astro14

In 1868 Fizeau noted that the purpose of the arrangement of mirrors or glass lenses in a conventional telescope was simply to provide an approximation to a Fourier transform of the optical wave field entering the telescope. As this mathematical transform was well understood and could be performed mathematically on paper, he noted that using an array of small instruments it would be possible to measure the diameter of a star with the same precision as a single telescope which was as large as the whole array, a technique which later became known as astronomical interferometry. It was not until 1891 that Michelson, successfully used this technique for the measurement of astronomical angular diameters, those of Jupiter's satellites (Michelson 1891). Finally, 30 years later, a direct interferometric measurement of a stellar diameter was realized by Michelson & Pease (1921) with their 20 ft (ca. 6.1 m) interferometer mounted on the 100 inch Hooker Telescope on Mount Wilson.

The next major development came in 1946 when Ryle and Vonberg (Ryle and Vonberg 1946) constructed a radio analogue of the Michelson interferometer and soon located a number of new cosmic radio sources. The signals from two radio antennas were added electronically to produce interference. Ryle and Vonberg's telescope used the rotation of the Earth to scan the sky in one dimension. With the development of larger arrays, and of computers which could rapidly perform the necessary Fourier transforms, the first aperture synthesis imaging instruments were soon developed, which could obtain high resolution images without the need of a giant parabolic reflector to perform the Fourier transform. This technique is now used in most radio astronomy observations. Radio astronomers soon developed the mathematical methods to perform aperture synthesis Fourier imaging using much larger arrays of telescopes, often spread across more than one continent. In the 1980s the aperture synthesis technique was extended to visible light and infrared astronomy providing the first very high resolution optical and infrared images of nearby stars.

In 1995 this imaging technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same techniques have now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer, the CHARA array and the IOTA array. A detailed description of the development of astronomical optical interferometry can be found here.