Data Set Overview
  =================
       This data set contains the spectral radiance (W/(m**2*sr*micron))
       data from the Dawn VIR instrument visible channel for all cruise
       mission phases. The data cover the time period between
       2007-10-17 and 2011-03-15.

  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
    =========================================================================
    2017-09-28 ICO    ICO        Initial Checkout
    -------------------------------------------------------------------------
    2007-10-17 ICO    FUNC       VIR Functional Test
    2007-12-11 ICO    STAR_CAL   33 IR & VIS Qubes centered on Arcturus
    -------------------------------------------------------------------------
    2008-01-19 EMC    EMC        Earth-Mars Cruise
    -------------------------------------------------------------------------
    2008-04-07 EMC    CAL        Full scan/slew qubes of Canopus  & Mars
    -------------------------------------------------------------------------
    2009-03-23 MVC    MVC        Mars-Vesta Cruise
    -------------------------------------------------------------------------
    2009-12-03 MVC    GEOCAL     Scan/slew qubes of Canopus (IR) to compute
                                 mounting geometry - unsuccessful
    2010-04-26 MVC    GEOCAL     Scan/slew qubes of Canopus (IR) to compute
                                 mounting geometry - unsuccessful
    2010-05-10 MVC    MMConfig   6Gb memory configuration test
    2010-07-21 MVC    GEOCAL     Full scan/slew qubes of Canopus (IR) to
                                 compute mounting geometry - succeeded
    2011-03-15 MVC    FUNC       Pre-Vesta VIR Functional Test
    --------------------------------------------------------------------------
    2012-09-10 VCC    VCC        Vesta-Ceres Cruise
    --------------------------------------------------------------------------
                             There are no calibrated data from the Vesta-Ceres
                             cruise phase. Activities during this phase did
                             not open the cover.  There are raw (EDR) versions
                             of the check-out data. Functional checkouts were
                             performed on 2012-352, 2013-320, and 2014-174.
    --------------------------------------------------------------------------
    The first two geometric calibrations failed to observe Canopus which
    was being used to determine the sensor alignments, because the cubes
    acquired were not large enough. The combination of ephemeris and pointing
    errors were larger than anticipated at the time when the observations
    were designed. The cube size was increased for the final observation
    which was successful.

  Processing
  ==========
    The data in this data set are calibrated, CODMAC Level 3 data. The
    data processing includes spectral, geometric, flat-field, and
    radiometric calibration.  The details of how these calibrations were
    performed are described in the VIR_CALIBRATION document contained
    the DOCUMENT/VIR_CALIBRATION directory of this archive. This document
    describes how the calibration parameters were derived an gives a
    procedure for generating the calibrated data set from the raw EDR
    data set DAWN-X-VIR-2-EDR-VIS-CRUISE-SPECTRA-V1.0.


  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.


  Calibration
  ===========
    These data all use the same calibration files and procedures that were
    used in the processing of the Vesta data set archived on the PDS volume
    DWNVVIR_V1B. The calibration tables described in the calibration
    procedure can be found on the Vesta volume in the /CALIB directory off
    of the root.  In particular, the labels for the data cubes on this volume
    refer to the instrument responsivity file 'DAWN_VIR_IR_RESP_V1.DAT'.
    This is one of the calibration tables provided on the Vesta data volume.


  Housekeeping Data
  =================
    Housekeeping data from the raw/EDR data set provided here are duplicated
    for the user's convenience.


  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.


  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. Note, however, that the geometry information
    provided in the PDS labels was updated to reflect the IAU (Claudia Double
    Prime) coordinate system and peer review issues have been resolved.
    A full PDS peer review was conducted of all products in this data set.


  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

    - the spectral dispersion may not be aligned with the detector pixels.
    - the conversion to radiance may be nonlinear and not well understood.
    - 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.


  Data Coverage and Quality
  =========================
    Data quality is good unless otherwise noted by the DATA_QUALITY
    keyword.

    Calibrated values in the spectral range [2.534E-06 m to 3.272E-06 m] are
    under verification, as a result these values have been set to null in
    the ITF.


  Limitations
  ===========
    These data were acquired to monitor instrument health and to
    determine various geometrical and spectral calibrations. These
    are not intended as science data.