VIRTIS is an imaging spectrometer that combines three observing channels in one instrument. Two of the channels are devoted to spectral mapping (mapper optical subsystem), while the third channel is devoted to spectroscopy (high resolution optical subsystem).

The optical subsystems are housed inside a common structure - the cold box - cooled to 130K by a radiative surface supported on a truss having low thermal conductivity. On the pallet supporting the truss, two sets of electronics and two cryogenic coolers for the detectors are mounted. The cold box is rigidly mounted on the pallet but thermally isolated from it. The pallet and cold box together form the optics module, which is mounted inside the spacecraft arranged so that the observing axes of the optical subsystems are normal to the nadir pointing wall of the spacecraft. The electronics module, containing the digital electronics and power supply, is mounted separately.

Mapping channel

The mapping channel optical system is a Shafer telescope matched through a slit to an Offner grating spectrometer. The Shafer telescope consists of five aluminium mirrors mounted on an aluminium optical bench. The primary mirror is a scanning mirror driven by a torque motor. The Offner spectrometer consists of a relay mirror and a spherical convex diffraction grating, both made of glass.

The mapping channel utilizes a silicon charge coupled device (CCD) to detect wavelengths from 0.25 micron to 1 micron and a mercury cadmium telluride (HgCdTe) infrared focal plane array (IRFPA) to detect from 0.95 micron to 5 microns. The IRFPA is cooled to 70K by a Stirling cycle cooler. The cold tip of the cooler is connected to the IRFPA by copper thermal straps. The CCD is operated at 155K and is mounted directly on the spectrometer.

High resolution channel

The high resolution channel is an echelle spectrometer. The incident light is collected by an off-axis parabolic mirror and then collimated by another off-axis parabola before entering a cross-dispersion prism. After exiting the prism, the light is diffracted by a flat reflection grating, which disperses the light in a direction perpendicular to the prism dispersion. The low groove density grating is the echelle element of the spectrometer and achieves very high spectral resolution by separating orders seven through sixteen across a two-dimensional detector array.

The high-resolution channel employs a HgCdTe IRFPA to perform detection from 2 to 5 microns. The detector is cooled to 70K by a Stirling cycle cooler.