VIRTIS: Visible and Infrared Thermal Imaging Spectrometer
VIRTIS is an imaging spectrometer that combines three data channels in one instrument. Two of the data channels are committed to spectral mapping and are housed in the Mapper (-M) optical subsystem. The third channel is devoted solely to spectroscopy and is housed in the High resolution (-H) optical subsystem.
The primary scientific objectives of the VIRTIS during the Rosetta mission are:
- To study the cometary nucleus and its environment
- Determine the nature of the solids of the nucleus surface
- Identify the gaseous species
- Characterise the physical conditions of the coma
- Measure the temperature of the nucleus
Secondary objectives include helping with the selection of landing sites and providing support to other instruments.
Tertiary objectives include detection and characterisation activities during asteroid flybys.
The optical subsystems are housed inside a common structure - the cold box - cooled to 130 K 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 (comet) pointing wall of the spacecraft. The electronics module, containing the digital electronics and power supply, is mounted separately.
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 μm to 1 μm and a mercury cadmium telluride (HgCdTe) infrared focal plane array (IRFPA) to detect from 0.95 μm to 5 μm. The IRFPA is cooled to 70 K 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 155 K 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 μm. The detector is cooled to 70 K by a Stirling cycle cooler.
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Last Update: 24 November 2016