The Optical Design of the High Resolution Imaging Channel for the SIMBIO-SYS experiment on the BepiColombo Mission to Mercury

This paper deals with the optical design of the High Resolution Imaging Channel (HRIC) of the SIMBIO-SYS instrument, selected as part of the scientific payload for the ESA cornerstone BepiColombo mission to Mercury. Under the lead of Italy, the project is based on an international cooperation with Institutes from France and Switzerland. The HRIC design starts from the scientific requirement of 5 m ground pixel scale at 400 km from the planet surface and is based on a catadioptric Ritchey-Chretien configuration modified with a dedicated three lens corrector. The optimised configuration is convenient in terms of image quality, number of optical elements, total length and mass budget. The channel guarantees a corrected FoV of about 1.47 and allows the achievement of the required resolution with the selected APS hybrid detector of 2k x 2k pixels with a pixel size of 10 m. The telescope is diraction limited, thanks to its focal ratio (F/8), and shows an optimised radiometric flux within the operative spectral range (400 - 900 nm), which guarantees a good S/N. The channel is equipped with 1 panchromatic and 3 selective filters. The operation plan foresees the coverage of at least 20% of the Hermean surface with the HRIC.