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 (https://github.com/lsst/gbdes/tree/lsst-dev), 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:

  1. Gathers information on the visit epochs, RAs and Decs, etc.

  2. 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 (https://github.com/lsst/gbdes/tree/lsst-dev).

#. 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.

Python API summary

from lsst.drp.tasks.gbdesAstrometricFit import GbdesAstrometricFitTask
classGbdesAstrometricFitTask(**kwargs)

Calibrate the WCS across multiple visits of the same field using the GBDES package...

attributeconfig

Access configuration fields and retargetable subtasks.

methodrun(inputCatalogRefs, inputVisitSummaries, instrumentName='', refEpoch=None, refObjectLoader=None, inputCameraModel=None)

Run the WCS fit for a given set of visits...

See also

See the GbdesAstrometricFitTask API reference for complete details.

Retargetable subtasks

referenceSelector

Default

lsst.meas.algorithms.sourceSelector.ReferenceSourceSelectorTask

Field type

ConfigurableField

How to down-select the loaded astrometry reference catalog.

sourceSelector

Default
'science'
Field type

Single-selection RegistryField

Choices
'science'

lsst.meas.algorithms.sourceSelector.ScienceSourceSelectorTask

'references'

lsst.meas.algorithms.sourceSelector.ReferenceSourceSelectorTask

'null'

lsst.meas.algorithms.sourceSelector.NullSourceSelectorTask

'objectSize'

lsst.meas.algorithms.objectSizeStarSelector.ObjectSizeStarSelectorTask

'flagged'

lsst.meas.algorithms.flaggedSourceSelector.FlaggedSourceSelectorTask

'astrometry'

lsst.meas.algorithms.astrometrySourceSelector.AstrometrySourceSelectorTask

'matcher'

lsst.meas.algorithms.matcherSourceSelector.MatcherSourceSelectorTask

How to select sources for cross-matching.

Configuration fields

allowSelfMatches

Default
False
Field type

bool Field

Allow multiple sources from the same visit to be associated with the same object.

applyRefCatProperMotion

Default
True
Field type

bool Field

Apply proper motion to shift reference catalog to epoch of observations.

connections

Data type

lsst.pipe.base.config.GbdesAstrometricFitConfigConnections

Field type

ConfigField

Configurations describing the connections of the PipelineTask to datatypes

deviceModel

Default
['BAND/DEVICE/poly']
Field type

str ListField

List of mappings to apply to transform from detector pixels to intermediate frame. Map namesshould match the format ‘BAND/DEVICE/<map name>’.

devicePolyOrder

Default
4
Field type

int Field

Order of device polynomial model.

excludeNonPMObjects

Default
True
Field type

bool Field

Exclude reference objects without proper motion/parallax information.

exposureModel

Default
['EXPOSURE/poly']
Field type

str ListField

List of mappings to apply to transform from intermediate frame to sky coordinates. Map namesshould match the format ‘EXPOSURE/<map name>’.

exposurePolyOrder

Default
6
Field type

int Field

Order of exposure polynomial model.

fitProperMotion

Default
False
Field type

bool Field

Fit the proper motions of the objects.

fitReserveFraction

Default
0.2
Field type

float Field

Fraction of objects to reserve from fit for validation.

fitReserveRandomSeed

Default
1234
Field type

int Field

Set the random seed for selecting data points to reserve from the fit for validation.

matchRadius

Default
1.0
Field type

float Field

Matching tolerance between associated objects (arcseconds).

minMatches

Default
2
Field type

int Field

Number of matches required to keep a source object.

modelComponents

Default
['INSTRUMENT/DEVICE', 'EXPOSURE']
Field type

str ListField

List of mappings to apply to transform from pixels to sky, in order of their application.Supported options are ‘INSTRUMENT/DEVICE’ and ‘EXPOSURE’.

referenceFilter

Default
'phot_g_mean'
Field type

str Field

Name of filter to load from reference catalog. This is a required argument, although the valuesreturned are not used.

referenceSystematicError

Default
0.0
Field type

float Field

Systematic error padding added in quadrature for the reference catalog (marcsec).

saveCameraModel

Default
False
Field type

bool Field

Save the ‘device’ part of the model to be used as input in future runs.

saveLogOutput

Default
True
Field type

bool Field

Flag to enable/disable saving of log output for a task, enabled by default.

saveModelParams

Default
False
Field type

bool Field

Save the parameters and covariance of the WCS model. Default to false because this can be very large.

sourceFluxType

Default
'apFlux_12_0'
Field type

str Field

Source flux field to use in source selection and to get fluxes from the catalog.

systematicError

Default
0.0034
Field type

float Field

Systematic error padding added in quadrature for the science catalogs (marcsec). The defaultvalue is equivalent to 0.02 pixels for HSC.

useInputCameraModel

Default
False
Field type

bool Field

Use a preexisting model for the ‘device’ part of the model. When true, the device part of the model will be held fixed in the fitting process.