Data Set Overview  :  This data set contains the digital numbers (DN) contained in the  telemetry (TM) packages of the VIR-IR instrument on board of the  spacecraft DAWN. The data are from the cruise phase from launch to  Vesta (Sep 2009-May 2011), and Vesta to Ceres (Sep 2012-Dec 2014).  The data in this dataset are stored in 3-D files (spatial, spectral,  time) referred to as cubes. These data were not acquired to do  science. Their function was to determine the instrument alignment  early in the mission and then to periodically verify the functionality  of the instrument and exercise various mechanisms including the  cryocooler, etc.   Required Reading  :  The format of all VIR data are described in detail by the SIS document  located in the /DOCUMENT directory. VIR team strongly recommends  reading this documentation before using the data.    VIR Cruise Observations  :  The following table contains a listing of VIR cruise calibrations   -------------------------------------------------------------------------  Dawn Cruise Calibration Observations  ------------------------------------------------------------------------  Start Mission Activity Activity  Date Phase Name Description  :  07-09-27 ICO ICO Begin Initial Checkout  -------------------------------------------------------------------------  07-10-17 ICO FUNC VIR Functional Test  07-12-11 ICO STAR_CAL 33 IR & VIS Cubes centered on Arcturus  -------------------------------------------------------------------------  08-01-19 EMC EMC Begin Earth-Mars Cruise  -------------------------------------------------------------------------  08-04-07 EMC CAL Full scan/slew cubes of Canopus & Mars  -------------------------------------------------------------------------  09-03-23 MVC MVC Begin Mars-Vesta Cruise  -------------------------------------------------------------------------  09-12-03 MVC GEOCAL Scan/slew cubes of Canopus (IR) to compute  mounting geometry - unsuccessful  10-04-26 MVC GEOCAL Scan/slew cubes of Canopus (IR) to compute  mounting geometry - unsuccessful  10-05-10 MVC MMConfig 6Gb memory configuration test  10-07-21 MVC GEOCAL Scan/slew cubes of Canopus (IR) to compute  mounting geometry - succeeded  11-03-15 MVC FUNC Pre-Vesta VIR Functional Test  -------------------------------------------------------------------------- 12-09-10 VCC VCC Begin Vesta-Ceres Cruise  -------------------------------------------------------------------------- 12-12-17 VCC FUNC VIR Functional Test  13-11-16 VCC FUNC VIR Functional Test  14-06-23 VCC FUNC VIR Functional Test ------------------------------------------------------------------------------  The first two geometry calibrations did not provide the data needed to  determine the sensor alignments, primarily because the cubes acquired  were not large enough to account for all of the pointing error and the  targets were not observed. The ephemeris related pointing error can be  especially large and unpredictable in a low thrust mission.   Processing  :  The data in this data set are raw, CODMAC Level 2 data. As such, no  processes have been applied to the data.    Data  :  The data are in PDS format. A detailed description of the data format  is given in the VIR SIS document located in the DOCUMENT directory of  this data set.    Ancillary Data  :  No ancillary data included in this data set.    Coordinate System  :  The geometry items SC_SUN_POSITION_VECTOR,  SC_TARGET_POSITION_VECTOR and SC_TARGET_VELOCITY_VECTOR  provided in the label of the data product are relative to the  Earth Mean Equator frame of J2000.

Confidence Level Overview  :  Data confidence is nominal, meaning that all data have been  validated by the VIR team and found as expected. Any exceptions  are noted by the DATA_QUALITY field in the label files.   Data Coverage and Quality  :  Data quality is good unless otherwise noted by the DATA_QUALITY  keyword.   Review  :  These are the same data files that were utilized by the Dawn Science  Team. Any errors or deficiencies reported by the Science Team were  corrected by the VIR team.   Unexplained variations in star spectra  --------------------------------------  At the PDS review of the VIR cruise data in July 2015, it was noted  that there are significant variations in the shape of the radiance  spectra for various observations of the star Canopus. Assuming the  instrument is well characterized and a good calibration is in place,  reviewers expected that all observations of the same star would  produce similar radiance curves. Reviewers requested an explanation  of source of the variation.   Inquiries to the VIR team produced this response: ''These kind of  targets have to be considered as sub-pixel targets, so they can't be  calibrated using the current responsivity file, suitable for full  pixels only. Indeed, our main purpose when observing these kind of  target, is the measurement of geometric alignment rather than radiance  value.''   The reviewers then noted that whereas radiance values are meaningless  for sub-pixel sources and that they would need to be converted to flux  to be meaningful, this would not change the shape of the spectrum but  would just scale it. They suggested that possible candidates for  effects which could change the shape of the spectrum include 1) the  spectral dispersion may not be aligned with the detector pixels;  2) the conversion to radiance may be nonlinear and not well understood;  or 3) the ITF may not be fully characterized outside the conditions  expected for 'normal' operations, which is understandable but which  should be documented so it is understood where the calibration can  be applied.   Reviewers requested a better explanation of the source of the variation,  and also requested that if there is a certain DN or radiance limit  under which the ITF is no longer appropriate, that this limit be  documented so users can understand under what circumstances the  calibration can be used.   The VIR team responded to this follow-up inquiry with this  information: 'The radiance values are meaningless for sub-pixel  sources (stars), as the responsivity matrix is not applicable for  such targets. The only purpose when observing this kind of targets  is the geometric calibration. So the Level 1B data of these  observations cannot be used to compare the spectra with other  observations performed in nominal conditions (a fully space resolved  target).'   An internal SBN reviewer noted that if images are undersampled in a  way that compromises the photometry, the astrometry is also compromised  as there is covariance between astrometry and photometry in undersampled  images.   These variations in the shape of the calibrated spectra of different  observations of the same star have not been satisfactorily explained,  so we provide this note to document the issue.  -----------------------------------------------   Limitations  :  These data were acquired to monitor instrument health and to  determine various geometrical calibrations. These are not intended as  science data.