Index: trunk/psphot/src/psphotStackImageLoop.c
===================================================================
--- trunk/psphot/src/psphotStackImageLoop.c	(revision 33030)
+++ trunk/psphot/src/psphotStackImageLoop.c	(revision 33690)
@@ -30,7 +30,16 @@
     }
 
+    psMetadata *recipe = psMetadataLookupPtr (NULL, config->recipes, PSPHOT_RECIPE);
+    bool useRaw = psMetadataLookupBool (NULL, recipe, "PSPHOT.STACK.USE.RAW");
+    if (useRaw && inputRaw == NULL) {
+        psLogMsg ("psphot", 1, "PSPHOT.STACK.USE.RAW set but no raw input.");
+        useRaw = false;
+    }
+
     pmFPAview *view = pmFPAviewAlloc (0);
 
-    // XXX for now, just load the full set of images up front
+
+    // XXX for now, just load the full set of images up front except for EXPNUM which we defer
+    pmFPAfileActivate (config->files, false, "PSPHOT.STACK.EXPNUM.RAW");
     if (!pmFPAfileIOChecks (config, view, PM_FPA_BEFORE)) ESCAPE ("failed input for fpa in psphot.");
 
@@ -86,6 +95,9 @@
 	UpdateHeadersForChip(config, view);
 
-	// save output which is saved at the chip level
-	if (!pmFPAfileIOChecks (config, view, PM_FPA_AFTER)) ESCAPE ("failed output for Chip in psphot.");
+        // Defer output until we have performed psphotSetNFrames()
+        pmFPAfileActivate (config->files, false, NULL);
+
+	// Iterate up
+	if (!pmFPAfileIOChecks (config, view, PM_FPA_AFTER)) ESCAPE ("failed pmFPAfileIOChecks for Chip in psphot.");
     }
     psMemDump("doneloop");
@@ -93,5 +105,36 @@
     UpdateHeadersForFPA(config, view);
 
-    // save output which is saved at the fpa level
+    // Iterate up output which is saved at the fpa level
+    if (!pmFPAfileIOChecks (config, view, PM_FPA_AFTER)) ESCAPE ("failed ouput pmFPAfileIOChecks FPA in psphot.");
+
+    // Load the appropriate EXPNUM image
+    pmFPAfileActivate (config->files, true, useRaw ? "PSPHOT.STACK.EXPNUM.RAW" : "PSPHOT.STACK.EXPNUM.CNV");
+
+    if (!pmFPAfileIOChecks (config, view, PM_FPA_BEFORE)) ESCAPE ("failed input for fpa EXPNUM in psphot.");
+
+    // for psphot, we force data to be read at the chip level
+    while ((chip = pmFPAviewNextChip (view, input->fpa, 1)) != NULL) {
+        psLogMsg ("psphot", 4, "Chip %d: %x %x\n", view->chip, chip->file_exists, chip->process);
+        if (! chip->process || ! chip->file_exists) { continue; }
+        if (!pmFPAfileIOChecks (config, view, PM_FPA_BEFORE)) ESCAPE ("failed input for Chip EXPNUM in psphotStack.");
+
+        // there is now only a single chip (multiple readouts?). loop over it and process
+        while ((cell = pmFPAviewNextCell (view, input->fpa, 1)) != NULL) {
+            psLogMsg ("psphot", 5, "Cell %d: %x %x\n", view->cell, cell->file_exists, cell->process);
+	    if (!pmFPAfileIOChecks (config, view, PM_FPA_BEFORE)) ESCAPE ("failed input for Cell in psphotStack.");
+
+            // process each of the readouts
+            while ((readout = pmFPAviewNextReadout (view, input->fpa, 1)) != NULL) {
+                psLogMsg ("psphot", 6, "Readout %d: %x %x\n", view->readout, cell->file_exists, cell->process);
+                if (! readout->data_exists) { continue; }
+
+                if (!psphotSetNFrames (config, view, useRaw ? inputRaw->name : inputCnv->name)) ESCAPE ("failed to setNFrames.");
+            }
+        }
+        // now activate all files to trigger final output
+        pmFPAfileActivate (config->files, true, NULL);
+
+        if (!pmFPAfileIOChecks (config, view, PM_FPA_AFTER)) ESCAPE ("failed output for Chip in psphot.");
+    }
     if (!pmFPAfileIOChecks (config, view, PM_FPA_AFTER)) ESCAPE ("failed ouput for FPA in psphot.");