lsst.meas.base.forcedMeasurement Namespace Reference

Classes
class	ForcedMeasurementConfig
class	ForcedMeasurementTask
class	ForcedPlugin
class	ForcedPluginConfig

Detailed Description

Base classes for forced measurement plugins and the driver task for these.

In forced measurement, a reference catalog is used to define restricted
measurements (usually just fluxes) on an image.  As the reference catalog may
be deeper than the detection limit of the measurement image, we do not assume
that we can use detection and deblend information from the measurement image.
Instead, we assume this information is present in the reference catalog and
can be "transformed" in some sense to the measurement frame.  At the very
least, this means that `~lsst.afw.detection.Footprint`\ s from the reference
catalog should be transformed and installed as Footprints in the output
measurement catalog. If we have a procedure that can transform "heavy"
Footprints (ie, including pixel data), we can then proceed with measurement as
usual, but using the reference catalog's ``id`` and ``parent`` fields to
define deblend families.  If this transformation does not preserve
heavy Footprints (this is currently the case, at least for CCD forced
photometry), then we will only be able to replace objects with noise one
deblend family at a time, and hence measurements run in single-object mode may
be contaminated by neighbors when run on objects with ``parent != 0``.

Measurements are generally recorded in the coordinate system of the image
being measured (and all slot-eligible fields must be), but non-slot fields may
be recorded in other coordinate systems if necessary to avoid information loss
(this should, of course, be indicated in the field documentation).  Note that
the reference catalog may be in a different coordinate system; it is the
responsibility of plugins to transform the data they need themselves, using
the reference WCS provided.  However, for plugins that only require a position
or shape, they may simply use output `~lsst.afw.table.SourceCatalog`\'s
centroid or shape slots, which will generally be set to the transformed
position of the reference object before any other plugins are run, and hence
avoid using the reference catalog at all.

Command-line driver tasks for forced measurement include
`ForcedPhotCcdTask`, and `ForcedPhotCoaddTask`.