DecorrelateALKernelTask¶

class lsst.ip.diffim.DecorrelateALKernelTask(*args, **kwargs)¶

Bases: lsst.pipe.base.Task

Decorrelate the effect of convolution by Alard-Lupton matching kernel in image difference

Notes

Pipe-task that removes the neighboring-pixel covariance in an image difference that are added when the template image is convolved with the Alard-Lupton PSF matching kernel.

The image differencing pipeline task @link ip.diffim.psfMatch.PsfMatchTask PSFMatchTask@endlink and @link ip.diffim.psfMatch.PsfMatchConfigAL PSFMatchConfigAL@endlink uses the Alard and Lupton (1998) method for matching the PSFs of the template and science exposures prior to subtraction. The Alard-Lupton method identifies a matching kernel, which is then (typically) convolved with the template image to perform PSF matching. This convolution has the effect of adding covariance between neighboring pixels in the template image, which is then added to the image difference by subtraction.

The pixel covariance may be corrected by whitening the noise of the image difference. This task performs such a decorrelation by computing a decorrelation kernel (based upon the A&L matching kernel and variances in the template and science images) and convolving the image difference with it. This process is described in detail in [DMTN-021](http://dmtn-021.lsst.io).

This task has no standalone example, however it is applied as a subtask of pipe.tasks.imageDifference.ImageDifferenceTask.

Methods Summary

`computeCorrectedDiffimPsf`(kappa, psf[, …])	Compute the (decorrelated) difference image’s new PSF.
`computeVarianceMean`(exposure)
`emptyMetadata`()	Empty (clear) the metadata for this Task and all sub-Tasks.
`getAllSchemaCatalogs`()	Get schema catalogs for all tasks in the hierarchy, combining the results into a single dict.
`getFullMetadata`()	Get metadata for all tasks.
`getFullName`()	Get the task name as a hierarchical name including parent task names.
`getName`()	Get the name of the task.
`getSchemaCatalogs`()	Get the schemas generated by this task.
`getTaskDict`()	Get a dictionary of all tasks as a shallow copy.
`makeField`(doc)	Make a `lsst.pex.config.ConfigurableField` for this task.
`makeSubtask`(name, **keyArgs)	Create a subtask as a new instance as the `name` attribute of this task.
`run`(exposure, templateExposure, …[, …])	Perform decorrelation of an image difference exposure.
`timer`(name[, logLevel])	Context manager to log performance data for an arbitrary block of code.

Methods Documentation

static computeCorrectedDiffimPsf(kappa, psf, svar=0.04, tvar=0.04)¶

Compute the (decorrelated) difference image’s new PSF. new_psf = psf(k) * sqrt((svar + tvar) / (svar + tvar * kappa_ft(k)**2))

Parameters:	kappa : `numpy.ndarray` A matching kernel array derived from Alard & Lupton PSF matching psf : `numpy.ndarray` The uncorrected psf array of the science image (and also of the diffim) svar : `float`, optional Average variance of science image used for PSF matching tvar : `float`, optional Average variance of template image used for PSF matching
Returns:	pcf : `numpy.ndarray` a 2-d numpy.array containing the new PSF

computeVarianceMean(exposure)¶

emptyMetadata()¶: Empty (clear) the metadata for this Task and all sub-Tasks.

getAllSchemaCatalogs()¶

Get schema catalogs for all tasks in the hierarchy, combining the results into a single dict.

Returns:	schemacatalogs : `dict` Keys are butler dataset type, values are a empty catalog (an instance of the appropriate lsst.afw.table Catalog type) for all tasks in the hierarchy, from the top-level task down through all subtasks.

Notes

This method may be called on any task in the hierarchy; it will return the same answer, regardless.

The default implementation should always suffice. If your subtask uses schemas the override Task.getSchemaCatalogs, not this method.

getFullMetadata()¶

Get metadata for all tasks.

Returns:	metadata : `lsst.daf.base.PropertySet` The `PropertySet` keys are the full task name. Values are metadata for the top-level task and all subtasks, sub-subtasks, etc..

Notes

The returned metadata includes timing information (if @timer.timeMethod is used) and any metadata set by the task. The name of each item consists of the full task name with . replaced by :, followed by . and the name of the item, e.g.:

topLevelTaskName:subtaskName:subsubtaskName.itemName

using : in the full task name disambiguates the rare situation that a task has a subtask and a metadata item with the same name.

getFullName()¶

Get the task name as a hierarchical name including parent task names.

