GbdesAstrometricFitTask#
GbdesAstrometricFitTask fits an astrometric solution, or world coordinate system (WCS), for a series of visits.
Sources in science images are matched with a reference catalog, which allows the fitting algorithm to break degeneracies in the astrometric model.
The fit model is a series of mappings, which can include (but are not limited to) a per-detector polynomial mapping and a per-visit polynomial mapping.
The fit is done using the WCSFit class from the gbdes package (lsst/gbdes), which is an implementation of the method described in Bernstein et al. (2017).
This task requires source catalogs for the input images, a reference catalog, and a visit summary table, which provides a starting point for the WCS fit.
The task produces the fitted WCSs, which are described in AST format and are held in lsst.afw.table.ExposureCatalogs called outputWcs.
The task also outputs a table for debugging, outputCatalog, which contains the sources used in the fit, along with their residuals with the best-fit model.
Processing summary#
GbdesAstrometricFitTask runs this sequence of operations:
Gathers information on the visit epochs, RAs and Decs, etc.
Associates sources in science images and the reference catalog to make matched objects using a friends-of-friends algorithm.
#. Uses the associated objects to fit the astrometric solution, which is a configurable sequence of mappings from the detector pixels to the sky.
This uses WCSFit from the gbdes package (lsst/gbdes).
#. Converts the internal astrometric mappings to AST format.
The output is in the form of one lsst.afw.table.ExposureCatalog for each visit, with a row for each detector.
The fit WCS is accessed by calling getWCS() on the catalog row corresponding to the desired visit and detector.