Index: trunk/psastro/src/psastroOneChipGrid.c
===================================================================
--- trunk/psastro/src/psastroOneChipGrid.c	(revision 42257)
+++ trunk/psastro/src/psastroOneChipGrid.c	(revision 42289)
@@ -84,5 +84,5 @@
     // adjust the chip.toFPA terms only
     pmAstromGridApply (chip->toFPA, stats);
-    psastroUpdateChipToFPA (fpa, chip); // updates PSASTRO.RAWSTARS and PSASTRO.REFSTARS
+    psastroUpdateChipToFPA (fpa, chip); // updates PSASTRO.RAWSTARS and PSASTRO.REFSTARS (see note below)
     psFree (gridStats);
     psFree (rawGridStars);
@@ -92,2 +92,8 @@
     return true;
 }
+
+/* if psastroUpdateChiptoFPA fails to invert the toFPA transformation, 
+   the ref->chip coordinates will not be set.  This is not a problem
+   at this stage since they are not used in the calculation.  Later
+   passes can still yield a valid solution.
+*/