ИСТИНА

#215.06 HDAC is part of the ultraviolet imaging spectrometer (UVIS) onboard the Cassini spacecraft. The instrument scans the Lyman-α emission lines of hydrogen and deuterium atoms. In the photometer mode only the CEM detector is used to register the signals within a 3 degree field of view. HDAC has been switched on in photometer mode most of the time producing a unique continuous data set for more than a decade. An analysis of the Lyman-α background data serves two purposes: determination of the parameters of the interstellar/interplanetary hydrogen and determination of the properties of the solar wind. The exhaustive pre-flight laboratory calibrations included evaluation of the absolute sensitivity of the instrument; evaluation of the instrument spectral sensitivity; evaluation of the off-axis response. During the mission these characteristics may change over time due to continuous time degradation of electronics and/or abrupt events. For example, three dramatic sensitivity breakdowns were observed in 2001. Thus the only chance to determine the current sensitivity of HDAC is to make in-flight comprehensive evaluation, e.g. measuring known fluxes from stars or other bodies. We systematically analyzed photometric observations of the star SPICA in order to perform in-flight calibrations. All three aspects listed above were explored. We found that the instrument is still in good condition. The current sensitivity of 12 count/s/Rayleigh is still sufficient to provide good signal to noise data. Off-axis responsivity is non-uniform and visibly differs from pre-flight determinations. At the same time the shape of the spatial sensitivity response is constant and can be used for all observations. The calibrated data are compared with sophisticated theoretical models describing the spatial distribution of interstellar/interplanetary hydrogen. First results will be reported.