The HST reference frame is effectively defined by the positions of the Guide Stars
that are selected for each pointing. The initial Guide Star Catalog (GSC1) was an all-sky catalog of stars down to 15th magnitude built from Schmidt Sky Survey plates. Since March 2006, an updated catalog (GSC2) has been used. GSC2 has been derived from more recent epoch plates and has been calibrated to be on the International Celestial Reference System (ICRS), which has been adopted by the IAU as the new fundamental reference frame. This simplifies the procedure for providing HST coordinates since it removes the necessity to tie the object coordinates back to the GSC1 and the plates used to construct it. Guidelines for obtaining the coordinates of your target in the ICRS are given in the HST Phase II Proposal Instructions.