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Document multi-band coadd processing.
Add documentation to multiBand.py and a high-level overview of multi-band coadd processing.
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| namespace lsst { namespace pipe { namespace tasks { | ||
| /** | ||
| \page pipeTasks_multiBand High-level Overview of Multi-Band Coadd Processing | ||
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| The goal of the multi-band coadd measurement procedure is to consistently detect and measure sources across | ||
| coadds constructed in multiple filter bands in a consistent and reasonable manner i.e. measure the same | ||
| sources, with consistent centroids and shapes, across all the photometric bands. This is useful, for | ||
| example: | ||
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| - When performing forced photometry of Lyman-break galaxies, which may disappear ('drop out') in bluer | ||
| filters; | ||
| - In obtaining consistent colors, which requires measuring model magnitudes based on identical model | ||
| parameters across all available bands. | ||
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| Consider how this workflow contrasts with a simpler approach in which detection and measurement is carried | ||
| out in each band independently. | ||
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| Our strategy for is broken up into a number of simple steps: | ||
| <DL> | ||
| <DT> i. \ref DetectCoaddSourcesTask_ "DetectCoaddSourcesTask": Detect Sources in Individual Coadds</DT> | ||
| <DD> Begin by detecting sources on each coadd. After scaling the variance plane in a coadd to correct for | ||
| the effects of warping, sources are detected using the \ref SourceDetectionTask_ "SourceDetectionTask" | ||
| subtask. This step produces sources with footprints in each band. | ||
| <DT> ii. \ref MergeDetectionsTask_ "MergeDetectionsTask": Merge Footprints and Peaks from Multiple Coadds | ||
| </DT> | ||
| <DD> Footprints and peaks from all the bands are merged together while keeping track of the band(s) that | ||
| each source comes from. This produces a cross-band master source catalog. During the merging process, we | ||
| cull garbage peaks around bright sources using the criteria described in \ref CullPeaksConfig_ | ||
| "CullPeaksConfig". | ||
| <DT> iii. \ref MeasureMergedCoaddSourcesTask_ "MeasureMergedCoaddSourcesTask": Deblend & Measure Sources | ||
| </DT> | ||
| <DD> The master catalog is used to deblend & measure sources in each coadd. This ensures that all bands | ||
| will have the same sources. First, the \ref SourceDeblendTask_ "SourceDeblendTask" subtask is used to | ||
| deblend sources. The deblender has to retain all peaks from the master catalog, deblending peaks that do not | ||
| correspond to sources in the given coadd (dropouts) as PSFs. Next, the | ||
| \ref SingleFrameMeasurementTask_ "SingleFrameMeasurementTask" subtask is used to measure the properties of | ||
| the deblended children. After this, the \ref SetPrimaryFlagsTask_ "SetPrimaryFlagsTask" subtask | ||
| is run to set 'is-primary' (i.e. source to select for a self-consistent catalog) and related flags. The | ||
| \ref PropagateVisitFlagsTask_ "PropagateVisitFlagsTask" subtask is used to propagate flags from individual | ||
| visits to the coadd (e.g. was the source used for PSF determination in the original visit etc). At this | ||
| point, the \ref ANetAstrometryTask_ "ANetAstrometryTask" subtask can be run (optional, not run by default) | ||
| to match the sources to an external reference catalog such as from PanSTARRS etc. | ||
| <DT> iv. \ref MergeMeasurementsTask_ "MergeMeasurementsTask" Merge Deblended Peaks</DT> | ||
| <DD> Deblended peaks from each band are merged. A 'reference' band for each peak is tracked. This reference | ||
