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Radio Astronomy

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Jennie McGrath
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« Reply #15 on: August 16, 2007, 11:14:38 pm »

History of astronomical interferometry

William Herschel knew as early as 1779 (Herschel 1805) that stars appeared much larger in telescopes than they really were but he did not know why. When Thomas Young demonstrated interference and the wave nature of light unambiguously, this was explained. As he stated in his Bakerian Lecture of 1803: "The proposition on which I mean to insist at present, is simply this, that fringes of colours are produced by the interference of two portions of light", and later: "that homogeneous light, at certain equal distances in the direction of its motion, is possessed of opposite qualities, capable of neutralizing or destroying each other, and of extinguishing the light, where they happen to be united" (Young 1804).

But it was not Young's researches that prompted Herschel to investigate the origin of the spurious diameters of stars. Instead it was the exactly contemporaneous discovery of the ­first minor planets: Ceres in 1801, Pallas in 1802 and Juno in 1803. Were their apparent diameters as real as those of planets or spurious as for stars? To address this question Herschel conducted an extensive series of experiments in his garden in Slough, examining through his telescope small globules of differing sizes and materials placed in a tree some 800 ft (ca. 244 m) away (Herschel 1805). His observations showed that for the smallest globules the diameters were all spurious and all of the same size. Furthermore, he found that, if just the inner part of the aperture of the telescope were used, the spurious diameters, whether of globules or of stars, were larger. If the whole aperture was employed, the diameters were smaller, and if only an outer annular aperture was used the diameters were smaller still. This experimental discovery that unfilled apertures can be used to obtain high angular resolution remains today the essential basis for interferometric imaging in astronomy (in particular Aperture Masking Interferometry). The theoretical justifi­cation of this result came with Airy's analysis of the diffraction pattern of a circular aperture 30 years later (Airy 1835), and it took a further 30 years before the idea of using multiple apertures was developed. In an early study the Reverend W. R. Dawes noted that he had `frequently found great advantage from the use of a perforated whole aperture' and that when observing Venus this produced `a central image of the planet perfectly colourless, and very sharply de­ned' (Dawes 1866). But it was left to Fizeau, in his submission to the Commission for the Prix Bordin the following year, to remark on `une relation remarquable et nŽecessaire entre la dimension des franges et celle de la source lumineuse' and suggest that by using an interferometric combination of light from two separated slits `il deviendra possible d'obtenir quelques donnŽees nouvelles sur les diametres angulaires de ces astres' (Fizeau 1868).

Steps towards the practical implementation of these techniques for optical astronomy were taken by Michelson, who defi­ned the `visibility' of interference fringes obtained from a source of ­finite angular size (Michelson 1890) and followed this a year later with the measurement of the angular diameters of Jupiter's satellites (Michelson 1891). Finally, 30 years later, Fizeau's predictions became a reality when the direct interferometric measurement of a stellar diameter was realized by Michelson & Pease (1921) with their 20 ft (ca. 6.1 m) stellar interferometer mounted on the 100 inch **** Telescope on Mount Wilson.

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