8.4 Determining Coordinates in the International Celestial Reference System (ICRS) Reference Frame

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. In March 2006, an updated catalog (GSC2) was introduced. GSC2 was derived from more recent epoch plates and was calibrated to be on the International Celestial Reference System (ICRS), which had been adopted by the IAU as the new fundamental reference frame. This simplified the procedure for providing HST coordinates since it removed the necessity to tie the object coordinates back to the GSC1 and the plates used to construct it. More recently, coordinates from Gaia have been incorporated – further reducing the relative positional errors between guide stars. Guidelines for obtaining the coordinates of your target in the ICRS are given in Chapters 3.1 and 3.2 of the HST Phase II Proposal Instructions.