Planck Telescope In-flight Retrieval
TICRA have done substantial work on the Planck Spacecraft telescope, during the design as well as the operational phases. One task was to determine the accurate shape of the telescope surfaces on-orbit, and use this information to accurately predict the patterns far beyond what can be measured from ground.
The retrieval of the shape of the reflector surfaces is performed by an optimization in which surface perturbations described by Zernike modes are optimized in order to fit the given patterns. The field is determined by Physical Optics and the optimization is based on least squares deviation using one of the best algorithms for non-linear least squares optimization. In the optimization the determined patterns are fitted to the measured patterns within the regions in which the measured patterns are higher than the noise level.
The goal of the Planck Space Telescope is to observe the cosmic microwave background radiation with an unprecedented accuracy. Therefore, it is very important to know the patterns of the antenna with a correspondingly high accuracy. The Telescope is an aplanatic reflector system equipped with 47 microwave detectors in RF bands from 30 GHz to 857 GHz positioned over the curved focal ‘plane’ of the antenna.
RF measurement could not be performed with sufficient accuracy on ground as the working temperature of the telescope is 40 K with the detectors cooled to 0.1 K. In-orbit measurements are thus requested. TICRA developed a technique for in-flight testing of the antenna system. The beams of each detector are obtained using the planets Jupiter, Saturn, and Mars as sources. These maps are fitted to antenna models whereby reflector deformations and misalignments can be retrieved. Accurate beam patterns of each detector may subsequently be determined. In order to test the algorithm, measurement 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.