sourceSelector: add selection by magnitude error

If you've got only a magnitude (not flux), you can't use the
SignalToNoiseLimit without some modification. However, for
magnitudes the error is the inverse of the signal-to-noise
ratio, so we can just put a limit on that.

python/lsst/meas/algorithms/sourceSelector.py

@@ -292,6 +292,37 @@ def apply(self, catalog):
         return selected
 
 
+class MagnitudeErrorLimit(BaseLimit):
+    """Select sources using a magnitude error limit
+
+    Because the magnitude error is the inverse of the signal-to-noise
+    ratio, this also works to select sources by signal-to-noise when
+    you only have a magnitude.
+
+    This object can be used as a `lsst.pex.config.Config` for configuring
+    the limit, and then the `apply` method can be used to identify sources
+    in the catalog that match the configured limit.
+    """
+    magErrField = pexConfig.Field(dtype=str, default="mag_err",
+                                  doc="Name of the source flux error field to use.")
+
+    def apply(self, catalog):
+        """Apply the magnitude limit to a catalog
+
+        Parameters
+        ----------
+        catalog : `lsst.afw.table.SourceCatalog`
+            Source to which to apply limits.
+
+        Returns
+        -------
+        selected : `numpy.ndarray`
+            Boolean array indicating whether each source is selected
+            (True means selected).
+        """
+        return BaseLimit.apply(self, catalog[self.magErrField])
+
+
 class RequireFlags(pexConfig.Config):
     """Select sources using flags
 
@@ -425,9 +456,11 @@ class ReferenceSourceSelectorConfig(pexConfig.Config):
     doMagLimit = pexConfig.Field(dtype=bool, default=False, doc="Apply magnitude limit?")
     doFlags = pexConfig.Field(dtype=bool, default=False, doc="Apply flag limitation?")
     doSignalToNoise = pexConfig.Field(dtype=bool, default=False, doc="Apply signal-to-noise limit?")
+    doMagError = pexConfig.Field(dtype=bool, default=False, doc="Apply magnitude error limit?")
     magLimit = pexConfig.ConfigField(dtype=MagnitudeLimit, doc="Magnitude limit to apply")
     flags = pexConfig.ConfigField(dtype=RequireFlags, doc="Flags to require")
     signalToNoise = pexConfig.ConfigField(dtype=SignalToNoiseLimit, doc="Signal-to-noise limit to apply")
+    magError = pexConfig.ConfigField(dtype=MagnitudeErrorLimit, doc="Magnitude error limit to apply")
     colorLimits = pexConfig.ConfigDictField(keytype=str, itemtype=ColorLimit, default={},
                                             doc="Color limits to apply; key is used as a label only")
 
@@ -462,6 +495,8 @@ def selectSources(self, catalog, matches=None):
             selected &= self.config.flags.apply(catalog)
         if self.config.doSignalToNoise:
             selected &= self.config.signalToNoise.apply(catalog)
+        if self.config.doMagError:
+            selected &= self.config.magError.apply(catalog)
         for limit in self.config.colorLimits.values():
             selected &= limit.apply(catalog)
 

tests/test_sourceSelector.py

@@ -198,6 +198,22 @@ def testMagnitudeLimit(self):
         self.config.magLimit.maximum = None
         self.check([True, True, False, True])
 
+    def testMagErrorLimit(self):
+        field = "flux_err"
+        # Note: using a field named "flux_err" as if it was a magnitude to save adding a new field
+        tooFaint = self.catalog.addNew()
+        tooFaint.set(field, 0.5)
+        tooBright = self.catalog.addNew()
+        tooBright.set(field, 0.00001)
+        good = self.catalog.addNew()
+        good.set(field, 0.2)
+
+        self.config.doMagError = True
+        self.config.magError.minimum = 0.01
+        self.config.magError.maximum = 0.3
+        self.config.magError.magErrField = field
+        self.check([False, False, True])
+
     def testColorLimits(self):
         num = 10
         for _ in range(num):