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'), orEQUINOX = 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.