PhotoCalTask#

class lsst.pipe.tasks.photoCal.PhotoCalTask(refObjLoader=None, schema=None, **kwds)#

Bases: Task

Calculate an Exposure’s zero-point given a set of flux measurements of stars matched to an input catalogue.

Parameters#

refObjLoaderlsst.meas.algorithms.ReferenceObjectLoader

A reference object loader object; gen3 pipeline tasks will pass None and call match.setRefObjLoader in runQuantum.

schemalsst.afw.table.Schema, optional

The schema of the detection catalogs used as input to this task.

**kwds

Additional keyword arguments.

Notes#

The type of flux to use is specified by PhotoCalConfig.fluxField.

The algorithm clips outliers iteratively, with parameters set in the configuration.

This task can adds fields to the schema, so any code calling this task must ensure that these columns are indeed present in the input match list; see pipe_tasks_photocal_Example.

Debugging:

The available lsstDebug variables in PhotoCalTask are:

display :

If True enable other debug outputs.

displaySources :

If True, display the exposure on ds9’s frame 1 and overlay the source catalogue.

red o :

Reserved objects.

green o :

Objects used in the photometric calibration.

scatterPlot :

Make a scatter plot of flux v. reference magnitude as a function of reference magnitude:

  • good objects in blue

  • rejected objects in red

(if scatterPlot is 2 or more, prompt to continue after each iteration)

Methods Summary

displaySources(exposure, matches, reserved)

Display sources we'll use for photocal.

extractMagArrays(matches, filterLabel, ...)

Extract magnitude and magnitude error arrays from the given matches.

getSourceKeys(schema)

Return a struct containing the source catalog keys for fields used by PhotoCalTask.

getZeroPoint(src, ref[, srcErr, zp0])

Flux calibration code, returning (ZeroPoint, Distribution Width, Number of stars).

run(exposure, sourceCat[, expId])

Do photometric calibration - select matches to use and (possibly iteratively) compute the zero point.

Methods Documentation

displaySources(exposure, matches, reserved, frame=1)#

Display sources we’ll use for photocal.

Sources that will be actually used will be green. Sources reserved from the fit will be red.

Parameters#

exposurelsst.afw.image.ExposureF

Exposure to display.

matcheslist of lsst.afw.table.RefMatch

Matches used for photocal.

reservednumpy.ndarray of type bool

Boolean array indicating sources that are reserved.

frameint, optional

Frame number for display.

extractMagArrays(matches, filterLabel, sourceKeys)#

Extract magnitude and magnitude error arrays from the given matches.

Parameters#

matcheslsst.afw.table.ReferenceMatchVector

Reference/source matches.

filterLabelstr

Label of filter being calibrated.

sourceKeyslsst.pipe.base.Struct

Struct of source catalog keys, as returned by getSourceKeys().

Returns#

resultlsst.pipe.base.Struct

Results as a struct with attributes:

srcMag

Source magnitude (np.array).

refMag

Reference magnitude (np.array).

srcMagErr

Source magnitude error (np.array).

refMagErr

Reference magnitude error (np.array).

magErr

An error in the magnitude; the error in srcMag - refMag. If nonzero, config.magErrFloor will be added to magErr only (not srcMagErr or refMagErr), as magErr is what is later used to determine the zero point (np.array).

refFluxFieldList

A list of field names of the reference catalog used for fluxes (1 or 2 strings) (list).

getSourceKeys(schema)#

Return a struct containing the source catalog keys for fields used by PhotoCalTask.

Parameters#

schemalsst.afw.table.schema

Schema of the catalog to get keys from.

Returns#

resultlsst.pipe.base.Struct

Results as a struct with attributes:

instFlux

Instrument flux key.

instFluxErr

Instrument flux error key.

getZeroPoint(src, ref, srcErr=None, zp0=None)#

Flux calibration code, returning (ZeroPoint, Distribution Width, Number of stars).

Returns#

resultlsst.pipe.base.Struct

Results as a struct with attributes:

zp

Photometric zero point (mag, float).

sigma

Standard deviation of fit of photometric zero point (mag, float).

ngood

Number of sources used to fit photometric zero point (int).

Notes#

We perform nIter iterations of a simple sigma-clipping algorithm with a couple of twists: - We use the median/interquartile range to estimate the position to clip around, and the “sigma” to use. - We never allow sigma to go _above_ a critical value sigmaMax — if we do, a sufficiently large estimate will prevent the clipping from ever taking effect. - Rather than start with the median we start with a crude mode. This means that a set of magnitude residuals with a tight core and asymmetrical outliers will start in the core. We use the width of this core to set our maximum sigma (see second bullet).

run(exposure, sourceCat, expId=0)#

Do photometric calibration - select matches to use and (possibly iteratively) compute the zero point.

Parameters#

exposurelsst.afw.image.Exposure

Exposure upon which the sources in the matches were detected.

sourceCatlsst.afw.table.SourceCatalog

Good stars selected for use in calibration.

expIdint, optional

Exposure ID.

Returns#

resultlsst.pipe.base.Struct

Results as a struct with attributes:

photoCalib

Object containing the zero point (lsst.afw.image.Calib).

arrays

Magnitude arrays returned be PhotoCalTask.extractMagArrays.

matches

ReferenceMatchVector, as returned by the matcher

matchMetametadata needed to unpersist matches, as returned

by the matcher (lsst.daf.base.PropertyList)

zp

Photometric zero point (mag, float).

sigma

Standard deviation of fit of photometric zero point (mag, float).

ngood

Number of sources used to fit photometric zero point (int).

Raises#

RuntimeError

Raised if any of the following occur: - No matches to use for photocal. - No matches are available (perhaps no sources/references were selected by the matcher). - No reference stars are available. - No matches are available from which to extract magnitudes.

Notes#

The exposure is only used to provide the name of the filter being calibrated (it may also be used to generate debugging plots).

The reference objects: - Must include a field photometric; True for objects which should be considered as photometric standards. - Must include a field flux; the flux used to impose a magnitude limit and also to calibrate the data to (unless a color term is specified, in which case ColorTerm.primary is used; See https://jira.lsstcorp.org/browse/DM-933). - May include a field stargal; if present, True means that the object is a star. - May include a field var; if present, True means that the object is variable.

The measured sources: - Must include PhotoCalConfig.fluxField; the flux measurement to be used for calibration.