For information on absolute astrometry, Hubble Advanced Products (HAP), and the DrizzlePac software, please see the following resources.

...

________________

TIMELINE:

December 2019:  MAST data products now include updated absolute astrometry for all WFC3 and ACS images. The World Coordinate System (WCS) in the image header were updated and may include  one or more corrections. The first makes use of a updated version of the 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 with knowledge of the instrument distortions, an an a priori correction has been is made for all WFC3 and ACS observations in order to lock all HST observations onto a common absolute reference frame. any data acquired prior to October 2017 (GSC240).  When possible, an additional second correction has been is applied by aligning sources in each HST image directly to the Gaia catalog; this fit is an external reference catalog catalog, referred to as an a posteriori correction. While some observing modes cannot be aligned to Gaia (e.g. grism and moving target observations) cannot be aligned to any catalog, or the alignment may fail due to a lack of sources in either the HST image or the Gaia reference 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).  On The software used to produce these drizzled products is described on the Pipeline Astrometric Calibration page.

December 2020: MAST began production of Hubble Advanced Products in the HST data calibration pipeline for WFC3 and ACS. 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 10 December 2020, further improvements to drizzled data products have been implemented as new Hubble Advanced Products referred to as Single Visit Mosaics (SVMs). These data are all drizzled onto the same north-up pixel grid and may have include improved relative alignment across filters within a given for datasets acquired within the same visit, enabling easy comparison of the images across multiple wavelengths. The algorithm used to derive 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 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.

November 2021: MAST began production of HAP 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.

April 2022:  A new Hubble Advanced Product (HAP) is now  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.

August 2022:  A new instrument science report was published: 'Improved Absolute Astrometry for ACS and WFC3 Data Products' (ACS ISR 2022-03; WFC3 ISR 2022-06). 

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.

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').

...

Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars as specified in the Guide Star Catalog. GSC 1.1 had nominal rms errors of ~0.5 arcsec per coordinate, with errors as large as ~1‐3 arcsec reported near the plate edges. This accuracy improved substantially in October 2005 (during Cycle 15) with the introduction of GSC 2.3.2, where rms errors per coordinate were reduced to ~0.3 arcsec over the whole sky.  An updated version of the catalog (GSC 2.4.0) was released in October 2017, improving the celestial coordinates with the positions from Gaia DR1 and reducing errors to < 30mas over the entire sky. After including uncertainties in the positions of the science instruments (SIs) in the alignment of the focal plane to the Fine Guidance Sensors (FGS), the total error in HST absolute astrometry is ~1 arcsec for observations made with GSC 1.1, ~0.3 arcsec for those with GSC 2.3.2, and ~0.2 arcsec for those with GSC 2.4.0. These errors are reduced to ~10 mas for observations with a posteriori alignment to Gaia. A summary of pointing errors over the HST lifetime and the expected accuracy of the updated WCS solutions is provided in Table 1. 

Table 1: Key Guide Star Catalog releases and associated errors 

HST Astrometry Project

The coordinates populated in the FITS headers of HST observations retrieved from DADS (the HST Data Archiving and Distribution Service) were derived based on the guide star coordinates in use at the time of the observation. As the accuracy in these catalogs were refined over time, the pointing accuracy of HST has also improved. Table 1 lists the catalog in use at the time of installation of the three main imaging cameras (WFPC2, ACS, and WFC3) and the typical errors at each epoch.

...

• a priori         : correct the coordinates of the guide stars in use at the time of observation to the coordinates of those guide stars as determined by Gaia by , applying a global offset to the WCS
• a posteriori  : identify sources in the HST image and cross-match with positions from an external reference catalog (such as Gaia) to derive an improved WCS based on improve the WCS (fitting x/y to RA/Dec)

Note that a priori corrections are only relevant for observations which executed prior October 2017 (eg. prior to the release of GSC 2.4.0), and these will still include small errors in the alignment of the science instruments to the HST focal plane. The a posteriori corrections are limited to imaging instruments for which there are an adequate number sources to define a reference catalog for matching. These solutions remove uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.

Implementation

The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be attached to a FITS file and applied to the primary WCS. An observation can have multiple headerlets, each of which may have astrometry derived by differing methods. As HST data is processed/reprocessed, all available headerlets will be present as FITS extensions in the archived image with the best solution applied to the primary WCS.  More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.

WCS Naming Conventions

Successfully aligning an observation to Gaia using the a posteriori processing will result in an update of the 'active' WCS of the image with the new solution and the new headerlet extension. This headerlet not only includes the WCS keywords which define the transformation from pixels to Gaia-aligned positions on the sky, but it also contains information about how this solution was derived along with the errors to be expected based on the fit. 

...

 and CorrectionType may have several forms

•    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 RefCat  : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'IMG' implies individual fits for each image each FLT is separately aligned to the reference catalog
•    FIT-REL-ReferenceCatalog RefCat   : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'REL' implies images within a given filter were aligned to one other that FLTs within the same filter within the same visit are aligned before a global catalog alignment
•    FIT-SVM-ReferenceCatalog RefCat : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'SVM' implies improved relative alignment across that FLTs in multiple filters within the same visit are aligned before a global catalog alignment

         and REFcat may be one of the following when an adequate number of matches are found in the HST frame to compute the linear transformations (shift, rotation, scale) to sky coordinates:

•    Gaia eDR3
•    GSC 2.4.2
•    2MASS

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.

...

Table 2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description. The best WCS is the FIT-SVM solution which has the best relative and absolute astrometry. 

Implementation

The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be attached to a FITS file and applied to the primary WCS. An observation can have multiple headerlets, each of which may have astrometry derived by differing methods. As HST data is processed/reprocessed, all available headerlets will be present as FITS extensions in the archived image with the best solution applied to the primary WCS.  More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.

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:

• Short and long exposures obtained in the same visit may no longer be aligned due 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.

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'.