Page History
...
Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars. 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 November released in __xxx____ 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 2.4.0. A summary of the GSC catalogs and associated errors over the HST lifetime is given in Table 1.
...
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 | ?? | 2015.0 | 0.03” | 0.1” | GSC 2.3.4 aligned to Gaia DR1DR1 Citation? |
**Please check cells highlighted in pink
HST Astrometry Project
The 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. As the accuracy in these catalogs were refined over time, the pointing accuracy of HST has also improved. Table 1 lists the catalog in use at the time of installation of the three main imaging cameras (WFPC2, ACS, and WFC3) and the typical errors at each epoch.
...
The various WCS solutions are identified by the WCSNAME keyword found in each FITS headerlet and use the following naming convention:
wcsName = OriginalSolution - CorrectionType
where OriginalSolution may be either
- OPUS : initial ground system wcs
- IDC_xxxxxxxxx : initial distortion corrected wcs (where xxxxxxxxx = geometric distortion model used, eg. the rootname of the IDCTAB reference file)
and CorrectionType may have several forms
...
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 including 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’ | Exposures individually aligned to Gaia DR1 |
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 HST observation epoch) |
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 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. 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 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.
...
Further refinements to the alignment will be available in the next release of Hubble Advanced Products, referred to as 'Single Visit Mosaics'. REMOVE THISthis 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.
...
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.) How Show how to determine which WCS is primaryfrom astropy.io import fits
fits.getval('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits','WCSNAME',1)
'IDC_0461802ej-FIT_REL_GAIADR2'
4.) Example for how to realign grism and direct imagesto back out the primary WCS in order to restore alignment between grism observations and their direct image counterparts.
