Add test for invariance of noise vs flux

python/lsst/cp/verify/verifyPtc.py

@@ -200,6 +200,18 @@ def verify(self, calib, statisticsDict, camera=None):
             verify['PTC_GAIN'] = bool(diffGain < self.config.gainThreshold)
             verify['PTC_NOISE'] = bool(diffNoise < self.config.noiseThreshold)
 
+            # Check that the noises measured in cpPtcExtract do not evolve
+            # as a function of flux.
+            # We fit a linear model to the noise as a function of flux
+            # and check that the maximum absolute residual of the model
+            # from 0 across the full signal range is within one standard
+            # deviation in noise
+            signals = calib.rawMeans[ampName]
+            noiseList = calib.noiseList[ampName]
+            params = np.polyfit(signals, noiseList, deg=1)
+            maxNoiseDiff = (signals[-1] - signals[0])*params[0]
+            verify['NOISE_SIGNAL_INDEPENDENCE'] = bool(maxNoiseDiff < np.std(noiseList))
+
             # DMTN-101: 16.3
             # Check that the measured PTC turnoff is at least greater than the
             # full-well requirement of 90k e-.
@@ -236,7 +248,8 @@ def verify(self, calib, statisticsDict, camera=None):
             verifyStats[ampName] = verify
 
         # Loop over amps to make a detector summary.
-        verifyDetStats = {'PTC_GAIN': [], 'PTC_NOISE': [], 'PTC_TURNOFF': [], 'PTC_BFE_A00': []}
+        verifyDetStats = {'PTC_GAIN': [], 'PTC_NOISE': [], 'PTC_TURNOFF': [], 'PTC_BFE_A00': [],
+                          'NOISE_SIGNAL_INDEPENDENCE': []}
         for amp in verifyStats:
             for testName in verifyStats[amp]:
                 if testName == 'SUCCESS':