CompareWarpAssembleCoaddTask

class lsst.drp.tasks.assemble_coadd.CompareWarpAssembleCoaddTask(*args, **kwargs)

Bases: AssembleCoaddTask

Assemble a compareWarp coadded image from a set of warps by masking artifacts detected by comparing PSF-matched warps.

In AssembleCoaddTask, we compute the coadd as an clipped mean (i.e., we clip outliers). The problem with doing this is that when computing the coadd PSF at a given location, individual visit PSFs from visits with outlier pixels contribute to the coadd PSF and cannot be treated correctly. In this task, we correct for this behavior by creating a new badMaskPlane ‘CLIPPED’ which marks pixels in the individual warps suspected to contain an artifact. We populate this plane on the input warps by comparing PSF-matched warps with a PSF-matched median coadd which serves as a model of the static sky. Any group of pixels that deviates from the PSF-matched template coadd by more than config.detect.threshold sigma, is an artifact candidate. The candidates are then filtered to remove variable sources and sources that are difficult to subtract such as bright stars. This filter is configured using the config parameters temporalThreshold and spatialThreshold. The temporalThreshold is the maximum fraction of epochs that the deviation can appear in and still be considered an artifact. The spatialThreshold is the maximum fraction of pixels in the footprint of the deviation that appear in other epochs (where other epochs is defined by the temporalThreshold). If the deviant region meets this criteria of having a significant percentage of pixels that deviate in only a few epochs, these pixels have the ‘CLIPPED’ bit set in the mask. These regions will not contribute to the final coadd. Furthermore, any routine to determine the coadd PSF can now be cognizant of clipped regions. Note that the algorithm implemented by this task is preliminary and works correctly for HSC data. Parameter modifications and or considerable redesigning of the algorithm is likley required for other surveys.

CompareWarpAssembleCoaddTask sub-classes AssembleCoaddTask and instantiates AssembleCoaddTask as a subtask to generate the TemplateCoadd (the model of the static sky).

Notes

Debugging: This task supports the following debug variables: - saveCountIm

If True then save the Epoch Count Image as a fits file in the figPath

• figPath

  Path to save the debug fits images and figures

Methods Summary

applyAltEdgeMask(mask, altMaskList)	Propagate alt EDGE mask to SENSOR_EDGE AND INEXACT_PSF planes.
filterArtifacts(spanSetList, ...[, ...])	Filter artifact candidates.
findArtifacts(templateCoadd, warpRefList, ...)	Find artifacts.
prefilterArtifacts(spanSetList, exp)	Remove artifact candidates covered by bad mask plane.
run(skyInfo, *, warpRefList, ...)	Assemble a coadd from input warps.

Methods Documentation

applyAltEdgeMask(mask, altMaskList)

Propagate alt EDGE mask to SENSOR_EDGE AND INEXACT_PSF planes.

Parameters

masklsst.afw.image.Mask

Original mask.

altMaskListlist of dict

List of Dicts containing spanSet lists. Each element contains the new mask plane name (e.g. “CLIPPED and/or “NO_DATA”) as the key, and list of SpanSets to apply to the mask.

filterArtifacts(spanSetList, epochCountImage, nImage, footprintsToExclude=None)

Filter artifact candidates.

Parameters

spanSetListlist [lsst.afw.geom.SpanSet]

List of SpanSets representing artifact candidates.

epochCountImagelsst.afw.image.Image

Image of accumulated number of warpDiff detections.

nImagelsst.afw.image.ImageU

Image of the accumulated number of total epochs contributing.

Returns

maskSpanSetListlist [lsst.afw.geom.SpanSet]

List of SpanSets with artifacts.

findArtifacts(templateCoadd, warpRefList, imageScalerList)

Find artifacts.

Loop through warps twice. The first loop builds a map with the count of how many epochs each pixel deviates from the templateCoadd by more than config.chiThreshold sigma. The second loop takes each difference image and filters the artifacts detected in each using count map to filter out variable sources and sources that are difficult to subtract cleanly.

Parameters

templateCoaddlsst.afw.image.Exposure

Exposure to serve as model of static sky.

warpRefListlist

List of dataset handles (data references) to warps.

imageScalerListlist

List of image scalers. Deprecated and will be removed after v29.

Returns

altMaskslist of dict

List of dicts containing information about CLIPPED (i.e., artifacts), NO_DATA, and EDGE pixels.

prefilterArtifacts(spanSetList, exp)

Remove artifact candidates covered by bad mask plane.

Any future editing of the candidate list that does not depend on temporal information should go in this method.

Parameters

spanSetListlist [lsst.afw.geom.SpanSet]

List of SpanSets representing artifact candidates.

explsst.afw.image.Exposure

Exposure containing mask planes used to prefilter.

Returns

returnSpanSetListlist [lsst.afw.geom.SpanSet]

List of SpanSets with artifacts.

run(skyInfo, *, warpRefList, imageScalerList, weightList, psfMatchedWarpRefList, supplementaryData)

Assemble a coadd from input warps.

Assemble the coadd using the provided list of coaddTempExps. Since the full coadd covers a patch (a large area), the assembly is performed over small areas on the image at a time in order to conserve memory usage. Iterate over subregions within the outer bbox of the patch using assembleSubregion to stack the corresponding subregions from the coaddTempExps with the statistic specified. Set the edge bits the coadd mask based on the weight map.

Parameters

skyInfoStruct

Struct with geometric information about the patch.

warpRefListlist

List of dataset handles (data references) to Warps (previously called CoaddTempExps).

imageScalerListlist

List of image scalers. Deprecated and will be removed after v29 in DM-49083.

weightListlist

List of weights.

psfMatchedWarpRefListlist, optional

List of dataset handles (data references) to psfMatchedWarps.

altMaskListlist, optional

List of alternate masks to use rather than those stored with warp.

maskint, optional

Bit mask value to exclude from coaddition.

supplementaryDataStruct, optional

Struct with additional data products needed to assemble coadd. Only used by subclasses that implement _makeSupplementaryData and override run.

Returns

resultStruct

Results as a struct with attributes:

coaddExposure

Coadded exposure (Exposure).

nImage

Exposure count image (Image), if requested.

inputMap

Bit-wise map of inputs, if requested.

warpRefList

Input list of dataset handles (data refs) to the warps (DeferredDatasetHandle) (unmodified).

imageScalerList

Input list of image scalers (list) (unmodified). Deprecated and will be removed after v29 in DM-49083.

weightList

Input list of weights (list) (unmodified).

Raises

lsst.pipe.base.NoWorkFound

Raised if no dataset handles (data references) are provided.

Notes

Assemble the coadd.

Find artifacts and apply them to the warps’ masks creating a list of alternative masks with a new “CLIPPED” plane and updated “NO_DATA” plane. Then pass these alternative masks to the base class’s run method.