New Absolute Astrometry for Some HST Data Products

About this Article

Improved Absolute Astrometry for WFC3 and ACS imaging data was released in MAST in December 2019. Calibrated data now include an updated WCS (world coordinate system) in the image header which is used to produce the combined visit-level drizzled data products. In the majority of cases, alignment is performed by matching stars in each HST frame directly to the Gaia DR2 reference catalog reducing astrometric errors to ~10 mas (see ‘Improvements in HST Astrometry’ for more detail). For fields with insufficient overlap with Gaia, the WCS is defined by the Hubble Guide Star Catalog (GSC 2.4) or the Hubble Source catalog (HSC 3.0), which correct the coordinates of HST’s guide stars with their positions from Gaia DR1 with typical ~200 mas accuracy.

In January 2020 the WFC3 team published a Newsletter article describing ‘New Astrometric Solutions in WFC3 Images’ and highlighted a Jupyter notebook for 'Using updated astrometry solutions' and evaluating the quality of the alignment. For some datasets, the new WCS solutions may not be optimal, and users may wish to reset the WCS to the original solution, as described in Section 4 of the notebook.  If desired, users may also manually align observations to the Gaia frame, following the workflow described in the notebook ‘Aligning HST images to an absolute reference catalog'.