Index: trunk/psphot/src/mark_psf_source.c
===================================================================
--- trunk/psphot/src/mark_psf_source.c	(revision 4129)
+++ trunk/psphot/src/mark_psf_source.c	(revision 4375)
@@ -14,4 +14,5 @@
 // any object which meets the criterion is marked as 
 // PS_SOURCE_BRIGHTSTAR 
+# define MIN_DS 0.01
 bool mark_psf_source (psSource *source, float shapeNsigma)
 { 
@@ -30,6 +31,9 @@
     dSY = source->modelPSF->dparams->data.F32[5];
 
-    nSx = dSX * SX * SN;
-    nSy = dSY * SY * SN;
+    nSx = dSX / hypot (MIN_DS, 1 / (SX * SN));
+    nSy = dSY / hypot (MIN_DS, 1 / (SY * SN));
+    // sigma = 1 / SX
+    // dsx = 1 / (SX * SN)
+    // dsx_o = hypot (1/SX*SN + MIN_DSX)
 
     // assign PS_SOURCE_BRIGHTSTAR to bright objects within PSF region of dparams[]