Data Set Overview
      =================
      This data set contains the infrared images collected and
      archived by the International Halley Watch (IHW) Infrared
      Studies Network (IRSN).  Raw and calibrated data are
      included, as well as calibration observations of standard
      stars.  These data have been reformatted and the
      accompanying documentation updated in this revised version.
 
      Four observatories contributed IR images to the IRSN: the
      Wyoming Infrared Observatory; the NASA Infrared Telescope
      Facility; the Kitt Peak National Observatory; and the
      Whipple Observatory.  The source of each image is indicated
      in both its FITS header and PDS label, as is instrumentation.
 
      Modification History
      ====================
      The PDS sections of this data set have undergone extensive
      revision, while the FITS data files themselves have remained
      almost completely untouched.  The following changes were
      made:
 
      o  The FITS images were originally archived with the header
         and data segment for each image in a separate files.
         In this version the FITS files have been restored to
         proper FITS format.
 
      o  All PDS labels were completely rewritten by extracting
         values from the FITS headers and editing by hand as
         needed.
 
      o  All catalog files have been rewritten from scratch.
 
      o  All index and volume support files have been regenerated.
 
      o  All reported errata were corrected.
 
      Specific changes and enhancements to the labels include:
 
      o  The PROCESSING_LEVEL_ID element has been added to allow
         programmatic differentiation between raw data ('1'),
         calibration data ('2') and calibrated data ('3').  These
         levels correspond roughly to the CODMAC data levels.
 
      o  For calibration target observations, the TARGET_NAME
         element contains 'CALIBRATION' and the specific name
         of the calibration star can be found in the TARGET_DESC
         element immediately following it.
 
      o  All comments and history notes from the FITS headers
         have been repeated in the PDS label as part of the
         DESCRIPTION text.  The only editing performed on these
         comments was to convert double-quotes to two consecutive
         single quotes (because of PDS formatting constraints).
 
      o  Details of observatory location and instrumentation
         have also been added to the DESCRIPTION field.
 
      In addition, known errata were corrected, including both the
      errata noted by Mikael Aronsson at JPL as he prapared the
      IHW Printed Archive and Printed Archive Index documents (and
      included on the HAL_0023 volume), and the errata collected
      after publication at the PDS Small Bodies Node.
 
      General Notes
      =============
      Times
      -----
      The observation times were reported as day fractions with
      anywhere from one to five decimal places of precision.
      These have been converted to the ISO standard format
      (YYYY-MM-DDThh:mm:ss) to a corrsponding level of accuracy.
 
      System Codes
      ------------
      The IHW directives for the SYSTEM keyword in the original
      FITS files mandated a format of 'dooottii', where
      d = discipline, ooo = IAU observatory code, tt = country
      code, and ii = (optional) telescope/instrument number
      assigned by the Infrared Studies Network.  For observatories
      that did not have an IAU code, ooo = 500 and in most of
      these cases ii = 00.  Thus, all U.S. observatories without
      and IAU code will have SYSTEM = '25001600' in their IRSN
      contributions.
 
      23 August 2006, A.C.Raugh.

Confidence Level Overview
      =========================
      The data are in several different stages of reduction, and
      those that have been reduced to physical units are not all
      in the same unit (milli-Jansky vs. micro-Jansky).  In a few
      cases, an error map is supplied for a reduced images.  In
      general, a user should read the entire PDS label and FITS
      header carefully for each image he is planning to reference
      to be sure of finding all the caveats and idiosyncracies of
      any particular image.