Index: /branches/eam_branches/ipp-20110213/ppStack/src/ppStackCombineFinal.c
===================================================================
--- /branches/eam_branches/ipp-20110213/ppStack/src/ppStackCombineFinal.c	(revision 30801)
+++ /branches/eam_branches/ipp-20110213/ppStack/src/ppStackCombineFinal.c	(revision 30802)
@@ -59,5 +59,5 @@
         }
 
-        // call: ppStackReadoutFinal(config, outRO, readouts, rejected)
+        // calls ppStackReadoutFinal(config, outRO, readouts, rejected) in ppStackReadout.c
         psThreadJob *job = psThreadJobAlloc("PPSTACK_FINAL_COMBINE"); // Job to start
         psArrayAdd(job->args, 1, thread);
Index: /branches/eam_branches/ipp-20110213/ppStack/src/ppStackSources.c
===================================================================
--- /branches/eam_branches/ipp-20110213/ppStack/src/ppStackSources.c	(revision 30801)
+++ /branches/eam_branches/ipp-20110213/ppStack/src/ppStackSources.c	(revision 30802)
@@ -206,4 +206,6 @@
         }
 
+	// XXX this is wrong, or at least inconsistent with the above: this needs to include 
+	// a value for the nominal system zero point to be consistent with zpExp
         zp->data.F32[i] = airmassTerm * airmass + 2.5 * log10(exptime);
     }
@@ -236,6 +238,12 @@
 
     if (zpExpNum == numGoodImages) {
+	psLogMsg ("ppStack", PS_LOG_INFO, "all zero points are finite; using reported zero points listed above");
         for (int i = 0; i < num; i++) {
             zp->data.F32[i] = zpExp->data.F32[i];
+        }
+    } else {
+	psLogMsg ("ppStack", PS_LOG_INFO, "missing some zero points; using guess values:");
+        for (int i = 0; i < num; i++) {
+	    psLogMsg("ppStack", PS_LOG_INFO, "Image %d: %.2f sec exposure with zero point %.2f", i, options->exposures->data.F32[i], zp->data.F32[i]);
         }
     }
@@ -265,9 +273,38 @@
             }
         }
-        // M = m + c0 + c1 * airmass - 2.5log(t) + transparency
-        // Want sources to have m corresponding to airmass = 1 and t = sumExpTime and transparency = 0
-        // m_0 + c1 * airmass_0 + 2.5log(t_0) - trans_0 = m_1 + c1 * airmass_1 + 2.5log(t_1) - trans_1
-        // m_0 = m_1 + zp_1 + trans_1 - c1 * airmass_0 - 2.5log(t_0)
-        // We don't need to know the magnitude zero point for the filter, since it cancels out
+
+	// EAM : the discussion here was not quite right (or at least sloppy).  Here is a replacement explanation:
+
+	// For any star, the observed instrumental magnitude on an image and the apparent magnitude are related by:
+	// M_app = m_inst + zp + c1 * airmass + 2.5log(t) + transparency
+
+	// we want to adjust the input images to be in a consistent flux system so that the
+	// final stack can be generated with a specific target zero point.  Any adjustment to
+	// the flux scale of the image must be made in coordination with the resulting
+	// zeropoint, exposure time, and airmass such that the above relationship yields the
+	// same apparent magnitude for a given star:
+
+	// m_inst_i : instrumental mags on input image (in)
+	// m_inst_o : instrumental mags on re-normalized image (out)
+
+	// m_inst_o + zp_o + c1 * airmass_o + 2.5log(t_o) + trans_o = m_inst_i + zp_i + c1 * airmass_i + 2.5log(t_i) + trans_i
+
+	// m_inst_o = m_inst_i + (zp_i - zp_o) + c1 * (airmass_i - airmass_o) + 2.5log(t_i) - 2.5log(t_o) + trans_i - trans_o
+
+	// zp_i, airmass_i, t_i, trans_i : reported or measured for input image
+
+	// zp_o      = zpTarget      (from recipe)
+	// airmass_o = airmassTarget (from recipe)
+	// t_o       = sumExpTime    [sum of input exposure times: once images are scale to this time, they can be avereaged]
+	// trans_o   = 0.0           [obviously!]
+
+	// we have 2 cases: (a) all reported ZPs are good or (b) some are bad:
+	// (a) FPA.ZP = zp_i + c1 * airmass_i
+	//  --> zp[i] = zp_i + c1 * airmass_i + 2.5log(exptime_i)
+	// (b)  zp[i] = c1 * airmass_i + 2.5log(exptime_i)
+	// NOTE: in case (b), the current code is equating the TARGET zp with the NOMINAL zp, which is wrong.
+
+	// m_inst_o - m_inst_i = zp[i] - zpTarget - c1 * airmassTarget - 2.5log(sumExpTime) + trans_i
+
         if (options->matchZPs) {
             options->norm = psVectorAlloc(num, PS_TYPE_F32);
@@ -278,6 +315,8 @@
                 psArray *sources = sourceLists->data[i]; // Sources of interest
                 float magCorr = zp->data.F32[i] - trans->data.F32[i] - 2.5*log10(options->sumExposure);
-                if (zpExpNum == numGoodImages) {
+                if (zpExpNum == numGoodImages) { // case (a)
                     // Using measured zero points, so attempt to set target zero point
+		    // XXX see NOTE above regarding case (b) : this is wrong.  the code should load a nominal zero point and supply it above
+		    // 
                     magCorr -= zpTarget;
                 }