Index: trunk/ppSim/src/ppSimMakeStars.c
===================================================================
--- trunk/ppSim/src/ppSimMakeStars.c	(revision 17616)
+++ trunk/ppSim/src/ppSimMakeStars.c	(revision 18011)
@@ -1,3 +1,7 @@
 # include "ppSim.h"
+
+# define ESCAPE(MSG) { \
+  psError(PS_ERR_BAD_PARAMETER_VALUE, true, MSG); \
+  return false; }
 
 bool ppSimMakeStars(psArray *stars, pmFPA *fpa, pmConfig *config, const psRandom *rng) {
@@ -23,5 +27,11 @@
     float zp           = psMetadataLookupF32(&status, recipe, "ZEROPOINT"); // Photometric zero point
     float seeing       = psMetadataLookupF32(&status, recipe, "SEEING"); // Seeing SIGMA (pixels)
-    float scale        = psMetadataLookupF32(&status, recipe, "SCALE") * M_PI / 3600.0 / 180.0; // Plate scale (radians/pixel)
+    float scale        = psMetadataLookupF32(&status, recipe, "PIXEL.SCALE") * M_PI / 3600.0 / 180.0; // Plate scale (radians/pixel)
+
+    if (isnan(darkRate)) darkRate = 0.0;
+    if (isnan(expTime))  ESCAPE("EXPTIME is not defined");
+    if (isnan(zp))       ESCAPE("ZEROPOINT is not defined");
+    if (isnan(seeing))   ESCAPE("SEEING is not defined");
+    if (isnan(scale))    ESCAPE("PIXEL.SCALE is not defined");
 
     bool flatLum       = psMetadataLookupBool(&status,recipe, "STARS.FLAT.LUM"); // were real stars generated?
@@ -47,5 +57,10 @@
 	refMag = psMetadataLookupF32(&status, fpa->concepts, "STARS.REAL.MAG.PEAK"); // Star brightest magnitude
 	refSum = psMetadataLookupF32(&status, fpa->concepts, "STARS.REAL.SUM.PEAK"); // Star brightest magnitude
-	assert (status);
+
+	// if we tried and failed to load reference stars, set more artificial limits
+	if (!status) {
+	    refMag = brightMag;
+	    refSum = 1;
+	}
     } else {
 	refMag = brightMag;