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Improved WFC3 and ACS Data Products
Starting on xx NovemberDecember, 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, and 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).
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- 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 the 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 any 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 enough an adequate number 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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wcsName = OriginalSolution - CorrectionType
where 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 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 fits
- FIT-REL-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'REL' implies images were aligned to each 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 2.
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Further refinements to the alignment will be available in the next release of Hubble Advanced Products, referred to as 'Single Visit Mosaics'. Should we keep this last sentence? These new products will correct the issues listed above, and they may further improve the relative alignment of exposures obtained in the same 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.
____Suggested basic examples for the Jupyter notebook___:
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