Index: trunk/psModules/src/imcombine/pmReadoutCombine.c
===================================================================
--- trunk/psModules/src/imcombine/pmReadoutCombine.c	(revision 10248)
+++ trunk/psModules/src/imcombine/pmReadoutCombine.c	(revision 10477)
@@ -9,4 +9,5 @@
 
 #include "pmFPA.h"
+#include "pmHDUUtils.h"
 #include "pmFPAMaskWeight.h"
 #include "pmConceptsAverage.h"
@@ -64,8 +65,30 @@
     }
 
+    bool first = !output->image || (output->row0 == 0 && output->col0 == 0); // First pass through?
+
+    pmHDU *hdu = pmHDUFromReadout(output); // Output HDU
+    if (!hdu) {
+        psError(PS_ERR_UNEXPECTED_NULL, false, "Unable to find HDU for readout.\n");
+        return false;
+    }
+
+    if (first) {
+        psString comment = NULL;        // Comment to add to header
+        psStringAppend(&comment, "Combining using statistic: %x", params->combine);
+        psMetadataAddStr(hdu->header, PS_LIST_TAIL, "HISTORY", PS_META_DUPLICATE_OK, comment, "");
+        psFree(comment);
+    }
+
     psStats *stats = psStatsAlloc(params->combine); // The statistics to use in the combination
     if (params->combine == PS_STAT_CLIPPED_MEAN) {
         stats->clipSigma = params->rej;
         stats->clipIter = params->iter;
+
+        if (first) {
+            psString comment = NULL;    // Comment to add to header
+            psStringAppend(&comment, "Combination clipping: %d iter, %f rej", params->iter, params->rej);
+            psMetadataAddStr(hdu->header, PS_LIST_TAIL, "HISTORY", PS_META_DUPLICATE_OK, comment, "");
+            psFree(comment);
+        }
     }
 
@@ -165,4 +188,9 @@
             psFree(output->weight);
             output->weight = newWeight;
+        }
+
+        if (first) {
+            psMetadataAddStr(hdu->header, PS_LIST_TAIL, "HISTORY", PS_META_DUPLICATE_OK,
+                             "Using input weights to combine images", "");
         }
 
@@ -210,8 +238,21 @@
 
     float keepFrac = 1.0 - params->fracLow - params->fracHigh; // Fraction of pixels to keep
+    if (keepFrac != 1.0 && first) {
+        psString comment = NULL;        // Comment to add to header
+        psStringAppend(&comment, "Min/max rejection: %f high, %f low, keep %d",
+                       params->fracHigh, params->fracLow, params->nKeep);
+        psMetadataAddStr(hdu->header, PS_LIST_TAIL, "HISTORY", PS_META_DUPLICATE_OK, comment, "");
+        psFree(comment);
+    }
+
     psMaskType maskVal = params->maskVal; // The mask value
+    if (maskVal && first) {
+        psString comment = NULL;        // Comment to add to header
+        psStringAppend(&comment, "Mask for combination: %x", maskVal);
+        psMetadataAddStr(hdu->header, PS_LIST_TAIL, "HISTORY", PS_META_DUPLICATE_OK, comment, "");
+        psFree(comment);
+    }
 
     #ifndef PS_NO_TRACE
-
     psTrace("psModules.imcombine", 3, "Iterating output: %d --> %d, %d --> %d\n",
             minInputCols - output->col0, maxInputCols - output->col0,