Radiometric model for the stereo camera STC onboard the BepiColombo ESA mission

The STereoscopic imaging Channel (STC) is one of the instruments on-board the BepiColombo mission, which is an ESA/JAXA Cornerstone mission dedicated to the investigation of the Mercury planet. STC is part of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) suite. STC main scientific objective is the 3D global mapping of the entire surface of Mercury with a mean scale factor of 55 m per pixel at periherm. To determine the design requirements and to model the on-ground and in-flight performance of STC, a radiometric model has been developed. In particular, STC optical characteristics have been used to define the instrument response function. As input for the model, different sources can be taken into account depending on the applications, i.e. to simulate the in-flight or on-ground performances. Mercury expected radiance, the measured Optical Ground Support Equipment (OGSE) integrating sphere radiance, or calibrated stellar fluxes can be considered. Primary outputs of the model are the expected signal per pixel expressed in function of the integration time and its signal-to-noise ratio (SNR). These outputs allow then to calculate the most appropriate integration times to be used during the different phases of the mission; in particular for the images taken during the calibration campaign on-ground and for the in-flight ones, i.e. surface imaging along the orbit around Mercury and stellar calibration acquisitions. This paper describes the radiometric model structure philosophy, the input and output parameters and presents the radiometric model derived for STC. The predictions of the model will be compared with some measurements obtained during the Flight Model (FM) ground calibration campaign. The results show that the model is valid, in fact the foreseen simulated values are in good agreement with the real measured ones.