Improve matching by culling non-good sources

MatchOptimisticBTask was not culling non-good sources from the match list
before returning. That was a bug I accidentally introduced while making
other changes.
I also changed the definition of good sources as those that are have no
interpolated pixels in their center, as recommended by Paul Price;
this catches cosmic rays and bad pixels, as well as saturation.
The result of re-introducing the culling is that the CFHT demo
not reaches 18.6 mas scatter of bright stars in only 3 iterations
of the outer loop in AstrometryTask, so I reduced the default accordingly
(still not quite the 2 Paul Price requested, but close).

python/lsst/meas/astrom/astrometry.py

@@ -34,7 +34,7 @@ class AstrometryConfig(pexConfig.Config):
         doc = "maximum number of iterations of match sources and fit WCS; " +
             "ignored if forceKnownWcs True",
         dtype = int,
-        default = 6,
+        default = 3,
         min = 1,
     )
     matchDistanceSigma = pexConfig.RangeField(

python/lsst/meas/astrom/matchOptimisticB.py

@@ -84,6 +84,8 @@ class SourceInfo(object):
     - centroidFlagKey  key for flag that is True if centroid is valid
     - edgeKey  key for field that is True if source is near an edge
     - saturatedKey  key for field that is True if source has any saturated pixels
+    - interpolatedCenterKey  key for field that is True if center pixels have interpolated values;
+        interpolation is triggered by saturation, cosmic rays and bad pixels, and possibly other reasons
     - fluxField  name of flux field
 
     @throw RuntimeError if schema version unsupported or a needed field is not found
@@ -103,9 +105,9 @@ def __init__(self, schema, fluxType="Ap"):
             self.edgeKey = schema["base_PixelFlags_flag_edge"].asKey()
             self.saturatedKey = schema["base_PixelFlags_flag_saturated"].asKey()
             self.fluxField = "slot_%sFlux_flux" % (fluxType,)
+            self.interpolatedCenterKey = schema["base_PixelFlags_flag_interpolatedCenter"].asKey()
             # extra keys that might be useful
             self.parentKey = schema["parent"].asKey()
-            self.interpolatedCenterKey = schema["base_PixelFlags_flag_interpolatedCenter"].asKey()
         else:
             raise RuntimeError("Version %r of sourceCat schema not supported" % (version,))
 
@@ -133,15 +135,15 @@ def isUsable(self, source):
         - have a valid centroid 
         - be not too near the edge
         """
-        return (
-            self.hasCentroid(source)
-            and not source.get(self.edgeKey)
-        )
+        return self.hasCentroid(source) and not source.get(self.edgeKey)
 
     def isGood(self, source):
-        """Return True if source is usable for matching (as per isUsable) and is not saturated
+        """Return True if source is usable for matching (as per isUsable) and likely has a good centroid
+
+        For a source to have a good centroid it should not be interpolated in the center;
+        this includes saturated sources so we don't have to test separately for that.
         """
-        return self.isUsable(source) and not source.get(self.saturatedKey)
+        return self.isUsable(source) and not source.get(self.interpolatedCenterKey)
 
 
 # The following block adds links to this task from the Task Documentation page.
@@ -245,7 +247,7 @@ def matchObjectsToSources(self, refCat, sourceCat, wcs, refFluxField, maxMatchDi
             if specified then min(config.maxMatchDistArcSec, maxMatchDistArcSec) is used
             if None then config.maxMatchDistArcSec is used
         @return an lsst.pipe.base.Struct with fields:
-        - matches  a list of matches, an instance of lsst.afw.table.ReferenceMatch
+        - matches  a list of matches, each instance of lsst.afw.table.ReferenceMatch
         - usableSourcCat  a catalog of sources potentially usable for matching.
             For this fitter usable sources include unresolved sources not too near the edge.
             It includes saturated sources, even those these are removed from the final match list,
@@ -283,7 +285,7 @@ def matchObjectsToSources(self, refCat, sourceCat, wcs, refFluxField, maxMatchDi
                             int(self.config.minFracMatchedPairs * min([len(refCat), len(usableSourceCat)])))
 
         # match usable (possibly saturated) sources and then purge saturated sources from the match list
-        matches = self._doMatch(
+        usableMatches = self._doMatch(
             refCat = refCat,
             sourceCat = usableSourceCat,
             wcs = wcs,
@@ -295,10 +297,19 @@ def matchObjectsToSources(self, refCat, sourceCat, wcs, refFluxField, maxMatchDi
             verbose = debug.verbose,
         )
 
+        # cull non-good sources
+        matches = []
+        for match in usableMatches:
+            if sourceInfo.isGood(match.second):
+                matches.append(match)
+
+        self.log.logdebug("Found %d usable matches, of which %d had good sources" %
+            (len(usableMatches), len(matches)))
+
         if len(matches) == 0:
             raise RuntimeError("Unable to match sources")
 
-        if self.log: self.log.info("Matched %d sources" % len(matches))
+        self.log.info("Matched %d sources" % len(matches))
         if len(matches) < minMatchedPairs:
             self.log.warn("Number of matches is smaller than request")