Following RFC-866 we have switched both the AP and DRP pipelines to use the new image differencing tasks. The old
lsst.pipe.tasks.ImageDifferenceTask and related
lsst.pipe.tasks.ImageDifferenceFromTemplateTask are now deprecated, and will eventually be removed.
Image differencing is now divided into three distinct steps, each with its own PipelineTask:
lsst.ip.diffim.GetTemplateTaskwhich constructs the warped and cropped (and possibly DCR-corrected) templateExp. This step was implemented previously as part of
lsst.ip.diffim.AlardLuptonSubtractTaskwhich PSF-matches either the template or science image, and subtracts the template from the science image. This Task can be replaced by other algorithms, such as a (yet to be written)
ZogySubtractTaskwith the appropriate input and output connections.
lsst.ip.diffim.DetectAndMeasureTaskwhich runs source detection and measurement on the difference image, including adding sky sources and forced measurement. For a future
ZogySubtractTaskor other image subtraction Task that produces a maximum likelihood image as an output connection we will create a new
DetectAndMeasureLikelihoodTaskthat inherits from
In order to simplify the transition to the new image differencing pipeline,
lsst.ip.diffim.AlardLuptonSubtractTask has been implemented with a temporary config option
forceCompatibility which currently defaults to
True. With this option enabled, the new task will produce results that are very similar to the old
lsst.pipe.tasks.ImageDifferenceFromTemplateTask. After a period of additional testing and verification we will disable the
The current implementation supports only image differencing based on the Alard&Lupton algorithm, though either the template or science image may be convolved and the decorrelation afterburner is included. In the future we will also implement Alard&Lupton with preconvolution, and ZOGY may be implemented.