A single very large collecting area would, by itself, lack angular resolution and be confusion limited for many observations and therefore inappropriate. Instead, a number of telescope elements will have to be joined interferometrically to synthesize a larger aperture. The need for aperture synthesis implies that long baseline tracks are required and that the individual telescope elements must be steerable over a large part of the sky. A telescope array with this capability can then also be combined with existing telescopes used for Very Long Baseline Interferometry (VLBI), including telescopes in space. Amongst the benefits would be a great improvement in VLBI polarization measurements which would bear on the understanding of the physical structure of jets in active galactic nuclei and other objects (Gabuzda et al. 1992; Kemball et al. 1996). It might also allow observation of the transverse velocity component of HO masers orbiting in external galaxies. This observation, when compared statistically with spectroscopically determined radial velocities, could lead to unbiased distance estimates for external galaxies (Reid & Moran 1988).
Reber (1940) built the first radio telescope to use a paraboloidal reflector almost 60 years ago. Fully steerable paraboloids have subsequently been the mainstay of radio astronomy for wavelengths ranging from metres to sub-millimetres. As a result of this widespread use, construction of steerable paraboloids has been well optimized to minimize cost and not very much additional saving can be expected.
Very little of the cost and weight of a steerable paraboloid is in the reflecting surface itself. The part that is expensive is the structure needed to support and move the reflector. It is clear that there are potentially very large savings in keeping the reflector close to the ground and supported by the ground and by avoiding the heavy rotating machinery of a steerable paraboloid. This was done with the Ohio State telescope (Kraus & Nash 1961), the Ratan 600 telescope in Russia (Korolkov & Pariiskii 1979), the Arecibo telescope (Gordon & Lalonde 1961), and Nançay telescope (Blum et al. 1963). All of these telescopes have provided large collecting areas relatively inexpensively, though at the price of limited sky coverage and therefore limited usefulness in aperture synthesis.
A different type of reflecting telescope is discussed here which may provide an economical means of achieving both a very large collecting area and the good sky coverage needed for modern aperture synthesis. Some early ideas on the scheme have already been briefly outlined (Legg 1996).
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