A novel optical design for planetary surface stereo-imaging: Preliminary design of the stereoscopic imaging channel of SIMBIOSYS for the BepiColombo ESA mission

The optical design of the STereoscopic imaging Channel (STC) of the imaging/spectroscopic system SIMBIOSYS for the ESA BepiColombo mission is presented. The main aim of this system is the global stereo mapping of planet Mercury surface during the BepiColombo mission lifetime. The instrument consists of two identical cameras looking at ±20° from nadir which are sharing some optical components and the detector. The instrument has a 23"/pixel scale factor, corresponding to 50 m/px at 400 km from the surface, on a 4° × 4° FoV; imaging in four different spectral bands, between 540 nm and 890 nm, is foreseen. The STC optical characteristics guarantee global stereo mapping of the whole Mercury surface with all the filters. The coupling of an achromatic air-spaced doublet with a relay lens system allows good aberration balancing over all the field of view: the diffraction Ensquared Energy inside one pixel of the detector is of the order of 80%. In addition, an intermediate field stop gives the possibility of designing an efficient baffling system for straylight rejection. To cope with the hazardous radiation environment in which the spacecraft will be immersed in during the mission, all the glasses selected for the design are rad-hard type. A preliminary tolerance analysis has also been undertaken showing a low criticality level for manufacturing, alignment and stability of the system.