Index: trunk/ppBackground/src/ppBackgroundStackMath.c
===================================================================
--- trunk/ppBackground/src/ppBackgroundStackMath.c	(revision 36642)
+++ trunk/ppBackground/src/ppBackgroundStackMath.c	(revision 36649)
@@ -55,5 +55,7 @@
 	  }
 	  psMetadata *chipData = psMetadataLookupPtr(NULL, expItem->data.md, workingChip);
+	  if (!chipData) { continue; }
 	  psImage *image      = psMetadataLookupPtr(NULL, chipData, "bkg image");
+	  if (!image) { continue; }
 	  psVectorAppend(tmp,image->data.F32[v][u]);
 	} // End loop over exposures
@@ -63,6 +65,17 @@
 
 	psFree(expIter);
+	psFree(tmp);
       } // End u
     } // End v
+    // Remove the median value from this data.  We just want the tilts, not the offsets
+    psStatsInit(stats);
+    psImageStats(stats,solution,NULL,0);
+    for (v = 0; v < solution->numRows; v++) {
+      for (u = 0; u < solution->numCols; u++) {
+	solution->data.F32[v][u] -= stats->robustMedian;
+      }
+    }
+    
+    
   } // End working chip scan
 
@@ -90,27 +103,37 @@
     psMetadataItem *chipItem;
     while ((chipItem = psMetadataGetAndIncrement(chipIter))) {
+      //      const char *chipName = chipItem->name;
+      
       psImage *image = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg image");
       psImage *ra    = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg ra");
       psImage *dec   = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg dec");
-      
+      //      psImage *camera= psMetadataLookupPtr(NULL, data->OTA_solutions, chipName);      
       psVector *obs  = psVectorAllocEmpty(image->numRows, PS_TYPE_F32);
       psVector *model= psVectorAllocEmpty(image->numRows, PS_TYPE_F32);
-      
+
+      int j = 0;
+      int used = 0;
       for (v = 0; v < image->numRows; v++) {
 	for (u = 0; u < image->numCols; u++) {
 	  if ((ra->data.F32[v][u] < data->x_min)||(ra->data.F32[v][u] > data->x_max)||
-	      (dec->data.F32[v][u] < data->y_min)||(dec->data.F32[v][u] > data->y_max)) { continue; }
-	  psVectorAppend(obs,image->data.F32[v][u]);
+	      (dec->data.F32[v][u] < data->y_min)||(dec->data.F32[v][u] > data->y_max)) { j++; continue; }
+	  psVectorAppend(obs,image->data.F32[v][u]);// - camera->data.F32[v][u]);
 	  psVectorAppend(model, psImageMapEval(data->modelMap,ra->data.F32[v][u],dec->data.F32[v][u]));
-	}
-      }
-
-      psPolynomial1D *poly = psPolynomial1DAlloc(PS_POLYNOMIAL_ORD,1);
-      int status = psVectorFitPolynomial1D(poly,NULL,0,model,NULL,obs);
-      if (!status) {
-	psMetadataAddF32(chipItem->data.md,PS_LIST_TAIL,"bkg offset", PS_META_REPLACE, "background offset for this exposure/ota pair", poly->coeff[0]);
-	psMetadataAddF32(chipItem->data.md,PS_LIST_TAIL,"bkg scale", PS_META_REPLACE, "background scale for this exposure/ota pair", poly->coeff[1]);
-      }
-      psFree(poly);
+	  j++;
+	  used++;
+	}
+      }
+
+      if (used > 0) {
+
+	psPolynomial1D *poly = psPolynomial1DAlloc(PS_POLYNOMIAL_ORD,1);
+	int status = psVectorFitPolynomial1D(poly,NULL,0,model,NULL,obs);
+	printf("in model fit loop: %d %d %d %f %f\n",status,j,used,poly->coeff[0],poly->coeff[1]);
+	if (status && (poly->coeff[1] != 0.0)) {
+	  psMetadataAddF32(chipItem->data.md,PS_LIST_TAIL,"bkg offset", PS_META_REPLACE, "background offset for this exposure/ota pair", poly->coeff[0]);
+	  psMetadataAddF32(chipItem->data.md,PS_LIST_TAIL,"bkg scale", PS_META_REPLACE, "background scale for this exposure/ota pair", poly->coeff[1]);
+	}
+	psFree(poly);
+      }
     } // End OTA loop
     psFree(chipIter);
@@ -153,5 +176,5 @@
 	      (dec->data.F32[v][u] < data->y_min)||(dec->data.F32[v][u] > data->y_max)) { continue; }
 
-	  model->data.F32[v][u] = scale * image->data.F32[v][u] - offset - camera->data.F32[v][u];
+	  model->data.F32[v][u] = scale * image->data.F32[v][u] + offset - camera->data.F32[v][u];
 	}
       }
@@ -168,8 +191,8 @@
 // This "averaging" is done using the psImageMapClipFit.
 bool ppBackgroundStackModelFit(ppBackgroundStackData *data) {
-  long j;
-  int u,v;
-
-  long used;
+  long j = 0;
+  int u,v;
+
+  long used = 0;
   psS16 N = psMetadataLookupS16(NULL, data->models, "N");
   psVector *X = psVectorAllocEmpty(N * 13 * 13,PS_TYPE_F32);
@@ -195,5 +218,12 @@
       psImage *ra    = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg ra");
       psImage *dec   = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg dec");
-      //      psImage *model = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg image");
+#define DUMP_DATA 0
+#if DUMP_DATA
+      psImage *model = psMetadataLookupPtr(NULL, chipItem->data.md, "bkg image");
+
+      psF32 offset = psMetadataLookupF32(NULL,chipItem->data.md,"bkg offset");
+      psF32 scale  = psMetadataLookupF32(NULL,chipItem->data.md,"bkg scale");
+
+#endif
       for (v = 0; v < calib->numRows; v++) {
 	for (u = 0; u < calib->numCols; u++) {
@@ -214,12 +244,13 @@
 	  used++;
 	  j++;
-	  
-/* 	  printf("DATA %ld %ld %f %f %f %f\n", */
-/* 		 j,used, */
-/* 		 ra->data.F32[v][u], */
-/* 		 dec->data.F32[v][u], */
-/* 		 calib->data.F32[v][u], */
-/* 		 model->data.F32[v][u]); */
-		 
+#if DUMP_DATA
+	  printf("DATA %d %ld %ld %f %f %f %f %f %f\n",
+		 data->model_iteration,j,used,
+		 offset,scale,
+		 ra->data.F32[v][u],
+		 dec->data.F32[v][u],
+		 calib->data.F32[v][u],
+		 model->data.F32[v][u]);
+#endif 
 	}
       }
@@ -230,5 +261,5 @@
   bool fitStatus;
   bool status = psImageMapClipFit(&fitStatus,data->modelMap,stats, mask, 1, X, Y, Z, E);
-
+  data->model_iteration++;
   psFree(expIter);
   psFree(X);