What is the difference between the RA and DEC NOM, TARG, and PNT header keywords?
(RA_PNT,DEC_PNT) is the time-averaged location of the optical axis, i.e. the on-axis position.
(RA_NOM, DEC_NOM) are the 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.
By default, (RA_NOM, DEC_NOM) are chosen to be equal to (RA_PNT, DEC_PNT).
(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.
The "What was the aimpoint of the observation?" FAQ has related information on the observation aimpoint.