Optical design of the single-detector planetary stereo camera for the BepiColombo European Space Agency mission to Mercury.

We present the catadioptric optical design solution for the stereo channel of the imaging system SIMBIOSYS for the BepiColombo European Space Agency mission to Mercury. The main scientific objectives of the instrument are the three-dimensional global mapping of the entire surface of Mercury in the panchromatic band and imaging of selected areas in four broad colored bands; both tasks have to be accomplished with a scale factor of 50?m per pixel at periherm. The system consists of an original compact layout in which the two stereo subchannels share a common detector; also, the optical components are common to the two subchannels, with the exception of the first element, which is a rhomboid prism. The field of view of each subchannel is about 5 degrees x5 degrees with a scale factor of 23 arcsec/pixel. The ray-tracing simulation of the system shows that the design guarantees optimal aberration balancing over the entire field of view and the entire wavelength range covered by the instrument, with ensquared energy of the order of 80% in one pixel.