What is the difference between the RA and DEC NOM, TARG, and PNT header keywords?
(RA_NOM, DEC_NOM) is the aimpoint of the detector,
i.e. the on-axis position of the telescope.
To be more precise, it is the set of coordinates defining the tangent point at the intersection of the 3-D sphere of the sky and its 2-D projection onto a tangent plane.
(RA_TARG, DEC_TARG) is the target requested by the PI.
These coordinates may not necessarily be coincident with any detectable source in the field, and are to the precision with which they were provided in the proposal.
(RA_PNT,DEC_PNT) is the average pointing of
the optical axis.
In practice, (RA_NOM, DEC_NOM) and (RA_PNT, DEC_PNT) should be the same.