| band is the first band from a priority-sorted list of bands (e.g. `["HSC-I", "HSC-R", "HSC-Z", "HSC-Y", | ||
| "HSC-G",]` in the case of HSC). Every source must have a centroid, shape, and CModel fit in every | ||
| band, even if the object was not detected in the canonical band. | ||
| <DT> v. \ref ForcedPhotCoaddTask_ "ForcedPhotCoaddTask": Run Forced Photometry on Coadds</DT> | ||
| <DD> Forced photometry is run on the coadd for each band using reference centroids and shapes but using | ||
| footprints from the previous de-blending step i.e. the footprints are band specific.</DD> | ||
| <DT> vi. \ref ForcedPhotCcdTask_ "ForcedPhotCcdTask": Run Forced Photometry on Exposures</DT> | ||
| <DD> Forced photometry is run on the individual calibrated exposures for each band for each visit using the | ||
| reference measurements to provide centroids and shapes (but no deblending).</DD> | ||
| </DL> | ||
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| We now provide a complete run through multiband processing using the | ||
| [ci_hsc](https://github.com/lsst/ci_hsc) package. We will process the HSC-I & -R band coadds generated from | ||
| HSC engineering test data provided in the ci_hsc package. To begin, assuming that the lsst stack has been | ||
| already set up, we must set up the obs_subaru and ci_hsc packages. This defines the environment variable | ||
| `$CI_HSC_DIR` which points at the location of the package on disc. The raw HSC exposures live in the | ||
| `$CI_HSC_DIR/raw` directory. Note that all the scons build commands are cumulative i.e. to build the coadd | ||
| in step 4, scons checks to see if the targets of step 3 exist. If not, it first builds step 3 etc. Stepwise, | ||
| we can process the data in ci_hsc as follows: | ||
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| <DL> | ||
| <DT> 1. \ref ProcessCcdTask_ "Process each CCD"</DT> | ||
| <DD> Process the individual ccds in `$CI_HSC_RAW` to produce calibrated exposures.</DD> | ||
| <DT> 2. \ref MakeSkyMapTask_ "Make a SkyMap"</DT> | ||
| <DD> Create a skymap that covers the area of the sky present in the raw exposures.</DD> | ||
| <DT> 3. \ref MakeCoaddTempExpTask_ "Make coaddTempExps"</DT> | ||
| <DD> Warp the individual calibrated exposures to the tangent plane of the coadd.</DD> | ||
| <DT> 4. \ref SafeClipAssembleCoaddTask_ "Assemble Coadds"</DT> | ||
| <DD> Coadd the calibrated exposures. | ||
| \code | ||
| $CI_HSC_DIR scons coadd-HSC-R coadd-HSC-I | ||
| \endcode | ||
| </DD> | ||
| <DT> 5. \ref DetectCoaddSourcesTask_ "Detect Sources in Individual Coadds"</DT> | ||
| <DD> Detect peaks and generate footprints in each coadd individually. | ||
| \code | ||
| $CI_HSC_DIR scons detect-HSC-I detect-HSC-I | ||
| \endcode | ||
| </DD> | ||
| <DT> 6. \ref MergeDetectionsTask_ "Merge Footprints and Peaks from Multiple Coadds"</DT> | ||
| <DD> Merge the detected sources in each coadd by running | ||
| \code | ||
| $CI_HSC_DIR scons mergeDetections | ||
| \endcode | ||
| </DD> | ||
| <DT> 7. \ref MeasureMergedCoaddSourcesTask_ "Deblend & Measure Sources"</DT> | ||
| <DD> Perform deblending and measure sources in each coadd by running | ||
| \code | ||
| $CI_HSC_DIR scons measure-HSC-I measure-HSC-R | ||
| \endcode | ||
| </DD> | ||
| <DT> 8. \ref MergeMeasurementsTask_ "Merge Deblended Peaks"</DT> | ||
| <DD> Create the catalog for driving forced photometry by running | ||
| \code | ||
| $CI_HSC_DIR scons mergeMeasurements | ||
| \endcode | ||
| </DD> | ||
| </DL> | ||
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| Having done this, we can run forced photometry on the coadds and individual visits using the master | ||
| catalog generated by step 8. | ||
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| */ | ||
| }}} // namespace lsst::pipe::tasks |
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