In August 2022, a new ACS/WFC3 instrument science report (ACS ISR 2022-03; WFC3 ISR 2022-06) titled 'Improved Absolute Astrometry for ACS and WFC3 Data Products'  was published. This ISR describes updated WCS solutions in MAST data as well two new types of Hubble Advanced Products.

Abstract:
As of late-2019, MAST data products for ACS and WFC3 include improved absolute astrometry in the image header World Coordinate System (WCS). The updated WCS solutions are computed during pipeline processing by aligning sources in the HST images to a select set of reference catalogs (e.g. Gaia eDR3). We compute statistics on the alignment fraction for each detector and estimate the uncertainties in the WCS solutions when aligning to different reference catalogs. We describe two new types of Hubble Advanced Products (HAP), referred to as Single Visit Mosaics (SVMs) and Multi Visit Mosaics (MVM), which began production in MAST in late-2020 and mid-2022, respectively. The SVM products include an additional relative alignment across filters in a visit, and the drizzled images are used to generate point source and segment catalogs during pipeline processing. These catalogs supersede those produced by the Hubble Legacy Archive and will be the basis of the next version of the Hubble Source Catalog. The MVM data products combine all ACS/WFC, WFC3/UVIS, or WFC3/IR images falling within a pre-defined 0.2° x 0.2° 'sky cell' for each detector+filter, which are drizzled to a common all-sky pixel grid. When combining observations over a large date range, MVMs may have photometric errors of several percent or systematic alignment errors when combining visits with different catalog solutions. We therefore recommend these to be used as ‘discovery images’ for comparing observations in different detectors and passbands and not for precise photometry.

PRIOR NEWS ITEMS:

On December 17, 2020, MAST began production of new ACS and WFC3 products in the HST data calibration pipeline. These Hubble Legacy Archive (HLA)-style mosaics comprise the data from a single HST visit and are aligned to a common astrometric reference frame. These mosaics will be available as they are produced in the pipeline; it will take some roughly 1-2 months before all archival data have been processed.

These new 'Hubble Advanced Products' (HAP) are referred to as 'Single Visit Mosaics' (SVMs). They are all drizzled onto the same north-up pixel grid and may have improved relative alignment across filters for images acquired within the same visit, enabling easy comparison of the images through multiple filters. When possible, sources in the images have been aligned directly to the Gaia source catalog to improve the image world coordinate system (WCS). SVM data products with both relative alignment (by filter) and absolute alignment to Gaia will contain the string 'FIT_SVM_GAIA' in the 'WCSNAME' keyword in the science extension of the image header. The software used to compute these new data products is described in the DrizzlePac documentation for Single Visit Mosaic Processing.

On December 3, 2019, the first set of improved astrometry data were released in MAST. All The World Coordinate System (WCS) in the image header of all WFC3 and ACS imaging filters were aligned to a common astrometric reference frame and include two corrections to the header WCSdatasets were updated and may include  one or more corrections. The first makes use of a new version of the 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 with knowledge of the instrument distortions, an a priori correction was made. When possible, an additional correction was applied by aligning sources in each HST image directly to the Gaia catalog, 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). The software used to produce these drizzled products is described on the Pipeline Astrometric Calibration page.

On December 17, 2020, the MAST began production of new ACS and WFC3 products in the HST data calibration pipeline (see the following MAST Newsletter article. These Hubble Legacy Archive (HLA)-style mosaics comprise the data from a single HST visit which are aligned to a common astrometric reference frame. These new 'Hubble Advanced Products' (HAP) are referred to as 'Single Visit Mosaics' (SVMs) and are described in a MAST Newsletter article from December 2020. The data products are all drizzled onto the same north-up pixel grid and may include improved relative alignment across filters for datasets acquired within the same visit, enabling easy comparison of the images through multiple filters. When possible, sources in the images have been aligned directly to the Gaia catalog to improve the WCS. SVM data products with both relative alignment (by filter) and absolute alignment to Gaia will contain the string 'FIT_SVM_GAIA' in the 'WCSNAME' keyword in the science extension of the image header. The software used to compute these new data products is described in the DrizzlePac documentation for Single Visit Mosaic Processing.

On November 23, 2021, MAST began producing source catalogs as part of the SVM data products. BecauseSVMproductsincludeanadditionalrelativealignmentacrossfiltersina visit,the drizzled imagesmayusedto generatepoint sourceand segmentcatalogsduring pipeline processing. Thesecatalogssupersedethoseproducedby theHubbleLegacy Archiveand will be the basis ofthenext version of the Hubble Source Catalog.

On April 26, 2022, the HST data calibration and archive pipelines began producing a new Hubble Advanced Product (HAP) to be distributed through MAST. These are cross-visit, cross-proposal mosaics called Multi-Visit Mosaics (MVM), which combine public observations of fields observed multiple times by ACS and WFC3 into a set of products drizzled onto a common, pre-defined pixel grid. These new products were described in a MAST Newsletter article from May 2022 and complement the existing HAP Single Visit Mosaics (SVM) released in December 2020.

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 'Caveats').

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

Caveats

While the majority of calibrated HST data products are now aligned to a common absolute reference frame, 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:

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Furthermore, 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'.