Returns:	fullName : `str` The full name consists of the name of the parent task and each subtask separated by periods. For example: The full name of top-level task “top” is simply “top”. The full name of subtask “sub” of top-level task “top” is “top.sub”. The full name of subtask “sub2” of subtask “sub” of top-level task “top” is “top.sub.sub2”.

getName()¶

Get the name of the task.

Returns:	taskName : `str` Name of the task.

See also

Task.getAllSchemaCatalogs

Notes

Warning

Subclasses that use schemas must override this method. The default implemenation returns an empty dict.

This method may be called at any time after the Task is constructed, which means that all task schemas should be computed at construction time, not when data is actually processed. This reflects the philosophy that the schema should not depend on the data.

Returning catalogs rather than just schemas allows us to save e.g. slots for SourceCatalog as well.

getTaskDict()¶

Get a dictionary of all tasks as a shallow copy.

Returns:	taskDict : `dict` Dictionary containing full task name: task object for the top-level task and all subtasks, sub-subtasks, etc..

classmethod makeField(doc)¶

Make a lsst.pex.config.ConfigurableField for this task.

Parameters:	doc : `str` Help text for the field.
Returns:	configurableField : `lsst.pex.config.ConfigurableField` A `ConfigurableField` for this task.

Examples

Provides a convenient way to specify this task is a subtask of another task.

Here is an example of use:

class OtherTaskConfig(lsst.pex.config.Config)
    aSubtask = ATaskClass.makeField("a brief description of what this task does")

makeSubtask(name, **keyArgs)¶

Create a subtask as a new instance as the name attribute of this task.

Parameters:	name : `str` Brief name of the subtask. keyArgs Extra keyword arguments used to construct the task. The following arguments are automatically provided and cannot be overridden: “config”. “parentTask”.

Notes

The subtask must be defined by Task.config.name, an instance of pex_config ConfigurableField or RegistryField.

run(exposure, templateExposure, subtractedExposure, psfMatchingKernel, preConvKernel=None, xcen=None, ycen=None, svar=None, tvar=None)¶

Perform decorrelation of an image difference exposure.

Decorrelates the diffim due to the convolution of the templateExposure with the A&L PSF matching kernel. Currently can accept a spatially varying matching kernel but in this case it simply uses a static kernel from the center of the exposure. The decorrelation is described in [DMTN-021, Equation 1](http://dmtn-021.lsst.io/#equation-1), where exposure is I_1; templateExposure is I_2; subtractedExposure is D(k); psfMatchingKernel is kappa; and svar and tvar are their respective variances (see below).

Parameters:

exposure : lsst.afw.image.Exposure: The science afwImage.Exposure used for PSF matching
templateExposure : lsst.afw.image.Exposure: The template exposure used for PSF matching
subtractedExposure :: the subtracted exposure produced by ip_diffim.ImagePsfMatchTask.subtractExposures()
psfMatchingKernel :: An (optionally spatially-varying) PSF matching kernel produced by ip_diffim.ImagePsfMatchTask.subtractExposures()
preConvKernel :: if not None, then the exposure was pre-convolved with this kernel
xcen : float, optional: X-pixel coordinate to use for computing constant matching kernel to use If None (default), then use the center of the image.
ycen : float, optional: Y-pixel coordinate to use for computing constant matching kernel to use If None (default), then use the center of the image.
svar : float, optional: image variance for science image If None (default) then compute the variance over the entire input science image.
tvar : float, optional: Image variance for template image If None (default) then compute the variance over the entire input template image.

Returns:

result : Struct

a lsst.pipe.base.Struct containing:

correctedExposure : the decorrelated diffim
correctionKernel : the decorrelation correction kernel (which may be ignored)

Notes

The subtractedExposure is NOT updated

The returned correctedExposure has an updated PSF as well.

Here we currently convert a spatially-varying matching kernel into a constant kernel, just by computing it at the center of the image (tickets DM-6243, DM-6244).

We are also using a constant accross-the-image measure of sigma (sqrt(variance)) to compute the decorrelation kernel.

Still TBD (ticket DM-6580): understand whether the convolution is correctly modifying the variance plane of the new subtractedExposure.

timer(name, logLevel=10000)¶

Context manager to log performance data for an arbitrary block of code.

Parameters:	name : `str` Name of code being timed; data will be logged using item name: `Start` and `End`. logLevel A `lsst.log` level constant.

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DecorrelateALKernelTask¶