Index: trunk/psModules/src/imcombine/pmSubtractionStamps.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionStamps.c	(revision 26037)
+++ trunk/psModules/src/imcombine/pmSubtractionStamps.c	(revision 26046)
@@ -669,10 +669,10 @@
             psKernel *var = psKernelAllocFromImage(varSub, size, size); // Variance postage stamp
             if (isfinite(stamps->sysErr) && stamps->sysErr > 0) {
-                float sysErr = 0.5 * stamps->sysErr; // Systematic error
+                float sysErr = 0.25 * PS_SQR(stamps->sysErr); // Systematic error
                 psKernel *image1 = stamp->image1, *image2 = stamp->image2; // Input images
                 for (int y = -size; y <= size; y++) {
                     for (int x = -size; x <= size; x++) {
-                        weight->kernel[y][x] = 1.0 / (var->kernel[y][x] +
-                                                      sysErr * (image1->kernel[y][x] + image2->kernel[y][x]));
+                        float additional = image1->kernel[y][x] + image2->kernel[y][x];
+                        weight->kernel[y][x] = 1.0 / (var->kernel[y][x] + sysErr * PS_SQR(additional));
                     }
                 }