4.5 Absolute Astrometry

HST Pointing Accuracy

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. Thus, 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 GSC1.1, ~0.3 arcsec for those made with GSC 2.3.2, and ~0.1 arcsec when using the new GSC 2.4.0.  A summary of the GSC catalogs and associated errors over the HST lifetime is provided in Table 4.1. 

Table 4.1: Key Guide Star Catalog releases and associated errors

Absolute Astrometry Improvements in MAST

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 4.1 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.

The goal of the HST Astrometry Project is to correct these inconsistencies in the archival data products as much as possible.  As observations are processed or reprocessed in the HST pipeline, their World Coordinate System (WCS) will be updated to use the most accurate solution available. There are two types of corrections that can be performed:

• 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 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.

New WFC and ACS Data Products

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 <~100 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 70% of the WFC3 and ACS frames have been aligned successfully. 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). 

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. 

The various WCS solutions are identified by the WCSNAME keyword found in each FITS headerlet and use the following naming convention: 

wcsName = OriginalSolution - CorrectionType

 where OriginalSolution may be either

•    OPUS : initial ground system wcs, no distortion correction
•    IDC_xxxxxxxxx : initial distortion corrected wcs  (where xxxxxxxxx = geometric distortion model used, eg. the rootname of the IDCTAB reference file)

 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 : 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 were aligned to one other 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 4.2.

Table 4.2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description

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).

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.

Improvements in Future Releases

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, exposures obtained in the same visit would have been aligned (relatively) to one another in prior archival products but may no longer aligned in the new HAP.

• 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.
• 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 Mosaics'.