The FGS Field of View is large (~ 60 square arcsec) and therefore sensitive to the
effects of differential velocity aberration (DVA), the field-dependent shifts of star positions owing to the telescope’s component of motion in the direction it is pointing. It is possible to maintain the pointing of HST so that the effects of DVA can be compensated for, but only at one place in the focal plane—the alignment point
. Because typical FGS astrometry programs measure the relative positions of stars spread widely throughout the pickle, there can be at most only one star that will not suffer from DVA. Although this effect is potentially large, up to 20 mas, it is simple to correct for this aberration. The correction algorithm requires HST's V1 pointing, V3 roll, and state vector, all obtained from the *.a*h
header files, plus the specification (in V2,V3) of the alignment point. For observations made before September 2001, the alignment point data for all FGS observations are retrieved from the reference library maintained by the STScI FGS staff. These data originate from the Science Mission Schedule (SMS) and are automatically extracted and entered into this library. The astrometry pipeline accesses this library when the differential velocity aberration connection is made. For observation executing after September 2001, the alignment point is specified by the FGSOFFV2 and FGSOFFV3 keywords in the *.a1h files. The FGS calibration pipeline extracts the values of these keywords for the computation of the differential velocity aberration correction.