Index: trunk/psModules/src/camera/pmFPAMaskWeight.c
===================================================================
--- trunk/psModules/src/camera/pmFPAMaskWeight.c	(revision 29544)
+++ trunk/psModules/src/camera/pmFPAMaskWeight.c	(revision 31451)
@@ -587,5 +587,7 @@
                 double imageValue, varianceValue; // Image and variance value from interpolation
                 psImageMaskType maskValue = 0; // Mask value from interpolation
-                psImageInterpolateStatus status = psImageInterpolate(&imageValue, &varianceValue, &maskValue, x, y, interp);
+
+		// interpolate to pixel center (index + 0.5)
+                psImageInterpolateStatus status = psImageInterpolate(&imageValue, &varianceValue, &maskValue, x + 0.5, y + 0.5, interp);
                 if (status == PS_INTERPOLATE_STATUS_ERROR || status == PS_INTERPOLATE_STATUS_OFF) {
                     psError(PS_ERR_UNKNOWN, false, "Unable to interpolate readout at %d,%d", x, y);