Improving the Absolute Astrometry of HST Data with GSC-II

We have used the new GSC-II to significantly improve the absolute astrometric accuracy of a large number of ACS datasets from the first years of ACS operations, reducing the initial levels of uncertainty (typically 1 2”, or more in some cases) to an accuracy of about 0.1 0.3”. There are typically enough GSC-II objects in the field of view of the ACS/WFC detectors to enable a robust solution to be determined by identifying the GSC-II objects and comparing their measured positions with those in the GSC-II. The resulting set of improved header coordinates are accurate enough to permit immediate multi-band comparisons between data from HST and other space-based and ground-based facilities, as well as a range of wavebands including X-ray, mid-IR, and radio data. The Institute has also begun planning to extend this work by including it in the automatic processing pipeline for ACS and eventually perhaps also for other HST instruments.