Merge pull request #68 from lsst/tickets/DM-32701

DM-32701: Add check and test for invalid PSF order.

python/lsst/meas/extensions/psfex/psfexPsfDeterminer.py

@@ -376,6 +376,16 @@ def determinePsf(self, exposure, psfCandidateList, metadata=None, flagKey=None):
 
         psf = psfex.PsfexPsf(psfs[0], geom.Point2D(avgX, avgY))
 
+        # If there are too few stars, the PSFEx psf model will reduce the order
+        # to 0, which the Science Pipelines code cannot handle (see
+        # https://github.com/lsst/meas_extensions_psfex/blob/f0d5218b5446faf5e39edc30e31d2e6f673ef294/src/PsfexPsf.cc#L118
+        # ).  The easiest way to test for this condition is trying to compute
+        # the PSF kernel and checking for an InvalidParameterError.
+        try:
+            _ = psf.getKernel(psf.getAveragePosition())
+        except pexExcept.InvalidParameterError:
+            raise RuntimeError("Failed to determine psfex psf: too few good stars.")
+
         #
         # Display code for debugging
         #

tests/test_PsfexPsf.py

@@ -272,6 +272,19 @@ def testPsfexDeterminer(self):
         self.assertEqual(psf.computeBBox(pos), psf.computeKernelImage(pos).getBBox())
         self.assertEqual(psf.computeBBox(pos), psf.getKernel(pos).getBBox())
 
+    def testPsfexDeterminerTooFewStars(self):
+        """Test the (Psfex) psfDeterminer with too few stars."""
+        self.setupDeterminer(self.exposure)
+        metadata = dafBase.PropertyList()
+
+        stars = self.starSelector.run(self.catalog, exposure=self.exposure)
+        psfCandidateList = self.makePsfCandidates.run(stars.sourceCat, exposure=self.exposure).psfCandidates
+
+        psfCandidateListShort = psfCandidateList[0: 3]
+
+        with self.assertRaisesRegex(RuntimeError, "Failed to determine"):
+            psf, cellSet = self.psfDeterminer.determinePsf(self.exposure, psfCandidateListShort, metadata)
+
 
 class TestMemory(lsst.utils.tests.MemoryTestCase):
     pass