Abstract :
The techniques used to remeasure the profile of the 25 m paraboloidal reflector on the UK Science Research Council steerable aerial at Chilbolton, Hants, England, are described, and the difficulties in obtaining such measurements are pointed out. Two methods are discussed: first, a specially constructed optical range finder for determining the co-ordinates of points on the reflector with respect to the vertex of the best-fit paraboloid, and secondly a laser system for determining the changes in reflector shape brought about by tilting the aerial through 90°. The procedures for calibrating these instruments which led to final r.m.s. errors of 0.36 and 0.4 mm, respectively, are outlined. The errors in profile of the reflector were found to be of a systematic rather than a random nature, and are discussed in relation to the radiation pattern measured at a wavelength of 3.1 cm. The change in reflector shape brought about by tilting through 90° is shown to be 1.3 mm r.m.s., which implies that, if the profile were readjusted to be perfect at the zenith, the gain at 0° elevation should be optimum (nearly 72 dB) at a wavelength of 9 mm.
