Index: trunk/ppStack/src/ppStackPrepare.c
===================================================================
--- trunk/ppStack/src/ppStackPrepare.c	(revision 27075)
+++ trunk/ppStack/src/ppStackPrepare.c	(revision 27400)
@@ -126,4 +126,6 @@
     options->inputMask = psVectorAlloc(num, PS_TYPE_VECTOR_MASK); // Mask for inputs
     psVectorInit(options->inputMask, 0);
+    options->exposures = psVectorAlloc(options->num, PS_TYPE_F32);
+    psVectorInit(options->exposures, NAN);
 
     pmFPAfileActivate(config->files, false, NULL);
@@ -134,28 +136,28 @@
     }
 
-    psMetadataIterator *fileIter = psMetadataIteratorAlloc(config->files, PS_LIST_HEAD, "^PPSTACK.INPUT$");
-    psMetadataItem *fileItem; // Item from iteration
     psArray *psfs = psArrayAlloc(num); // PSFs for PSF envelope
     int numCols = 0, numRows = 0;   // Size of image
-    int index = 0;                  // Index for file
-    while ((fileItem = psMetadataGetAndIncrement(fileIter))) {
-        assert(fileItem->type == PS_DATA_UNKNOWN);
-        pmFPAfile *inputFile = fileItem->data.V; // An input file
+    options->sumExposure = 0.0;
+    for (int i = 0; i < num; i++) {
+        pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPSTACK.INPUT", i); // File of interest
+        pmCell *cell = pmFPAviewThisCell(view, file->fpa); // Cell of interest
+
+        options->exposures->data.F32[i] = psMetadataLookupF32(NULL, cell->concepts, "CELL.EXPOSURE");
+        options->sumExposure += options->exposures->data.F32[i];
 
         // Get list of PSFs, to determine target PSF
         if (options->convolve) {
-            pmChip *chip = pmFPAviewThisChip(view, inputFile->fpa); // The chip: holds the PSF
+            pmChip *chip = pmFPAviewThisChip(view, file->fpa); // The chip: holds the PSF
             pmPSF *psf = psMetadataLookupPtr(NULL, chip->analysis, "PSPHOT.PSF"); // PSF
             if (!psf) {
                 psError(PPSTACK_ERR_PROG, false, "Unable to find PSF.");
                 psFree(view);
-                psFree(fileIter);
                 psFree(psfs);
                 return false;
             }
-            psfs->data[index] = psMemIncrRefCounter(psf);
+            psfs->data[i] = psMemIncrRefCounter(psf);
             psMetadataRemoveKey(chip->analysis, "PSPHOT.PSF");
 
-            pmCell *cell = pmFPAviewThisCell(view, inputFile->fpa); // Cell of interest
+            pmCell *cell = pmFPAviewThisCell(view, file->fpa); // Cell of interest
             pmHDU *hdu = pmHDUFromCell(cell);
             assert(hdu && hdu->header);
@@ -165,5 +167,4 @@
                 psError(PPSTACK_ERR_PROG, false, "Unable to determine size of image from PSF.");
                 psFree(view);
-                psFree(fileIter);
                 psFree(psfs);
                 return false;
@@ -180,5 +181,5 @@
         pmDetections *detections = NULL;
         if (options->convolve || options->matchZPs || options->photometry || redoPhot) {
-            pmReadout *ro = pmFPAviewThisReadout(view, inputFile->fpa); // Readout with sources
+            pmReadout *ro = pmFPAviewThisReadout(view, file->fpa); // Readout with sources
             detections = psMetadataLookupPtr(NULL, ro->analysis, "PSPHOT.DETECTIONS"); // Sources
             if (!detections) {
@@ -188,16 +189,16 @@
             psAssert (detections->allSources, "missing sources?");
 
-            options->sourceLists->data[index] = psMemIncrRefCounter(detections->allSources);
+            options->sourceLists->data[i] = psMemIncrRefCounter(detections->allSources);
         }
 
         // Re-do photometry if we don't trust the source lists
         if (redoPhot) {
-            psTrace("ppStack", 2, "Photometering input %d of %d....\n", index, num);
+            psTrace("ppStack", 2, "Photometering input %d of %d....\n", i, num);
             pmFPAfileActivate(config->files, false, NULL);
-            ppStackFileActivationSingle(config, PPSTACK_FILES_CONVOLVE, true, index);
+            ppStackFileActivationSingle(config, PPSTACK_FILES_CONVOLVE, true, i);
             if (options->convolve) {
                 pmFPAfileActivate(config->files, true, "PPSTACK.CONV.KERNEL");
             }
-            pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPSTACK.INPUT", index); // File
+            pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPSTACK.INPUT", i); // File
             pmFPAview *photView = ppStackFilesIterateDown(config);
             if (!photView) {
@@ -223,8 +224,5 @@
             ppStackFileActivation(config, PPSTACK_FILES_PREPARE, true);
         }
-
-        index++;
-    }
-    psFree(fileIter);
+    }
 
     psString log = psStringCopy("Input seeing FWHMs:\n"); // Log message