The Narrow Angle Camera of the MPCS suite for the MarcoPolo ESA mission: requirements and optical design solutions

Possible optical designs of a Narrow Angle Camera (NAC) suitable for being the high resolution channel of the MarcoPolo Camera System for the MarcoPolo ESA mission are presented. The MarcoPolo mission objective is the rendezvous with a Near Earth Asteroid in order to fully characterize the body, to land on the surface and to return to Earth a sample of the asteroid soil. Science goals for the NAC are global mapping of the object, detailed investigations of the surface at high spatial resolution (order of millimeters), and deep examination of possible landing sites from a close distance. The instrument has a 3”/pixel scale factor, corresponding to 80 mm/px at 5 km from the surface, on a 1.75° × 1.75° FoV; imaging in 5 to 8 different spectral bands (panchromatic and broadband), in the range between 400 and 900 nm, is foreseen. Since the target is an extended low contrast object, to avoid image contrast degradation, only off-axis unobstructed optical layouts have been considered. Solutions with two mirrors plus a refractive corrector, or allreflective three mirrors ones, have been studied, both allowing to reach good aberration balancing over all the field of view: the diffraction Ensquared Energy inside one pixel of the detector is of the order of 70%. 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