Index: /trunk/Mars/hawc/plot_callisto.C
===================================================================
--- /trunk/Mars/hawc/plot_callisto.C	(revision 19840)
+++ /trunk/Mars/hawc/plot_callisto.C	(revision 19841)
@@ -1,6 +1,10 @@
+// ==========================================================================
+// ============ see plot_callisto function at the end of the file ===========
+// ==========================================================================
 int HandleInput(int evtnum)
 {
+    // This is a pure man's command line interface to wait for a key input
+    // and allow exit but at the same time allow interaction with the GUI
     TTimer timer("gSystem->ProcessEvents();", 50, kFALSE);
-
     while (1)
     {
@@ -22,25 +26,62 @@
 }
 
+// ==========================================================================
+//
+// Run the macro with
+//
+//      root hawc/plot_callisto.C\(\"00000001.003_Y_MonteCarlo003_Events.root\"\)
+//
+// The default file name is a Y-file (either as Monte Carlo-truth from ceres
+// or from the callisto)
+//
+// From within root, the escape characters can be omitted, e.g.
+//
+//      root
+//      [0] .x hawc/plot_callisto.C("00000001.003_Y_MonteCarlo003_Events.root")
+//
+// ==========================================================================
+
 void plot_callisto(const char *fname)
 {
-    TFile f(fname);
+    // Create the file for reading and get the tree
+    TFile file(fname);
+    if (file.IsZombie())
+    {
+        cout << "Could not open file." << endl;
+        return;
+    }
 
-    TTree *T = f.Get("Events");
+    TTree *T = 0;
+    file.GetObject("Events", T);
+    if (!T)
+    {
+        cout << "Could not access tree." << endl;
+        return;
+    }
 
+    // Setup the branch with the calibrated datafor reading
     MSignalCam *signal = NULL;
-    T->SetBranchAddress("MSignalCam.", &signal);
+    chain.SetBranchAddress("MSignalCam.", &signal);
 
+    // Create the FAMOUS style camera with a
+    // focal distance of 0.5m and 61 pixels
     MGeomCamFAMOUS geom(0.5, kFALSE);
 
+    // Instantiate three camera histograms
     MHCamera cam_signal(geom);
     MHCamera cam_cleaned(geom);
     MHCamera cam_time(geom);
 
+    // Setup names for the cameras
     cam_signal.SetName("Signal");
     cam_cleaned.SetName("Cleaned");
     cam_time.SetName("Time");
+
+    // Fix minimum for amplitude at 0 ("disable zero suppression")
     cam_signal.SetMinimum(0);
     cam_cleaned.SetMinimum(0);
 
+    // Create a canvas and divide it into 4 (2x2) pads
+    // Add all four camera displays in the pads
     TCanvas c;
     c.Divide(2,2);
@@ -55,11 +96,14 @@
     cam_time.Draw();
 
+    // Loop over the data
     int evtnum = 0;
     while (evtnum>=0)
     {
+        // Infinite loop (start over at the beginning)
         if (evtnum>=T->GetEntries())
             evtnum = 0;
 
-        T->GetEntry(evtnum);
+        // Get event
+        chain.GetEntry(evtnum);
 
         //  0: Number of Photons*PixRatio <default>
@@ -75,8 +119,14 @@
         // 11: as 8, but returns kFALSE if signal <=0
 
+        // This is a function which directly copies the entries
+        // from *signal into the camera histograms. The number
+        // is an ID which contents to be copied. This could also
+        // be done in a manula loop over all pixels.
         cam_signal.SetCamContent(*signal, 10); // num phot uncleaned
         cam_cleaned.SetCamContent(*signal, 3); // num phot cleaned
         cam_time.SetCamContent(*signal, 8); // arr time cleaned
 
+        // Signal root that the objects displayed in all four pads
+        // were changed and the pads have to be updated
         for (int i=0; i<4; i++)
         {
@@ -85,4 +135,5 @@
         }
 
+        // Wait for user intput
         evtnum = HandleInput(evtnum);
     }