In-flight retrieval of reflector anomalies for the Planck Space Telescope

It is important to know the antenna patterns for the Planck Space Telescope with high accuracy for detectors operating at frequencies from 30 to 857 GHz. This could not be achieved with sufficient accuracy on ground as the working temperature of the telescope is 40 K with the detectors cooled to 0.1K Accurate in-orbit measurements are thus requested. Simulations in which the reflector is displaced and the surface distorted with Zernike modes have been carried out for noise contaminated amplitude measurements of Jupiter by 5 and 10 different detectors. First, the main-beams of the antenna patterns are retrieved in a regular grid. Here the accuracy is limited by the noise level. Then, by a Physical Optics optimization the distortions of the telescope´s reflector are determined so that the radiation patterns of the antenna are correlated to those measured. The patterns for the optimized, retrieved, reflector geometry are shown to be precise at levels far below the noise in the measurements.¹