Since 20 October 1994, STScI’s Observatory Monitoring System (OMS) has been
populating the STScI data archives with the Observation Logs. These logs are of secondary use for the processing of FGS astrometry data because the guide star data are directly available to the astrometry pipeline. The item of interest in the *_jif.fits
header is the OMS computed HST roll angle. Recall that the .a*h
header files contain the predicted roll angle. The details of how the telescope is pointed on the sky reveal that the roll angle is not set by the pointing control system using FGS data, rather it is set using less accurate fixed head star tracker data. Whatever roll angle results is then maintained by the roll guide star. OMS uses the FGS data, guide star RA, Dec, and HST/FGS alignment matrices to compute the actual roll of the telescope, which can differ from the predicted roll by as much as 0.4 degrees. The more accurate OMS roll angle is best used for the final computation of astrometric parallaxes and binary star position angles.