolution is crude by optical standards. The application and development of radio interferometry, building on the rapidly developing arts of electronics and signal processing, overcame this handicap. In their 1947 studies of the Sun, Pawsey, McCready and Payne-Scott recognized that an interferometer's response to an extended source amounted to determining a particular value of the Fourier transform of the source brightness distribution. This insight was broadly recognized in the radio-astronomy community, and informed much of the work in Sydney, Cambridge and Manchester.

As the art of interferometry progressed, interferometers were used with multiple spacings to develop approximate Fourier transforms of extended sources, which could be inverted to give maps of the brightness distribution. Fourier concepts, reviewed in Chapter 3, became the natural language for discussing the brightness distribution across sources. Finally, Ryle formulated Earth-rotation synthesis, in which the rotation of the Earth is used to vary the orientation and length of interferometer baselines, yielding an extensive sampling of the Fourier transform of the sources each day. The aperture-synthesis arrays, such as the Westerbork Synthesis Radio Telescope in the Netherlands, the Merlin array in the UK, the Australia Telescope and the Very Large Array in the USA, all use this principle, with each possible pair of array elements forming a separate two-element interferometer.