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, 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
'objectSize': lsst.meas.algorithms.objectSizeStarSelector.ObjectSizeStarSelectorTask
'flagged': lsst.meas.algorithms.flaggedSourceSelector.FlaggedSourceSelectorTask
'astrometry': lsst.meas.algorithms.astrometrySourceSelector.AstrometrySourceSelectorTask
'matcher': lsst.meas.algorithms.matcherSourceSelector.MatcherSourceSelectorTask
'diaCatalog': lsst.ip.diffim.diaCatalogSourceSelector.DiaCatalogSourceSelectorTask

How to select sources for cross-matching.

wcsFitter¶

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

WCS fitter

Configuration fields¶

doMagnitudeOutlierRejection¶

Default: False
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 rerference objects post-fit. A mean distance greater than this threshold raises a TaskError 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. 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: 'Calib'
Field type: str Field

Source flux type to use in source selection.

Examples¶

See lsst.pipe.tasks.photoCal.PhotoCalTask.

Note

Task documentation is currently in the process of being converted from the old (Doxygen) system to this site. Until that conversion is complete, refer to the Doxygen documentation for PhotoCalTask.

Debugging¶

The lsst.pipe.base.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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