AstrometryTask¶

AstrometryTask matches a source catalog with objects from a reference catalog and attempts to fit a WCS.

Processing summary¶

AstrometryTask runs this sequence of operations:

Find position reference stars that overlap the exposure.
Match input source catalog to the position reference stars.
Fit a WCS based on the matches.

Python API summary¶

from lsst.meas.astrom.astrometry import AstrometryTask

classAstrometryTask(refObjLoader=None, schema=None, **kwargs): Match an input source catalog with objects from a reference catalog and solve for the WCS...

attributeconfig: Access configuration fields and retargetable subtasks.

methodrun(sourceCat, exposure): Load reference objects, match sources and optionally fit a WCS...

Retargetable subtasks¶

matcher¶

Default: lsst.meas.astrom.matchPessimisticB.MatchPessimisticBTask
Field type: ConfigurableField

reference object/source matcher

referenceSelector¶

Default: lsst.meas.algorithms.sourceSelector.ReferenceSourceSelectorTask
Field type: ConfigurableField

How to select reference objects for cross-matching.

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.

wcsFitter¶

Default: lsst.meas.astrom.fitTanSipWcs.FitTanSipWcsTask
Field type: ConfigurableField

WCS fitter

Configuration fields¶

doMagnitudeOutlierRejection¶

Default: True
Field type: bool Field

If True then a rough zeropoint will be computed from matched sources and outliers will be rejected in the iterations.

forceKnownWcs¶

Default: False
Field type: bool Field

If True then load reference objects and match sources but do not fit a WCS; this simply controls whether ‘run’ calls ‘solve’ or ‘loadAndMatch’

magnitudeOutlierRejectionNSigma¶

Default: 3.0
Field type: float Field

Number of sigma (measured from the distribution) in magnitude for a potential reference/source match to be rejected during iteration.

matchDistanceSigma¶

Default: 2
Field type: float RangeField
Range: [0,inf)

the maximum match distance is set to mean_match_distance + matchDistanceSigma*std_dev_match_distance; ignored if not fitting a WCS

maxIter¶

Default: 3
Field type: int RangeField
Range: [1,inf)

maximum number of iterations of match sources and fit WCSignored if not fitting a WCS

maxMeanDistanceArcsec¶

Default: 0.5
Field type: float RangeField
Range: [0,inf)

Maximum mean on-sky distance (in arcsec) between matched source and reference objects post-fit. A mean distance greater than this threshold raises BadAstrometryFit and the WCS fit is considered a failure. The default is set to the maximum tolerated by the external global calibration (e.g. jointcal) step for conceivable recovery; the appropriate value will be dataset and workflow dependent.

minMatchDistanceArcSec¶

Default: 0.001
Field type: float RangeField
Range: [0,inf)

the match distance below which further iteration is pointless (arcsec); ignored if not fitting a WCS

sourceFluxType¶

Default: 'Psf'
Field type: str Field

Source flux type to use in source selection.

Debugging¶

The lsst.pipe.base.cmdLineTask.CmdLineTask command line task interface supports a flag -d to import debug.py from your PYTHONPATH; see lsstDebug for more about debug.py files.

The available variables in AstrometryTask are

display (bool): If True display information at three stages: after finding reference objects, after matching sources to reference objects, and after fitting the WCS; defaults to False
frame (int): frame to use to display the reference objects; the next two frames are used to display the match list and the results of the final WCS; defaults to 0

To investigate the meas_astrom_astrometry_Debug, put something like

import lsstDebug
def DebugInfo(name):
    debug = lsstDebug.getInfo(name) # N.b. lsstDebug.Info(name) would call us recursively
    if name == "lsst.meas.astrom.astrometry":
        debug.display = True

    return debug

lsstDebug.Info = DebugInfo

into your debug.py file and run this task with the --debug flag.

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