Index: trunk/ppSim/src/ppSimMakeStars.c
===================================================================
--- trunk/ppSim/src/ppSimMakeStars.c	(revision 14817)
+++ trunk/ppSim/src/ppSimMakeStars.c	(revision 16498)
@@ -18,5 +18,4 @@
     float starsDensity = psMetadataLookupF32(NULL, config->arguments, "STARS.DENSITY"); // Density of fakes
 
-    float skyRate = psMetadataLookupF32(NULL, config->arguments, "SKY.RATE"); // Sky rate
     float darkRate = psMetadataLookupF32(NULL, config->arguments, "DARK.RATE"); // Dark rate
     float expTime = psMetadataLookupF32(NULL, config->arguments, "EXPTIME"); // Exposure time
@@ -25,4 +24,10 @@
     float seeing = psMetadataLookupF32(NULL, config->arguments, "SEEING"); // Seeing sigma (pix)
     float scale = psMetadataLookupF32(NULL, config->arguments, "SCALE") * M_PI / 3600.0 / 180.0; // Plate scale (radians/pixel)
+
+    float skyRate = psMetadataLookupF32(&mdok, config->arguments, "SKY.RATE"); // Sky rate
+    if (isnan(skyRate)) {
+	float skyMags = psMetadataLookupF32(&mdok, config->arguments, "SKY.MAGS");  assert (mdok);
+	skyRate = scale * scale * pow (10.0, -0.4*(skyMags - zp));
+    }
 
     if (starsDensity <= 0) return true;