Page History
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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 2.4.0 + Gaia Fit | Dec 2019 | 2015.5 | 0.01" | 0.01" | WCSNAME= 'IDC*_FIT_*_GAIADR*' | |
GSC 2.4.0 | Oct 2017 | 2015.0 | 0.03” | ~0.2” | GSC2.3.4 aligned to Gaia DR1 DR1 | |
GSC 2.3.3 | Oct 2009 | WFC3 installed May 2009 | ||||
GSC 2.3.2 | Oct 2005 | 1992.5 | 0.3” | 0.75” | ~0.3” | |
GSC 2.2.0 | Jun 2001 | Public Release | ||||
GSC 2.0 | Jan 2000 | Science target fields only; GSC2 summary | ||||
GSC 1.1 | Aug 1992 | 1981.8 | 0.5” | ~1” | ~1” | First version published for the user community |
GSC 1.0 | Jun 1989 | 1-2” |
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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 stars as determined by Gaia, 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 improve the WCS (fitting x/y to RA/Dec)
Note that 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 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 an adequate number sources to define a reference catalog for matching. These solutions remove uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.
Implementation
The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be 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. More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.
WCS Naming Conventions
Successfully aligning an observation to Gaia using the a posteriori processing will result in an update of the 'active' WCS of the image with the new solution and the new headerlet extension. This headerlet not only includes the WCS keywords which define the transformation from pixels to Gaia-aligned positions on the sky, but it also contains information about how this solution was derived along with the errors to be expected based on the fit.
The various WCS solutions are identified by the WCSNAME keyword found in each FITS headerlet and use the following naming convention:
wcsName = OriginalSolution - CorrectionType
WCS Naming Conventions
Successfully aligning an observation to Gaia using the a posteriori processing will result in an update of the 'active' WCS of the image with the new solution and the new headerlet extension. This headerlet not only includes the WCS keywords which define the transformation from pixels to Gaia-aligned positions on the sky, but it also contains information about how this solution was derived along with the errors to be expected based on the fit.
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 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)
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Table 2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description. The best WCS is the FIT-SVM solution which has the best relative and absolute astrometry.
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 |
IDCTAB reference file '0461802ej_idc.fits', but not aligned to |
any 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_ |
IMG_catalog | ‘undistorted a posteriori solution |
aligned image-by-image to catalog’ | Exposures individually aligned to |
the reference catalog (not as a set |
) |
IDC_0461802ej-FIT_REL_ |
catalog | ‘undistorted a posteriori solution relatively aligned to |
catalog’ | Exposures |
aligned to one another, |
and then aligned as a set to the reference catalog |
IDC_0461802ej-FIT_ |
SVM_catalog |
‘undistorted a posteriori solution relatively aligned |
filter-by- |
filter to catalog’ | HAP-SVM solution; Exposures |
aligned to |
a reference catalog |
IDC_0461802ej-FIT_IMG_NONE
‘undistorted a posteriori solution aligned image-by-image to NONE’
and include improved relative alignment across filters in a visit. BEST WCS SOLUTION |
Implementation
The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be 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. More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.
Caveats
While the majority of calibrated HST data products are now aligned to a common absolute reference frame, further improvements may be possible via manual realignment using the drizzlepac tools. This is particularly true for exposures acquired in the same visit where the WCSNAMEs does not contain the string 'FIT_SVM_GAIA***'. For standard drizzled data products:
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