Radioastronomical Interferometers

Abstract

As described above, an interferometer requires two similar antenna systems, which are separated by some distance (6,15). The first interferometer constructed for radioastronomy, the sea interferometer of McCready, Pawsey and Scott at Dover Heights near Sydney (8,44) made use of a single antenna, since this instrument made use of a principle familiar in the field of optics. The single antenna had a horizontal beam, and was situated on a high cliff overlooking the sea. A wave packet arriving with a small angle to the horizontal would then arrive at the antenna in two ways, both directly and after reflection by the sea, which is a good reflector for short radio waves at low angles of incidence. This reflected wave packet has, when it reaches the antenna situated 80 metres above the sea, the same time difference relative to direct reception as it would have had if it had continued to an antenna at a height h = -80 metres. The effective interferometer baseline was thus D = 160.m. An antenna which is movable in azimuth allows observation of objects at various positions on the sky. Since the method only functions when the inclination of the arriving radiation to the horizontal is small, one is restricted to observations approximately an hour after rising and before setting of the celestial object. For such measurements at low elevations it is essential to apply corrections for atmospheric refraction (27). This instrument was used for solar measurements and for position and intensity measurements of radio sources (Fig. 7.1). This type of interferometer is analogous to the Lloyd’s mirror interferometer (35,44).