This paper deals with the optical design and preliminary optomechanical tolerances of HRIC, the High Resolution Imaging Channel 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 (Principal Investigator: E. Flamini), the project is based on an international co-operation with Institutes from France and Switzerland. Starting from the stringent scientific requirement of 5m ground pixel scale at 400 km from the planet surface, a robust optical design based on a catadioptric Ritchey-Chretien configuration modified with a dedicated corrector camera has been achieved. The optimized configuration is convenient in terms of image quality, number of optical elements, and total length. The channel guarantees a corrected FoV of about 1.5° and allows the achievement of the required resolution with a detector of 2k × 2k pixels. The telescope is diffraction limited, thanks to its focal ratio (F/8), and shows an optimised radiometric flux within the operative spectral range (400 - 900 nm). The channel is equipped with one panchromatic and 3 selective filters. The operation plan foresees the coverage of at least 20% of the whole Hermean surface with the HRIC. The preliminary optomechanical tolerances and the corresponding image quality have been analyzed. Further thermo-mechanical analysis is in progress, which is being analyzed by means of ray-tracing tools for image quality evaluation.