Some technical details
VIR is a Visual and Infrared Imaging Spectrometer in a single optical head. A Shafer telescope is mated to an Offner slit spectrometer. Spectral images are formed on two matrix detectors to cover the 0.25-1 µm and 1-5 µm ranges. The design uses a dual arm optical and focal design with mapping ability to 5 µm.The Dawn mapping spectrometer (VIR) is a rebuild of the VIRTIS mapping spectrometer on board the ESA Rosetta mission with an operational durations of 2y and flight time of 13y. The MS spectrometer will only use the – M channel of VIRTIS, thus allowing a great mass reduction. VIR also derives much of its design from the Cassini VIMS spectrometer with an operational lifetime of >4y and a mission life>10y. The spectrometer consists of only three modules: the optical system, 5.0 kg mass; the proximity electronics, 3.0 kg and 5 W; the cryocooler including driving electronics, 1.3 kg and 12.6 W. A mechanical and thermal mounting of 5.0 kg mass accommodates the spectrometer subsystems. The optical system, which includes foreoptics, dispersive elements, filters, focal plane assemblies as well as the cryocooler and proximity electronics is a complete re-build of the VIRTIS M instrument The data processing unit box (DPU) is shared with the framing camera to save mass. The DPU handles the data for both camera and spectrometer, compresses and buffers them, and controls both the FCs and MS. Flight software is based on Rosetta VIRTIS and ROLIS developments. The camera is operated only when the spectrometer is not operating. As VIRTIS-M, VIR does not use beam-splitters. Two different groove densities are ruled on a single grating. The central part of the grating (about 30% of the pupil) is ruled with a higher groove density, which generate the higher spectral resolution needed in the visible channel. The infrared channel utilizes the outer 70% of the grating, which is ruled with a lower groove density. The larger collecting area in the IR compensates for the lower solar irradiance in this region. The visible detector array is based on the Thomson-CSF type TH 7896 CCD detector. The IR detector used in the MS is based on a bidimensional array of IR-sensitive photovoltaic Mercury Cadmium Telluride coupled to silicon CMOS multiplexer. This detector can operate at temperatures of the order of 75 K. For this reason cryocoolers have to be used. The calibration unit will make use of the cover, which -inside is coated. It will diffuse the light coming from two internal calibration lamps (one for the VISFPA and one for the IR-FPA), calibrated at IAS. The VIRTIS instrument combines a double capability: (1) high-resolution visible and infrared imaging in the 0.25-5 mm range at moderate spectral resolution (VIRTIS-M channel) and (2) high-resolution spectroscopy in the 2-5 mm range (VIRTIS-H channel). This improved capability considerably enlarges the scientific return of the instrument. The two channels will observe the same comet areas in combined modes to take full advantage of their complementarities. VIRTIS-M (named -M in the following) is characterised by a single optical head consisting of a Shafer telescope combined with an Offner imaging spectrometer and by two bidimensional FPAs: the VIS (0.25-1 mm) and IR (1-5 mm). VIRTIS-H (-H) is a high-resolution infrared cross-dispersed spectrometer using a prism and a grating. The 2-5 mm spectrum is dispersed in 9 orders on a focal-plane detector array. Both channels re state of the art and their tested performances are extremely good. The instrument is divided into 4 separate modules: the Optics Module – which houses the two -M and -H optical heads and the Stirling cycle cryocoolers used to cool the IR detectors to 70 °K -, the two Proximity Electronics Modules (PEM) required to drive the two optical heads, the Main Electronics Module – which contains the Data Handling and Support Unit, for the data storage and processing, the power supply and control electronics of the cryocoolers and the power supply for the overall instrument.