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10.5 SMS Support Data
The FGS is unique among the science instruments onboard HST in that its total field of view (the pickle) is large 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 relatively simple to correct for this aberration. The algorithm requires HST's V1 pointing, V3 roll, and state vector, all obtained from the .a*h header files, plus the specification of the alignment point. The alignment point data for all FGS observations are stored in a reference library maintained by the STScI FGS staff. The 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.
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Last updated: 11/13/97 16:54:22