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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).
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.
Guide Star Catalogs
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 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 catalogs and associated errors over the HST lifetime is provided in Table 1.
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WCSNAME | WCSTYPE | Comment |
OPUS | ‘distorted not aligned’ | No distortion correction has been applied; analysis of these FLT/FLC files may only be performed if corrected by the instrument-specific pixel area map |
IDC_0461802ej | ‘undistorted not aligned' | Distortion corrected using the IDCTAB reference file '0461802ej_idc.fits', but not aligned to an external catalog |
IDC_0461802ej-GSC240 | ‘undistorted a priori solution based on GSC240' | Alignment based on Guide Star Catalog v2.4.0 (GSC240). Absolute errors ~0.1" |
IDC_0461802ej-HSC30 | ‘undistorted a priori solution based on HSC30’ | Alignment based on Hubble Source Catalog v3.0. HSC30 errors are typically smaller than GSC240. If both corrections are available, HSC takes precedence. |
IDC_0461802ej-FIT_REL_GAIADR1 | ‘undistorted a posteriori solution relatively aligned to GAIADR1’ | Exposures relatively aligned to one another, and subsequently aligned as a set to Gaia DR1 |
IDC_0461802ej-FIT_REL_GAIADR2 | ‘undistorted a posteriori solution relatively aligned to GAIADR2’ | Exposures relatively aligned to one another, and subsequently aligned as a set to Gaia DR2 (including proper motion corrections to HST observation epoch) |
IDC_0461802ej-FIT_REL_NONE | ‘undistorted a posteriori solution relatively aligned to NONE’ | Exposures relatively aligned to one another, but the quality of the fit to an absolute reference catalog is unverified and should be checked by the user |
IDC_0461802ej-FIT_IMG_GAIADR1 | ‘undistorted a posteriori solution aligned image-by-image to GAIADR1’ | Exposures individually aligned to Gaia DR1 (not as a set) |
IDC_0461802ej-FIT_IMG_GAIADR2 | ‘undistorted a posteriori solution aligned image-by-image to GAIADR2’ | Exposures individually aligned to Gaia DR2 (including proper motion corrections to the HST observation epoch) |
IDC_0461802ej-FIT_IMG_NONE | ‘undistorted a posteriori solution aligned image-by-image to NONE’ | Exposures individually aligned to an astrometric reference catalog, but the quality of the fit is unverified and should be checked by the user |
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).
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Improvements in Future Releases
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