Some technical details
VIRTIS combines three data channels in one compact instrument. Two of them are devoted to spectral mapping (Mapper optical subsystem: -M), the third channel is devoted to spectroscopy (High resolution optical subsystem:-H). The -M utilizes a silicon charge coupled device (CCD) to image from 0.25 micron (1 micron is 1/1.000.000 m) to 1 micron and a mercury cadmium telluride infrared focal plane array (IRFPA) to image from 0.95 micron to 5 micron. The -H employs the same HgCdTe IRFPA to perform spectroscopy from 2 to 5 micron. The -M and -H optical subsystems are housed inside the Cold Box of the Optics Module. The Optics Module is externally mounted to the -X panel of the spacecraft with the -M and -H co-aligned and boresighted in the positive Z direction. Both optical systems have their slits parallel to the Y axis; the -M has the ability to point and scan by rotating the primary mirror around the Y axis. The Optics Module is electrically connected by the Inter-Unit Harness.to the -M and -H Proximity Electronics Modules and to the Main Electronics Module, which are internally mounted to the spacecraft on its -Y panel. The electronics to drive the CCD and the two IRFPAs is housed inside the Proximity Electronics Modules, while the remaining electronics boards are housed inside the Main Electronics Module. Both IRFPAs require active cooling to minimize the detector dark current (thermally generated Johnson noise). To minimize the thermal background radiation seen by these two IRFPAs, the Cold Box must be passively cooled to less than 130 K by radiating one of its surfaces toward cold space. While the coolers are housed inside the Optics Module Pallet, which directly interfaces with the warm spacecraft, the cold detectors and optical systems are housed in a cold structure that must be rigidly mounted to the much warmer Pallet while remaining thermally insulated from it.