Ghost images determination for the stereoscopic imaging channel of SIMBIOSYS for the BepiColombo ESA mission

BepiColombo is the fifth cornerstone mission of the European Space Agency (ESA) foreseen to be launched in August 2014 with the aim of studying in great detail the Mercury planet. One of the BepiColombo instruments is the STereoscopic imaging Channel (STC), which is part of the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS) suite: an integrated system for imaging and spectroscopic investigation of the Mercury surface. STC is a stereo camera consisting of two sub-channels, which are looking at +/-20° from nadir direction, that share the detector and some of the optical components. The main scientific camera objective is the 3D global mapping of the entire surface of Mercury with a scale factor of 50 m per pixel at periherm. Five different spectral bands are foreseen, a panchromatic and four intermediate bands, in the range between 410 and 930 nm. To avoid mechanisms, the technical solution chosen for wavelength selection is the single substrate stripe-butted filter in which different glass pieces, with different transmission properties, are glued together. This peculiar choice for the filter architecture, coupled with the fact that the filter is mounted very close to the detector, creates multiple ghosts which can affect seriously the quality of the images. The intensity and the position of the ghost images generated by each filter strip are discussed. An analysis of the ghosts impact on the imaging performance and the solution adopted to limit the degradation are presented.