urn:esa:psa:context:instrument:ro.virtis 1.0 1.11.0.0 Product_Context urn:nasa:pds:context:instrument:virtis.ro 2021-02-24 1.0 Changed inst LIDs from u:n:p:c:i:instID.scID to u:n:p:c:i:scID.instID Changed LIDs from urn:nasa:pds: to urn:esa:psa: And per "Guide toPDS4 Context Products" v1.7, changed all lidvid_reference to lid_reference urn:esa:psa:context:instrument_host:spacecraft.ro instrument_to_instrument_host Coradini, A., F. Capaccioni, P. Drossart, G. Arnold, E. Ammannito, F. Angrilli, M.A. Barucci, G. Bellucci, J. Benkhoff, G. Bianchini, and 35 others, Virtis : an imaging spectrometer for the Rosetta mission, Space Science Review, Volume 128, Issue 1-4, 529-559, 2007. reference.CORADINIETAL2007 VISIBLE AND INFRARED THERMAL IMAGING SPECTROMETER Imager Spectrometer not applicable not applicable Instrument Overview =================== VIRTIS stands for Visible Infra Red Thermal Imaging Spectrometer, and is part of the ROSETTA orbiter payload. The VIRTIS instrument [CORADINIETAL2007] combines a double capability: (1) high-resolution visible and infrared imaging in the 0.25-5 micron range at moderate spectral resolution (VIRTIS-M channel) and (2) high-resolution spectroscopy in the 2-5 micron range (VIRTIS-H channel). The two channels observe the same 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 micron) and IR (1-5 micron). VIRTIS-H (-H) is a high-resolution infrared cross-dispersed spectrometer using a prism and a grating. The 2-5 micron spectrum is dispersed in 9 orders on a focal-plane detector array. Technical Description ===================== 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. Proximity Electronics Modules Description ----------------------------------------- Each optical head requires specific electronics to drive the CCD, the two IRFPAs, the covers, the thermal control; the PEMs are two small boxes interfaced directly to the S/C and placed in the vicinity of the Optics Module to minimize interference noise. Optics Module Description ------------------------- The -M imaging spectrometer and the -H echelle spectrometer optical heads are located inside the Optics Module, which in turn is divided into two regions thermally insulated from each other by means of MultiLayer Insulation (MLI): the Cold Box and the Pallet. The Pallet is mechanically and thermally connected to the SpaceCraft; inside the Pallet are located the two Stirling cycle cryocoolers. The heat produced by the cryocoolers compressors (a maximum of 24 W in closed loop mode) is dissipated to the spacecraft. The Cold Box contains the two optical heads and its main function is to act as a thermal buffer between the Optical Heads, working at 130 K, and the external environment (the S/C temperature ranges from 250 to 320 K). The Cold Box is mechanically connected to the Pallet through 8 Titanium rods, whose number and size were selected to minimize conductive heat loads and to avoid distorsion upon cooling from room temperature. The structural part of the cold box is a ledge which is supported by the 8 titanium rods; on the ledge the two optical heads are mechanically fixed. Thermal insulation of the Cold Box is improved by means of MLI, while thermal dissipation from the Cold Box is achieved by means of a two stage passive radiator: the first stage keeps the Cold Box temperature in the range 120-140 K, while the second stage is splitted in two parts, one for each optical head, and allows to reach the required 130 K. Another important component of the instrument are the two covers; they provide a double function: protection against dust contamination, internal calibration by means of an internally reflecting surface finish. They use a step motor and their operation is controlled by the PEMs. VIRTIS-M Description -------------------- The VIRTIS-M optical head perfectly matches a Shafer telescope to an Offner grating spectrometer to disperse a line image across two FPAs. The Shafer telescope produces an anastigmatic image, while Coma is eliminated by putting the aperture stop near the center of curvature of the primary mirror and thus making the telescope monocentric. The result is a telescope system that relies only on spherical mirrors yet remains diffraction limited over an appreciable spectrum and field: at +/- 1.8 degrees the spot diameters are less than 6 microns in diameter, which is 7 times smaller than the slit width. The Offner grating spectrometer allows to cover the visible and IR ranges by realising, on a single grating substrate, two concentric separate regions having different groove densities: the central one, approximately covering 30% of the grating area is devoted to the visible spectrum, while the external region is used for the IR range. The IR region has a larger area as the reflected infrared solar irradiance is quite low and is not adequately compensated by the infrared emissions of the cold comet. The visible region of the grating is laminar with rectangular grooves profile, and the groove density is 268 grooves/mm. Moreover, to compensate for the low solar energy and low CCD quantum efficiency in the ultra-violet and near infrared regions, two different groove depths have been used to modify the spectral efficiency of the grating. The resulting efficiency improves the instrument's dynamic range by increasing the S/N at the extreme wavelengths and preventing saturation in the central wavelengths. Since the infrared channel does not require as high a resolution as the visible channel, the lower MTF caused by the visible zone's obscuration of the infrared pupil is acceptable; the groove density is 54 grooves/mm. In any case, the spot diagrams for all visible and infrared wavelengths at all field positions are within the dimension of a 40 microns pixel. For the infrared zones, a blazed groove profile is used that results in a peak efficiency at 5 micron to compensate for the low signal levels expected at this wavelength. VIRTIS-H Description -------------------- In -H the light is collected by an off-axis parabola and then collimated by another off-axis parabola before entering a cross-dispersing prism made by Lithium Fluoride. After exiting the prism the light is diffracted by a flat reflection grating which disperses in a direction perpendicular to the prism dispersion. The prism allows the low groove density grating, which is the echelle element of the spectrometer, to achieve very high spectral resolution by separating orders 9 through 13 across a two-dimensional detector array: the spectral resolution varies in each order between 1200 and 3500. Since the -H is not an imaging channel, it is only required to achieve good optical performance at the zero field position. The focal length of the objective is set by the required IFOV and the number of