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In this paper the adopted optical design solution for the Stereo Channel of the imaging system SIMBIOSYS for the BepiColombo ESA mission to Mercury is presented. The optical design of the camera together with its performance, expressed in terms of optical quality, tolerance and stray-light analysis, are fully described. The main scientific camera objective is the tridimensional 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. The Stereo Channel consists of two sub-channels looking at ±20° from nadir direction, which share the detector and most of the optical components. The field of view of each channel is 4.8° x 5.3° with a scale factor of 22"/pixel. The chosen modified Schmidt configuration guarantees an optimal aberration balancing over all the field of view and all the wavelength range; in addition the technical solution chosen for the filter manufacturing, i.e. single substrate with stripe-butted filters, allows to further optimize chromatic aberration. For stray-light suppression, an efficient baffling system, able to well separate the two optical paths over the common optical elements, has been designed and an appropriate ‘filter masking’ has been foreseen to cope with ghosts and cross talk between adjacent filter stripes. The tolerance analysis shows that manufacturing, alignment and stability tolerances are rather relaxed. Thus concluding, the analysis of the global optical performance of the camera assures that the scientific requirements are optimally fulfilled.
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