FITS celestial coordinate keywords.

[pence@tetra.gsfc.nasa.gov (William Pence)]

The local OGIP FITS Panel has been attempting to invent some multi-mission
FITS keywords to represent the celestial position of astronomical data
sets.  The following keywords have been proposed; any comments,
complaints, or suggestions about this choice of keyword names are
welcome.

Note that these keywords are provided for general descriptive
information and should not be used for precise position determination.
These new keywords in no way replace the standard CRVAL, CRPIX, etc
FITS keywords.  Note also that not all of these keywords will be
relevant for all observations (e.g. for observations taken with
a scanning instrument)

RA_OBJ    /  position of the object specified by the OBJECT keyword (degrees)
DEC_OBJ   /

RA_NOM   / nominal pointed position of the observation (degrees)
DEC_NOM

RA_PNT   / actual pointed position of the optical axis (degrees)
DEC_PNT  /  (or the mean value if the pointing is not stable)

RA_PNTE  / the 1-signa error on RA_PNT
DEC_PNTE / the 1 sigma error on DEC_PNT

PA_NOM     / the nominal Position Angle (degrees from N to the E)

PA_PNT     / the actual measured Position Angle of the pointing
PA_PNTE    / the 1-sigma error on PA_PNT

For a 2-D detector these position angles refer to the angle
between the +Y axis of the image and North measured towards the East.