pixels allowed for summing. While the telescope is F/1.6, the objective is F/1.67 and requires five pixels to be summed in the spatial direction to achieve a 1 mrad2 IFOV (5 x .45 mrad x .45 mrad). Main Electronics Module Description ----------------------------------- The Main Electronics is physically separated from the Optics Module. It houses the Power supply for all the experiment, the cooler electronics, the Spacecraft interface electronics, for telemetry and telecommanding, the interfaces with the Optics Module subsystems, and the DHSU (Data Handling and Support Unit) which is the electronics for the data handling, processing and for the instrument control. The data processing and the data handling activities into the DHSU are performed using an on-line philosophy. The data are processed and transferred to the spacecraft in real time. The mass memory (SSR) of the spacecraft is used to store or buffer a large data volume. The Main Electronics contains no additional hardware component for data processing and compression. All data processing is performed by software. Scientific requirements and activity in the mission phases ========================================================== A summary of the scientific requirements needed to define the characteristics of VIRTIS is given hereafter. Nucleus science objectives: 1- Identification of different ices and ice mixtures and determination of their spatial distribution. This must be done in spite of the very low albedo ( < 0.1), due to dark inclusions present in the ice and the resulting weak ice spectral features, as well as at large distances from the Sun during some phases of the mission. 2- Identification of the carbonaceous materials that probably are mixed with water ice, even in low percentages (<20%). The associated spectral features will be very subdued and their identification, from their shape and centre, will require high S/N (>100). It will be important also to determine the overall continuum slopes of the spectra, for the presence of organic compounds will redden the spectra in diagnostic ways between 0.4 and 1.1 ?m and into the IR as well. 3- Determination of the physical microstructure and nature of the surface grains by measuring the spectrophotometric phase curve with a relative radiometric accuracy of about 1-%. Identification of the silicates, hydrates and other minerals which are expected to be found. Strong but broad absorption features should be observed from the visible (0.25-1.0 micron) to the near IR (1.0-5.0 micron), so a spectral resolution of 100 is adequate. 4- Determination of the spatial distribution of the various mineralogical types and their mixtures using both the spectral features and the overall brightness. Local fluctuations of the reflectivity and spectral features related to small scale compositional differences are expected. For global mapping of the nucleus, a spatial resolution of a few tens of metres is needed. 5- Detection and monitoring of active areas on the comet surface (cryo-volcanism) to understand the physical processes operating and to identify the material types associated. Determination of the composition of ices on the nucleus surface. With the exception of H2O, condensed molecules display diagnostic narrow (as well as broad) absorption bands in the reflected component (1-3 micron). S/N higher than 100 and a minimum resolving power of 1000 is needed to resolve the bands fully. The -H FOV must be within the field of view of one -M frame (accuracy of few arcminutes) to allow combined operation at both high spatial and spectral resolution. Coma science objectives: 1- Determination of the global distribution of gas and dust in the inner coma. Radiometric accuracy of 10% absolute and 1% relative with a resolving power of 100 are required. 2- Identification and mapping of the strong molecular emissions in the near ultraviolet and visible portions of the spectrum. These include the main water dissociation product OH (0.28 and 0.31 micron), CN, C3, NH, CH and CO+ ions, and the neutral radicals CN and C2. The combination of high spatial resolution with moderate spectral resolution will allow correlation of the evolution of radicals with that of their parent molecules. 3- Mapping of the composition and evolution of gas and dust jets in the coma and comparison with the mineralogical composition and spatial morphology of active regions on the surface of the nucleus. 4- Determination of the composition of the dust grains in the coma by observing emission features in the fundamental bands between 2.5-5 micron. These emissions will be observed mainly at the end of the mission, at distances from the Sun of less than 2 AU. 5- Identification of the gas molecules. The 2-5 micron range corresponds to the maximum efficiency of molecular resonant fluorescence at fundamental rotational/vibrational bands. A major objective is to identify the hydrocarbon emission in the 3-4 micron range. Such identification requires a resolving power of the order of 2000 at 3.5 micron. 6- Assessment of the coma thermodynamic with the determination and mapping the rotational temperatures for the individual lines of H2O with an accuracy of 1 K and a nominal S/N of 200. 7- Detection of early gaseous activity: Spectra will be obtained at long integration times, starting before the mapping phase, to search for CO emission as an indicator of beginning activity. Determination of ortho/para ratio for H2O. Long integration should allow a determination of the OPR with a S/N ratio of 50 to discriminate among different scenarios of comet formation. 8- Determination of isotopic ratios for selected molecules, such as 13CO, H217O, H218O or HDO. Calibration measurements ======================== Before the integration of the VIRTIS experiment aboard Rosetta, a full calibration of the instrument was performed to completely characterize the instrumental performances. The fundamental calibrations necessary to correctly retrieve the scientific information from VIRTIS data are: 1- Geometric calibration: measurement of IFOV, FOV and in field distorsions; 2- Spectral calibration: correlation between spectral dispersion axis of the focal planes with wavelength; 3- Spatial calibration: evaluation of the flat-field matrices necessary to homogenize the focal planes responses; 4- Radiometric calibration: determination of the Instrument Transfer Function (ITF), that allows to convert digital numbers (DN) in physical units of spectral radiance (W m-2 micron-1 sterad-1). These quantities, continuously checked during the flight at each switch on of the experiment, are used in the data pipeline before the scientific analysis. Principal Investigator ====================== Angioletta Coradini Istituto Nazionale di Astrofisica, Rome