Measuring the surface accuracy of the Effelsberg 100-m radio telescope

As described by Ruze (Proc. IEEE 54, p. 633, 1966), the gain of a radio telescope does strongly depend on the surface accuracy of the main dish. When a large reflector is being tilted, gravitational effects lead to changing deformations of the surface, which have to be taken into account for a proper calibration of the astronomical observations. Therefore, the design of the Effelsberg 100-m radio telescope incorporated a specific construction scheme, called “homologous distortion”: With changing elevation, the deformations of the dish led always to a new paraboloid (with different focus position).