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Beginning 3 December 2019, improved astrometry for WFC3 and ACS imaging data is available from MAST and includes two new corrections to the header world coordinate system (WCS). The first includes an updated Hubble Guide Star Catalog (GSC version 2.4.0) which updates the coordinates of the guide stars with the positions from Gaia DR1. This reduces the typical uncertainties in the positions of the guide stars to ~200 mas over the entire sky. Combining this new information with the knowledge of the instrument distortions, an a priori correction has been made for all WFC3 and ACS observations in order to lock all HST observations onto a common absolute reference frame. When possible, an additional correction has been applied by aligning sources in each HST image directly to the Gaia catalog; this fit is referred to as an a posteriori correction. While some observing modes cannot be aligned to Gaia (e.g. grism and moving target observations) or the alignment may fail due to a lack of sources in either the HST image or the Gaia catalog, approximately 80% of ACS/WFC and 50% of WFC3/IR frames have been directly aligned. For these data products, the typical pointing uncertainty is reduced to ~10 mas, although the uncertainties increase for observations further in time from the Gaia reference epoch (2015.0 for DR1, 2015.5 for DR2). 

In mid-December 2020, further improvements have been implemented as Hubble Advanced Products referred to as Single Visit Mosaics (SVMs). These new data products are all drizzled onto the same north-up pixel grid and may have improved relative alignment across filters within a given visit, enabling easy comparison of the images across multiple wavelengths. The algorithms used to derive these new data products is described in the drizzlepac documentation for Single Visit Mosaic Processing. For comparison, the algorithm used to produce standard drizzled data products is described in the documentation for Pipeline Astrometric Calibration.

Usage 

Images downloaded from the archive after reprocessing with the new Enhanced Pipeline Products code will have headerlets added as extra extensions to the FITS file. A new python notebook, 'Using updated astrometry solutions', will familiarize users with the structure of the new FITS images and demonstrate how the primary WCS may be changed to any other preferred solution. These instructions will also show how to back out the new WCS updates entirely if desired (see the section below on Future Improvements'Caveats').

Alternatively, any of the new WCS solutions may be downloaded from MAST/STScI as separate headerlet files and applied to existing data. For users who wish to manually reprocess existing data, the 'updatewcs' task in the STWCS package as used by the Enhanced Pipeline Products code will be able to automatically connect to the astrometry database to retrieve and apply the headerlets. Python functions for creating, updating, and applying headerlets to FITS images are described via the Headerlet User Interface.

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•    GSC240 : a priori WCS where guide star coordinates are corrected from the original reference frame (e.g. GSC1.1 or GSC2.3) to the Gaia DR1-based GSC2.4.0
•    HSC30 :   a priori WCS corrected from the original reference frame to the Hubble Source Catalog (HSC v3.0) frame which is based on Gaia DR1
•    FIT-IMG-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'IMG' implies individual fits for each image to the reference catalog
•    FIT-REL-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'REL' implies images within a given filter were aligned to one other before a global catalog alignment
•    FIT-SVM-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'SVM' implies improved relative alignment across multiple filters before a global catalog alignment

More details on interpreting the WCS names may be found on the Astrometry in Drizzled Products page. A list of possible 'active' WCSNAME values populated in the image headers is provided in Table 2.

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Table 2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description 

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Caveats

While the majority of calibrated HST data products are now aligned to a common absolute reference frame, there are still improvements which will be available in the next data release.  For example, further improvements may be possible via manual realignment using the drizzlepac tools.  This is particularly true for exposures acquired in the same visit where the WCSNAMEs does not contain the string 'FIT_SVM_GAIA'.  For standard drizzled data products:

• Short and long exposures obtained in the same visit

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• may no longer

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• Grism images will now be offset from their direct image counterparts, where only the later of which may be aligned to an external reference catalogdue to potentially different number of Gaia matches.
• Exposures in different filters (eg. narrowband vs broadband) which were obtained in the same visit may no longer be aligned to one another, for example, if each filter had a different number of matches to Gaia.
• Short and long exposures obtained in the same visit may no longer be aligned due to potentially different number of Gaia matches.

Further refinements to the alignment will be available in the next release of Hubble Advanced Products, referred to as 'Single Visit MosaicsFurthermore, grism images will now be offset from their direct image counterparts, where only the later of which may be aligned to an external reference catalog. In order to preserve relative alignment between grism and direct images, users may wish to back out the updated WCS solutions entirely, as described in Section 5 of the python notebook, 'Using updated astrometry solutions'.