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    HFWG Recommendation R3:

    On standard RA and DEC keywords giving the position of the observed object, the telescope pointing, and the spacecraft axes

    (Approved: 1993 Jun 30)

    A) Reference Frame Keywords

    Both of the following keywords are mandatory if one or more pairs of keywords specifying RA and dec (as defined in B) are given.

    • RADECSYS - a string denoting the stellar reference frame in use
      e.g. values: 'FK4', 'FK5' etc
    • EQUINOX - a real giving the date (in decimal years) of the Besselian epoch or Julian epoch appropriate for the specified RADECSYS.
      It is further recommended that the value of this keyword is restricted be either
      EQUINOX = 1950.0 (for RADECSYS = 'FK4'), or
      EQUINOX = 2000.0 (for RADECSYS = 'FK5')
      with the latter strongly recommended for all new datasets.

    B) The RA and Dec Coordinates

    In all cases listed below, the values of keywords are reals expressed in decimal degrees. Obviously only those keyword pairs considered necessary need be specified. However, the values of ALL such keyword pairs MUST be given in the same reference frame as specified by the values of the RADECSYS and EQUINOX keywords (see A).

    i) The Positions of Astronomical Objects/Sources

    The position of an astronomical source should be given using the keywords:

    • RA_OBJ
    • DEC_OBJ

    ii) The Pointing Direction of the Instrument.

    The pointing direction of the instrument should be given using the keywords:

    • RA_PNT
    • DEC_PNT

    where
    - in the case of imaging instruments, the values of these keywords give the direction of the optical axis of the instrument (after all borsighting corrections etc have been applied).
    - in the case of non-imaging instrumentation, the values of these keywords give the direction of some other (instrument-specific) vector. It is anticipated that in most cases this will be the direction of maximum instrument sensitivity.

    For instruments with less than 100% stable pointing accuracy, the above pair of keywords should be the mean values during the observation.

    iii) The Orientation of the Spacecraft

    The orientation of the spacecraft (or telescope platform) should be given by the specification of any/all of the following keyword pairs:

    • RA_SCX
    • DEC_SCX
    • RA_SCY
    • DEC_SCY
    • RA_SCZ
    • DEC_SCZ

    giving the orientation of the spacecraft X-, Y- and Z-axes respectively. For instruments with less than 100% stable pointing accuracy, the above pairs of keywords should be the mean values during the observation.