The RA, DEC, and position angle of the observation are recorded in several ways. The FITS header includes information on both the requested pointing and the actual position of the telescope, as measured during the time of the observation and corrected for proper motion, as entered in SPOT. All reconstructed values are averaged over the time interval of the DCE. The pointing-related keywords include:
RA_RQST, DEC_RQST, PA_RQST, and UNCRTPA give the requested values for the current DCE. Here the position angle is given for the +Z axis of the telescope, measured East of North. Its uncertainty can be found in the keyword UNCRTPA.
RA_REF and DEC_REF give the original target position. If the observation is part of a group (e.g., cluster target type or Spectral Mapping mode), then the central target position may be different from the requested position of the current DCE.
RA_FOV, DEC_FOV, and PA_FOV give the field-of-view pointing, while RA_SLT, DEC_SLT, and PA_SLT give the center of the slit pointing. The reconstructed pointing of the telescope is recorded for both the re-quested field-of-view and the center of the slit. So, in a Staring Mode observation of a single target, the pointing on the center of the slit will change at the two nod positions, but the two fields-of-view will each be pointed at the same position (with some small pointing uncertainty). Field-of-view and center of the slit positions will be the same for Spectral Mapping mode in which the slit centers have been requested.
PTGDIFF is the difference between the reconstructed pointing (for the field-of-view) and the requested pointing. The requested and reconstructed pointings are computed with double-precision, but the recon- structed pointing and, therefore, the value of PTGDIFF is accurate only to 0.2 arcsec (1σ radial). This uncertainty applies equally to the above FOV and SLT keywords. For data taken before December 2004, pointing control was based on gyros, resulting in relatively large values of PTGDIFF. Since December 2004, pointing control has been based on the star tracker. If an IRS or PCRS peak-up operation failed, then the telescope could not make a correction to the attitude, and by default assumed the correction to be zero. In this case, the value of PTGDIFF may be misleadingly small. This is also the result if no peak-up is requested. A false positive peak-up, on the other hand, may lead to large PTGDIFF values by virtue of differences between the reference (commanded) positioning and the attitude measured from the star tracker.