Data Set Overview
      =================
      The Astrometry Net of the International Halley Watch (IHW)
      compiled astrometric observations of comet 1P/Halley from
      observations made by professional astronomers from
      ground-based observatories all over the world.  The data
      set comprises 6475 astrometric observations over the
      period 16 Oct 1982 through 9 Jan 1989, in addition to 158
      observations from the 1835 apparition and 1696 observations
      from the 1910 apparition. The data from previous apparitions
      was re-reduced using modern star catalogs.
 
      The data were edited by the Astrometry Net Discipline
      Specialists: D.K. Yeomans, R.M. West, R.S. Harrington and
      B.G. Marsden.  The results of the original version of
      the modern data have been consolidated into single tables
      for each month of observations in this revised data set.
      For the previous apparitions, the single table for each
      apparition has been reformatted slightly to use the same
      formats as the modern data.
 
      1986 Apparition
      ===============
      Observations were contributed by 430 observers working at
      148 observatories around the world.  The complete list
      of observers and observatories is included in ''The Comet
      Halley Archive Summary Volume'' (JPL D-400-450, 1991).
 
      These observations played an essential role in supporting
      the various flight projects dispatched to 1P/Halley,
      allowing for rapid updates of ephemerides and orbit
      determinations before, during and after encounter.
 
      Observers submitted their data to the orbit determination
      center at the Jet Propulsion Laboratory (JPL).  The orbit
      determination center added the ephemeris time (ET)
      corresponding to the reported UTC and the observatory
      coordinates, as well as applying small corrections for
      elliptical aberration.  FITS headers were added to the
      resulting records before delivery.
 
      The original FITS files were prepared and submitted to the
      IHW Lead Center at JPL by R.N. Wimberly.  The version 1.0
      archive was prepared under the original version of the PDS
      standards by E. Grayzeck, D.A. Klinglesmith III, and
      M.B. Niedner, Jr.
 
      1835 and 1910 Apparitions
      =========================
      S.F. Roeser (Astronomisches Rechen-Institut, Heidelberg)
      and T.A. Morley re-reduced the visual observations available
      from the earlier apparitions using modern star catalogs. The
      revised data were gathered into a single table for each
      apparition, in a format similar to the data tape format used
      for the individual observations submitted to the Astrometry
      Net.
 
 
      Modification History
      ====================
      These data were originally part of the IHW 'Mixed Data'
      archives on CD volumes HAL_0019-0023.  In their original
      form they consisted of files in a prototype FITS format
      (never adopted as part of the standard) which provided a
      simple FITS header above a set of ASCII records.  The
      ASCII records were in the format of the 80-byte card
      images fed into the JPL ephemeris software.  They included
      observed and corrected coordinates for the comet, error
      estimates, the location of the observatory, quality
      indicators, and comments from the observers.
 
      Personnel at the Small Bodies Node (SBN) at the University
      of Maryland have performed extensive reformatting of the
      original data.  This editing includes:
 
        o FITS headers have been dropped.
        o All information from the FITS headers and ephemeris
          records have been combined into a single record
          for each observation in an ASCII table.
        o Right ascension and declination have been converted to
          decimal degrees (from sexigesimal notation), but left
          in the B1950.0 system.
        o Observatory latitude and longitude have been converted
          from sexigesimal notation to decimal degrees.
        o Comments from the ephemeris records have been included,
          unedited, as a text field in the new table records.
        o The observation records have been gathered into tables
          by the month of observation.
        o All reported errata have been corrected.
 
      The data from previous apparitions where in simple FITS
      ASCII tables.  PDS personnel converted the FITS tables
      into flat ASCII, and reformatted the data so that individual
      field formats matched those used in the monthly tables for
      the modern data.

Confidence Level Overview
      =========================
      Each observation in this archive was specifically chosen
      for inclusion by the Astrometry Net Discipline Specialists.
      Note that there are flag fields in the data record to give
      some additional indication of quality for each record, and
      also that details on the ancient observations are frequently
      scant.