Index: trunk/ppMerge/src/ppMergeMask.c
===================================================================
--- trunk/ppMerge/src/ppMergeMask.c	(revision 21365)
+++ trunk/ppMerge/src/ppMergeMask.c	(revision 23166)
@@ -16,4 +16,5 @@
                       const pmFPAview *view, ///< View to chip
                       bool writeOut,     ///< Write output?
+                      pmHDU **lastHDU,   ///< HDU last updated
                       psRandom *rng,    ///< Random number generator
                       psMetadata *stats ///< Statistics output
@@ -96,4 +97,13 @@
                     i, inView->chip, inView->cell);
 
+            // Update the header
+            {
+                pmHDU *hdu = pmHDUGetHighest(outCell->parent->parent, outCell->parent, outCell); // File HDU
+                if (hdu && hdu != *lastHDU) {
+                    ppMergeVersionHeader(hdu->header);
+                    *lastHDU = hdu;
+                }
+            }
+
             if (!pmFPAfileIOChecks(config, inView, PM_FPA_BEFORE)) {
                 psFree(inView);
@@ -395,4 +405,5 @@
     assert(output && output->fpa);
     pmFPA *outFPA = output->fpa;        ///< Output FPA
+    pmHDU *lastHDU = NULL;              // Last HDU updated
 
     if (!pmFPAfileIOChecks(config, view, PM_FPA_BEFORE)) {
@@ -409,5 +420,5 @@
 
         for (int i = 0; i < iter; i++) {
-            if (!mergeMask(config, view, (i == iter - 1), rng, stats)) {
+            if (!mergeMask(config, view, (i == iter - 1), &lastHDU, rng, stats)) {
                 psError(PS_ERR_UNKNOWN, false, "Unable to merge chip %d", view->chip);
                 goto PPMERGE_MASK_ERROR;