PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2018-03-14 NOTE = "Description of the CALIB directory contents." END_OBJECT = TEXT END The CALIB directory contains all of the files required to prroduce the calibrated VIR IR (1b/RDR) data from the uncalibrated (1a/EDR) data. A document describing the calibration procedure is provided in this archive in the DOCUMENT/VIR_CALIBRATION directory. Additional information concerning instrument calibration and function which may affect data quality (including a description of the contamination issue associated with the IR channel) is provided in the data set catalog file, which is in the CATALOG directory. After operations at the asteroid Vesta, the VIR team identified some artifacts in the instrument responsivity in the 2.5-3.5 micron region, which is where several absorption bands of OH and H2O occur. Those artifacts were systematic, and therefore they did not prevent the detection of relative spectral variations associated with OH and H2O. Actually, the only paper published [DESANCTISETAL2012] in which are discussed spectra in this range used a different calibration method described in the paper itself. This method used a simple correction of the response function based on an empirical calibration correction that is appropriate to recover the signal in that specific range. However, the absolute absorption band depth of OH and H2O could not be calculated. To compute a new instrument responsivity the team used the internal lamp of the spectrometer. This lamp, made of a tungsten filament, is characterized by a black body-like emission at about 2400K (see [MELCHIORRIETAL2003]; a document containing excerpts from MELCHIORRIETAL2003 is provided on this volume in the directory DOCUMENT/VIR_LAMP_DESCRIPTION). Since the spectrum of the infrared radiation emitted by the tungsten filaments is featureless, a polystyrene filter was inserted for a wavelength calibration of the IR channel. The idea was to use the black body radiation of this lamp to retrieve a relative Instrument Responsivity in the 2.5-3.5 micron region. First the signal from the internal calibration lamp was calibrated with the on-ground (V1) responsivity, and the equivalent temperature of the radiation fitting a Planck curve was determined. The next step was to compute the Planck function at the equivalent temperature. The new Instrument Responsivity is the result of the ratio between the raw signal of the lamp and the Planck function. This ratio must be multiplied by a factor to take into account the integration time used to acquire the signal, the transmittance of the polystyrene filter, and the viewing geometry. Examples of polystyrene spectra can be found in [MELCHIORRIETAL2003]. The new instrument responsivity function minimizes most calibration residuals that were showing as artifact peaks between 2.5 and 2.9 microns in the previous calibration. It now allows the use of VIR spectra to identify absorption bands of OH and H2O, without requiring additional empirical corrections. As calibration is always an ongoing process, the instrument team is looking forward to enable the interpretation of absolute absorption band depths, which is a required step for the calculation of surface material abundances. It is VIR team practice to consider the entire collection of calibration files as a single unit so the version number of every file was incremented to V2 even though only the IR responsivity file was updated. References ---------- DESANCTISETAL2012: De Sanctis, M.C., J.P. Combe, E. Ammannito, E. Palomba, A. Longobardo, T.B. McCord, S. Marchi, F. Capaccioni, M.T. Capria, D.W. Mittlefehldt, C.M. Pieters, J. Sunshine, F. Tosi, F. Zambon, F. Carraro, S. Fonte, A. Frigeri, G. Magni, C.A. Raymond, C.T. Russell, D. Turrini, Detection of Widespread Hydrated Materials on Vesta by the VIR Imaging Spectrometer on board the Dawn Mission, Astrophys. J. Lett. 758 L36, doi:10.1088/2041-8205/721/2/L172, 2012. MELCHIORRIETAL2003: Melchiorri, R., G. Piccioni, VIRTIS-M flight lamps, Rev. Sci. Instrum., Vol. 74, Iss. 8, p. 3796, doi: 10.1063/1.1593784, 2003. The following files and their PDS labels are provided here: DAWN_VIR_IR_HIGHRES_SPECAL_V2.TAB Wavelengths of the 432 bands of the infrared focal plane for high spectral modes. DAWN_VIR_IR_NOMRES_SPECAL_V2.TAB Wavelengths of the 144 bands of the infrared focal plane for low spectral modes DAWN_VIR_IR_RESP_V2.DAT DAWN-VIR IR responsivity in (DN*m^2*um*sterad)/(W*s) DAWN_VIR_IR_SOLAR_SPECTRUM_V2.TAB Solar spectrum irradiance for the IR channel, obtained from the convolution of the Cebula+Kurucz spectrum with VIR spectrum resolution using the ENVI_RESAMPLE_SPECTRA procedure. DAWN_VIR_IR_WIDTH144_V2.TAB Wavelengths of the 144 widths of the infrared focal plane for low spectral modes DAWN_VIR_IR_WIDTH432_V2.TAB Wavelengths of the 432 widths of the infrared focal plane for high spectral modes. VIR_IR_CORRECTION_FACTOR.TAB Correction factors that may be applied to each band of the VIR L-1B IR cubes to correct for a positive slope in the VIS-NIR range when compared to ground based spectra of the same target (Vesta and Ceres)