From fe09c0d214a29526543faf94a0ffc657a4e98967 Mon Sep 17 00:00:00 2001
From: Jim Bosch <jbosch@astro.princeton.edu>
Date: Thu, 23 Jul 2015 14:13:06 -0400
Subject: [PATCH] Fix PhotoCal error calculation to use inverse variance
 weighting.

Previous versions used the uncertainty as the weight, which is completely
backwards.
---
 python/lsst/pipe/tasks/photoCal.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/python/lsst/pipe/tasks/photoCal.py b/python/lsst/pipe/tasks/photoCal.py
index e50f993d3..83418ba47 100644
--- a/python/lsst/pipe/tasks/photoCal.py
+++ b/python/lsst/pipe/tasks/photoCal.py
@@ -810,8 +810,10 @@ def getZeroPoint(self, src, ref, srcErr=None, zp0=None):
 
         dmag = dmag[good]
         dmagErr = dmagErr[good]
+        zp, weightSum = np.average(dmag, weights=1/dmagErr**2, returned=True)
+        sigma = np.sqrt(1.0/weightSum)
         return pipeBase.Struct(
-            zp = np.average(dmag, weights=dmagErr),
-            sigma = np.std(dmag, ddof=1),
+            zp = zp,
+            sigma = sigma,
             ngood = len(dmag),
         )