Merge pull request #297 from lsst/tickets/DM-48945

DM-48945: Remove postLinearFit intercept from linearity residual computation.
lsst · Feb 14, 2025 · 6cb8c26 · 6cb8c26
2 parents e51c0c8 + 00b95d7
commit 6cb8c26
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Showing 2 changed files with 7 additions and 6 deletions.
diff --git a/python/lsst/cp/pipe/cpLinearitySolve.py b/python/lsst/cp/pipe/cpLinearitySolve.py
@@ -722,7 +722,11 @@ def run(self, inputPtc, dummy, camera, inputDims,
                     linearizeModel[mask],
                     funcPolynomial,
                 )
-                residuals = linearizeModel - (postLinearFit[0] + postLinearFit[1] * inputAbscissa)
+                # When computing residuals, we only care about the slope of
+                # the postLinearFit and not the intercept. The intercept
+                # itself depends on a possibly unknown zero in the abscissa
+                # (often photodiode) which may have an arbitrary value.
+                residuals = linearizeModel - (postLinearFit[1] * inputAbscissa)
                 # We set masked residuals to nan.
                 residuals[~mask] = np.nan
 

diff --git a/tests/test_linearity.py b/tests/test_linearity.py
@@ -465,12 +465,9 @@ def _check_linearity_spline(
             np.testing.assert_array_equal(lin_mask[outlier_indices], False)
 
             # Check the turnoff and max values.
-
-            # The first point at very low flux is noisier and so we exclude
-            # it from the test here.
-            resid_atol = 1.1e-3
+            resid_atol = 4e-4
             self.assertFloatsAlmostEqual(
-                (linearizer.fitResiduals[amp_name][lin_mask] / mu_linear[lin_mask])[1:],
+                linearizer.fitResiduals[amp_name][lin_mask] / mu_linear[lin_mask],
                 0.0,
                 atol=resid_atol,
             )