From the optical characteristics of Herschel-hifi to in-orbit spatial response calibration using Mars

Making use of all Mars observations taken by Herschel-HIFI, we present a detailed comparison of the measured and predicted beam profiles observed in space. Starting with a number of instrument beam profile models, including one based on actual wavefront measurements in the 480-1625 GHz frequency range, we developed a forward model predicting the detailed characteristics of the beam profile at the sky. Once convolved with the actual brightness temperature distribution and appearance of Mars, this forward model can be compared in detail with the observed spatial response towards Mars, using the science data of HIFI. Since the HIFI instrument is a single-mode coherent receiver, and given the availability of phase-sensitive measurements at instrument level, the results suggest that it is possible to extract some interesting optical characteristics of the Herschel telescope in addition. We discuss a possible way to extract the location of the telescope focus from these observations, which could provide useful feedback to the thermo-elastic properties of the SiC telescope assembly under zero-g and cryogenic conditions.