Investigating LSST Measurements¶

Before the LSST software stack can make measurements on a source identified within an image, it must first mask out any surrounding sources so that they do not contaminate the measurements. The software accomplishes this by replacing every other source in an image with noise that corresponds to the measured background distribution.

When attempting to understand the output of measurements after the analysis is complete, it is often useful to put an exposure back into the same state it was in when the measurement was made. What this means in practice is that the noise used to mask out adjacent sources will need to be reconstructed in the same manner (using the same seed) as in the original measurement. This returns the image to a bitwise identical state, so any measurements will produce the exact same output. The example below demonstrates how to do this using a fully processed ci-hsc dataset.

from lsst.meas.base.measurementInvestigationLib import rebuildNoiseReplacer
from lsst.daf.persistence import Butler

ciHscDataPath = "" # Set this to the path to a ci-hsc data repository.
ciHscDataPath = "/ssd/nlust/repos_lsst/ci_hsc/DATA/rerun/ci_hsc/"

# Create a butler object for loading in the data.
butler = Butler(ciHscDataPath)

# Create a data Id for a single ccd.
dataId = {"visit":903334, "ccd":16, "filter":"HSC-R"}

# Load in the calibrated exposure, and the associated source catalog.
exposure = butler.get("calexp", dataId)
srcCat = butler.get("src", dataId)

# Reconstruct a noise replacer from the loaded data.
noiseReplacer = rebuildNoiseReplacer(exposure, srcCat)

The NoiseReplacer object, when created, records the pixel values for all sources in the exposure internally and then sets the pixel values in the exposure to random noise drawn from the background distribution. This allows individual sources to be visualized by adding them source back in into the exposure with a call to the NoiseReplacer’s insertSource method. The source can be removed again with a call to the removeSource method. When all operations involving the NoiseReplacer are completed, calling end will reset the exposure back to its original state.

Rather than inserting and viewing individual sources, a NoiseReplacer object is more commonly used in the context of making measurements. The following example shows how to rerun measurements, using a reconstructed NoiseReplacer. The example selects a subset of objects for which measurements are to be rerun, and places the outputs into a new catalog, retaining the same id for each object.

# Continued from the above example
from lsst.afw.table import SourceTable
from lsst.meas.base.measurementInvestigationLib import makeRerunCatalog
from lsst.meas.base import (SingleFrameMeasurementConfig,
                            SingleFrameMeasurementTask)

# Make a list of ids of objects to remeasure
idsToRerun = [775958066192449538, 775958066192449539,
              775958066192449540, 775958066192449541]

# Fields to copy from old catalog, these are generally fields added outside
# the measurement framework, that may be desirable to maintain
fields = ["deblend_nChild"]

# Create a new schema object, and use it to initialize a measurement task
schema = SourceTable.makeMinimalSchema()

# Configure any plugins at this stage.
measConfig = SingleFrameMeasurementConfig()

measTask = SingleFrameMeasurementTask(schema, config=measConfig)

# Create a Measurement catalog containing only the ids to remeasure
newSrcCatalog = makeRerunCatalog(schema, srcCat, idsToRerun, fields=fields)

# Re-run measure on the sources selected above, using the reconstructed
# noise replacer.
measTask.runPlugins(rebuildNoiseReplacer(exposure, srcCat), newSrcCatalog, exposure)

At the time of writing, measTask.runPlugins will not run on any child objects if their parents are not also in the catalog. The makeRerunCatalog function has two options to resolve this issue. The resetParents flag (True by default) will reset the parent keys of any children whose parents are not included in the idList, effectively turning them into parents. The addParents flag (False by default) will add these parents to the idList.

import numpy as np
# Get only the child ids
idsToRerun = srcCat["id"][srcCat[srcCat.getParentKey()] != 0]

# Previously, it would skip all children - only the noiseReplacer takes time
newSrcCatalog = makeRerunCatalog(
    schema, srcCat, idsToRerun, fields=fields, resetParents=False
)
measTask.runPlugins(rebuildNoiseReplacer(exposure, srcCat), newSrcCatalog, exposure)
# None of these objects have centroids
print(len(newSrcCatalog), np.sum(np.isnan(newSrcCatalog["base_NaiveCentroid_x"])))

# resetParents=True (default) resets parents and takes a few seconds longer
newSrcCatalog = makeRerunCatalog(
    schema, srcCat, idsToRerun, fields=fields, resetParents=True
)
measTask.runPlugins(rebuildNoiseReplacer(exposure, srcCat), newSrcCatalog, exposure)
# Now none of the objects have nan centroids
print(len(newSrcCatalog), np.sum(np.isnan(newSrcCatalog["base_NaiveCentroid_x"])))

# Setting addParents=True adds all parents and takes a little longer still
newSrcCatalog = makeRerunCatalog(
    schema, srcCat, idsToRerun, fields=fields, addParents=True
)
measTask.runPlugins(rebuildNoiseReplacer(exposure, srcCat), newSrcCatalog, exposure)
# None of the objects have nan centroids and the catalog is larger than above
print(len(newSrcCatalog), np.sum(np.isnan(newSrcCatalog["base_NaiveCentroid_x"])))

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Investigating LSST Measurements¶