ForcedMeasurementTask

class lsst.meas.base.ForcedMeasurementTask(refSchema, algMetadata=None, **kwds)

Bases: BaseMeasurementTask

Measure sources on an image, constrained by a reference catalog.

A subtask for measuring the properties of sources on a single image, using an existing “reference” catalog to constrain some aspects of the measurement.

Parameters

refSchemalsst.afw.table.Schema

Schema of the reference catalog. Must match the catalog later passed to ‘ForcedMeasurementTask.generateMeasCat` and/or ForcedMeasurementTask.run.

algMetadatalsst.daf.base.PropertyList or None

Will be updated in place to to record information about each algorithm. An empty PropertyList will be created if None.

**kwds

Keyword arguments are passed to the supertask constructor.

Notes

Note that while SingleFrameMeasurementTask is passed an initial Schema that is appended to in order to create the output Schema, ForcedMeasurementTask is initialized with the Schema of the reference catalog, from which a new Schema for the output catalog is created. Fields to be copied directly from the reference Schema are added before Plugin fields are added.

Methods Summary

attachPsfShapeFootprints(sources, exposure)	Attach Footprints to blank sources prior to measurement, by creating elliptical Footprints from the PSF moments.
attachTransformedFootprints(sources, refCat, ...)	Attach Footprints to blank sources prior to measurement, by transforming Footprints attached to the reference catalog.
generateMeasCat(exposure, refCat, refWcs[, ...])	Initialize an output catalog from the reference catalog.
run(measCat, exposure, refCat, refWcs[, ...])	Perform forced measurement.

Methods Documentation

attachPsfShapeFootprints(sources, exposure, scaling=3)

Attach Footprints to blank sources prior to measurement, by creating elliptical Footprints from the PSF moments.

Parameters

sourceslsst.afw.table.SourceCatalog

Blank catalog (with all rows and columns, but values other than coord_ra, coord_dec unpopulated). to which footprints should be attached.

exposurelsst.afw.image.Exposure

Image object from which peak values and the PSF are obtained.

scalingint, optional

Scaling factor to apply to the PSF second-moments ellipse in order to determine the footprint boundary.

Notes

This is a utility function for use by parent tasks; see attachTransformedFootprints for more information.

attachTransformedFootprints(sources, refCat, exposure, refWcs)

Attach Footprints to blank sources prior to measurement, by transforming Footprints attached to the reference catalog.

Notes

Footprints for forced photometry must be in the pixel coordinate system of the image being measured, while the actual detections may start out in a different coordinate system. This default implementation transforms the Footprints from the reference catalog from the WCS to the exposure’s WCS, which downgrades HeavyFootprints into regular Footprints, destroying deblend information.

See the documentation for run for information about the relationships between run, generateMeasCat, and attachTransformedFootprints.

generateMeasCat(exposure, refCat, refWcs, idFactory=None)

Initialize an output catalog from the reference catalog.

Parameters

exposurelsst.afw.image.exposureF

Image to be measured.

refCatiterable of lsst.afw.table.SourceRecord

Catalog of reference sources.

refWcslsst.afw.geom.SkyWcs

Defines the X,Y coordinate system of refCat. This parameter is not currently used.

idFactorylsst.afw.table.IdFactory, optional

Factory for creating IDs for sources.

Returns

meascatlsst.afw.table.SourceCatalog

Source catalog ready for measurement.

Notes

This generates a new blank SourceRecord for each record in refCat. Note that this method does not attach any Footprints. Doing so is up to the caller (who may call attachedTransformedFootprints or define their own method - see run for more information).

run(measCat, exposure, refCat, refWcs, exposureId=None, beginOrder=None, endOrder=None)

Perform forced measurement.

Parameters

exposurelsst.afw.image.exposureF

Image to be measured. Must have at least a lsst.afw.geom.SkyWcs attached.

measCatlsst.afw.table.SourceCatalog

Source catalog for measurement results; must be initialized with empty records already corresponding to those in refCat (via e.g. generateMeasCat).

refCatlsst.afw.table.SourceCatalog

A sequence of lsst.afw.table.SourceRecord objects that provide reference information for the measurement. These will be passed to each plugin in addition to the output SourceRecord.

refWcslsst.afw.geom.SkyWcs

Defines the X,Y coordinate system of refCat.

exposureIdint, optional

Optional unique exposureId used to calculate random number generator seed in the NoiseReplacer.

beginOrderint, optional

Beginning execution order (inclusive). Algorithms with executionOrder < beginOrder are not executed. None for no limit.

endOrderint, optional

Ending execution order (exclusive). Algorithms with executionOrder >= endOrder are not executed. None for no limit.

Notes

Fills the initial empty SourceCatalog with forced measurement results. Two steps must occur before run can be called:

• generateMeasCat must be called to create the output measCat argument.

• Footprints appropriate for the forced sources must be attached to the measCat records. The attachTransformedFootprints method can be used to do this, but this degrades “heavy” (i.e., including pixel values) Footprints to regular Footprints, leading to non-deblended measurement, so most callers should provide Footprints some other way. Typically, calling code will have access to information that will allow them to provide heavy footprints - for instance, ForcedPhotCoaddTask uses the heavy footprints from deblending run in the same band just before non-forced is run measurement in that band.