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Text revised and expanded from this page:  HST Astrometry Project Overview

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Improved astrometry for WFC3 and ACS imaging data is now available from MAST , which and includes two new corrections to the header world coordinate system. First, The first includes an updated Hubble Guide Star Catalog (GSC 2.4.0) was released which updated updates the coordinates of the guide stars with the positions from Gaia DR1. The This reduces the typical uncertainties in the positions of the guide stars is reduced 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 a posteriori an additional correction has been applied to data by aligning sources in the images each HST image directly to the Gaia catalog, and this is referred to as an a posteriori correction. While some observing modes cannot be aligned to Gaia (e.g. , grism and moving target observations) , and or the alignment can fail due to a lack of sources in either the HST data or the Gaia catalog, ~70% approximately 70% of the WFC3 and ACS data were have been aligned successfully. The For these observations, the typical uncertainty in the pointing is reduced to ~10 mas, although the uncertainties increase the farther for observations further in time from the Gaia reference epoch (2015.5 for DR2). 

Guide Star Catalogs

Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars. GSC1 GSC 1.1 had nominal rms errors per coordinate 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, which had where rms errors per coordinate of were reduced to ~0.3 arcsec over the whole sky.As of November 2019, an   An updated version of the catalog (GSC 2.4) has now been released.0) was released  in November 2019, improving the celestial coordinates with the positions from GAIA Gaia DR1 and reducing errors to < 30mas over the entire sky. Thus, after including uncertainties in the positions of the science Instruments 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 made with GSC 2.3.2, and ~0.1 arcsec when using the new GSC 2.4.0.  A summary of the GSC properties catalogs and associated errors over the HST lifetime is given in Table 1. 

Table 1: GSC Properties Guide Star Catalog history and associated errors 

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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 2 two types of corrections that can be performed.:

• a priori 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 GAIA and apply applying a global offset to the WCS
• a posteriori posteriori  : identify sources in the HST image and cross-match with positions from an external reference catalog (such as GAIAGaia) to derive a new an improved WCS based on fitting x/y to RA/Dec

Note that the a priori corrections will still include any errors in the HST focal plane alignment and are only relevant for observations which executed prior to the release of GSC 2.4.0 and will still include any errors in the alignment of the science instruments to the HST focal plane. The a posteriori corrections corrections are limited to imaging instruments for which there are enough sources to define a good reference catalog for matching. These solutions remove uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.

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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.A database has been created to contain all the headerlets that is used by the xxxtask  to save and retrieve headerlets during pipeline processing. (Do we need this part?) This database is accessed using a restful web service as well as a simple web-form. Prior to enabling this database in the HST pipeline, it was pre-populated with headerlets derived from:

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Pipeline processing now includes a source finding step to create additional a posteroiri solutions via cross-identification with a limited set of external reference catalogs; namely, GAIA DR1 and DR2.  Alternate astrometric catalogs (like Pan-STARRS, SDSS, 2MASS, GSC 2.4 etc) may eventually be added to the pipeline in order to obtain the best a posteriori WCS, but only after they have been determined to provide high quality alignment.

WCS Naming Conventions

Successfully aligning an observation to GAIA 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 GAIAGaia-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. 

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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 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 on Gaia DR1
•    FIT-IMG-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'IMG' implies individual image catalog fits
•    FIT-REL-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'REL' implies images were aligned to each other before a catalog alignment

More details on interpreting the WCS names may be found on the Astrometry in Drizzled Products page. Several examples are listed below in Table 2.

Table 2: Sample WCSWCSNAME keyword keywordsvalues inand the imagecorresponding WCSTYPE headerdescription

Usage 

(Still needs revision)

Images downloaded from the HST archive after reprocessing with the new Enhanced Pipeline Products code will already have headerlets added as extra extensions to the FITS file. A new python notebook <insert LINK> has been developed to familiarize users with the structure of the new FITS images and to demonstrate how the primary WCS may be changed to any other preferred solution. Alternately. 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 available headerlets 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 software linked above will be able to automatically connect to the database to retrieve and retrieve/ apply the headerlets. Python functions for creating, updating, and applying headerlets to FITS images are described via the Headerlet User Interface.

Improvements in Future

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Releases

While the majority of calibrated HST data products are now aligned to a common absolute reference frame, there are still some improvements in the alignment which will be made available in the next data release.  For example, exposures obtained in different filters that 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'. These new products will correct the issues listed above, and they will further improve the relative alignment of exposures GAIA. Additionally, grism images may be now offset from their direct image counterparts, where only the later of which may be aligned to an external reference catalog. These refinements in the alignment process will be coming soon in the upcoming release for Hubble Advanced Products: Single Visit Mosaics. These new products will likely be able to improve the alignment for exposures which were obtained in the same visit, but which had guide star lock issues. They will also correct for small errors in the pointing of the telescope when very large dithers are commanded withing a visit for example for datasets with very large commanded dithers (eg. half the detector FOV) where small residual shifts and rotations are required to align frames.

____Some suggested very basic examples for the Jupyter notebook___:

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