Improvements in HST Astrometry

Text modified from HST Astrometry Project Overview

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 had nominal rms errors per coordinate of ~0.5 arcsec, 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 rms errors per coordinate of ~0.3 arcsec over the whole sky.

An updated version of the catalog (GSC 2.4) has now been released as of November 2019, 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 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.  A summary of the GSC properties and associated errors is given in Table 1. 

Table 1: GSC Catalog Properties 

*Comparison between GSC1.1 and GSC2.3.2

HST Astrometry Project

The celestial 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, and thus improve in accuracy with date. Table 1 lists the catalog in use at the time of installation of the three main imaging cameras: WFPC2, ACS, and WFC3.

The goal of the HST Astrometry Project is to correct these inconsistencies in the archive as much as possible.  As observations are processed or reprocessed in the HST pipeline, they will have their World Coordinate System (WCS) updated to use the most accurate solution available. There are 2 types of correction that can be performed.

• a priori : correct the Guide Star positions in use at the time of observation to the GAIA reference frame and apply that global offset to the image WCS.
• a posteriori : identify sources in the image with objects in the GAIA catalog or another reference catalog on the same frame and derive a new WCS based on fitting x/y to RA/Dec.

A priori corrections will still include any errors in the HST focal plane alignment and are only relevant for observations which executed prior to GSC 2.4. A posteriori corrections are limited to imaging instruments for which there are enough sources to define a good reference catalog for matching. These solutions remove the uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.

Implementation

The key to being able to carry out this project is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can to 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. A user can easily apply any of the solutions to the primary wcs using astropy if they wish to use a different one.

LINK TO JUPYTER NOTEBOOK

A database was created to contain all the headerlets that is used by the xxxtask 'create_headerlet'? task to save and retrieve headerlets during pipeline processing. 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:

• The original pipeline astrometry based on original guide star positions
• a priori corrected astrometry based on updating the guide star positions to the GAIA DR1 reference frame
• a posteriori corrected astrometry based on sources in the Hubble Legacy Archive (HLA) images fitted to the Hubble Source Catalog (HSC). These positions are primarily based on the Pan-STARRS catalog which is on the GAIA reference frame but with larger errors. These are only available for a subset of the ACS & WFC3 images that were public prior to November 2017.

Currently, the processing pipeline now includes a source finding step and cross-identification to a limited set of catalogs; namely, GAIA DR1 and GAIA DR2.  Additional astrometric catalogs (like Pan-STARRS, SDSS, 2MASS, GSC 2.4 etc) can be added to the pipeline in order to obtain the best a posteriori solution only after they have been vetted to provide high-enough quality alignment.  In addition to loading this headerlet to the database it will append all other available headerlets to the image before it gets archived and available to users.

Table 2: Sample WCS keywords in the image header