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Improved astrometry for WFC3 and ACS imaging data is now available from MAST and includes two new corrections to the header world coordinate system. The first includes an updated Hubble Guide Star Catalog (GSC 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 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 can fail due to a lack of sources in either the HST data or the Gaia catalog, approximately 70% of the WFC3 and ACS data have been aligned successfully. For these observations, the typical uncertainty in the pointing 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).
Guide Star Catalogs
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Catalog | Release Date | Mean Epoch of catalog positions | Typical errors | Worst errors | Total Error (including SI to FGS alignment) | Comment |
|---|---|---|---|---|---|---|
GSC 1.0 | Jun 1989 | 1-2” | ||||
GSC 1.1 | Aug 1992 | 1981.8 | 0.5” | ~1” | ~1” | First version published to the community Used by HST operations prior to Cycle 15 WFPC2 installed Dec 1993 |
GSC 2.0 | Jan 2000 | Science target fields only | ||||
GSC 2.2.0 | Jun 2001 | ACS installed Mar 2002 | ||||
GSC 2.3.2 | Oct 2005 | 1992.5 | 0.3” | 0.75” | 0.3” | |
| GSC 2.3.3 | Oct 2009 | WFC3 installed May 2009 | ||||
GSC 2.3.4 | ?? | 'Current version' Citation? | ||||
GSC 2.4.0 | Nov 2019?? | 2015.0 | 0.03” | 0.1” | GSC 2.3.4 aligned to Gaia DR1 |
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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 active WCSNAME keyword values and the corresponding WCSTYPE description
WCSNAME | WCSTYPE | Comment |
OPUS | ‘distorted not aligned’ | No distortion correction, not recommended for analysis |
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. Absolute errors ~0.1" |
IDC_0461802ej-HSC30 | ‘undistorted a priori solution based on HSC30’ | Alignment based on Hubble Source Catalog v3.0. These positions are primarily based on the Pan-STARRS catalog, which is matched to the Gaia reference frame but with larger errors. HSC30 errors are typically smaller than GSC240. If both corrections are available, HSC takes precedence. |
IDC_0461802ej-FIT_REL_NONE | ‘undistorted a posteriori solution relatively aligned to NONE’ | ?Exposures relatively aligned to one another, but not to an absolute reference catalog |
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 (includes proper motion corrections to HST observation epoch) |
IDC_0461802ej-FIT_IMG_NONE | ‘undistorted a posteriori solution aligned image-by-image to NONE’ | ?? |
IDC_0461802ej-FIT_IMG_GAIADR1 | ‘undistorted a posteriori solution aligned image-by-image to GAIADR1’ | |
IDC_0461802ej-FIT_IMG_GAIADR2 | ‘undistorted a posteriori solution aligned image-by-image to GAIADR2’ |
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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 software linked above (?) 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.
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While the majority of calibrated HST data products are now aligned to a common absolute reference frame, there are still some 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.
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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'. REMOVE THIS? These new products will correct the issues listed above, and they will 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.
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from stwcs.wcsutil.headerlet import headerlet_summary
headerlet_summary('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits',columns=['HDRNAME','WCSNAME'])
EXTN HDRNAME WCSNAME
8 j8c041sdq_flt_OPUS-hlet.fits OPUS
9 OPUS2019-06-04 IDC_0461802ej
10 j8c041sdq_flt_OPUS-GSC240-hlet.fits OPUS-GSC240
11 j8c041sdq_flt_IDC_0461802ej-GSC240-hlet.fits IDC_0461802ej-GSC240
12 j8c041sdq_flt_OPUS-HSC30-hlet.fits OPUS-HSC30
13 j8c041sdq_flt_IDC_0461802ej-HSC30-hlet.fits IDC_0461802ej-HSC30
22 IDC_0461802ej IDC_0461802ej
23 IDC_0461802ej-FIT_REL_GAIADR2 IDC_0461802ej-FIT_REL_GAIADR2
3.) Alternate way to get the keyword names. Is the last one always primary?How to determine which is primary
from astropy.io import fits
fits.getvalfrom stwcs.wcsutil import headerlet
headerlet.get_headerlet_kw_names('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits',kw='WCSNAME',1)
['OPUS', 'IDC_0461802ej', 'OPUS-GSC240', 'IDC_0461802ej-GSC240', 'OPUS-HSC30', 'IDC_0461802ej-HSC30', 'IDC_0461802ej', 'IDC_0461802ej-FIT_REL_GAIADR2']
headerlet.get_headerlet_kw_names('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041051_drc.fits',kw='WCSNAME')
4.) Example for how to realign grism and direct images>This doesnt seem to work for the DRC image...
