diff --git a/src/ctobssim/ctobssim.cpp b/src/ctobssim/ctobssim.cpp
index 5ca3814..0f2949c 100644
--- a/src/ctobssim/ctobssim.cpp
+++ b/src/ctobssim/ctobssim.cpp
@@ -762,7 +762,9 @@ void ctobssim::simulate_source(GCTAObservation* obs, const GModels& models,
                         // number of simulated photons to m_max_photons.
                         // The photon rate is estimated from the model flux
                         // and used to set the duration of the time slice.
-                        double flux     = model->spectral()->flux(emin, emax);
+                        //double flux     = model->spectral()->flux(emin, emax);
+                        double flux     = get_model_flux(model, emin, emax,
+                                                         dir, rad);
                         double rate     = flux * m_area;
                         double duration = 1800.0;  // default: 1800 sec
                         if (rate > 0.0) {
@@ -960,6 +962,67 @@ void ctobssim::simulate_source(GCTAObservation* obs, const GModels& models,
 
 
 /***********************************************************************//**
+ * @brief Determine sky model flux
+ *
+ * @param[in] model Sky model.
+ * @param[in] emin Minimum energy.
+ * @param[in] emin Maximum energy.
+ * @param[in] centre Centre of region for photon rate determination.
+ * @param[in] radius Radius of region for photon rate determination.
+ * @return Model flux (photons/cm2/sec).
+ ***************************************************************************/
+double ctobssim::get_model_flux(const GModelSky* model,
+                                const GEnergy&   emin,
+                                const GEnergy&   emax,
+                                const GSkyDir&   centre,
+                                const double&    radius)
+{
+    // Initialise de-allocation flag
+    bool free_spectral = false;
+
+    // Get pointer to spectral model
+    const GModelSpectral* spectral = model->spectral();
+
+    // If the spatial model is a diffuse cube then create a node function
+    // spectral model that is the product of the diffuse cube node function
+    // and the spectral model evaluated at the energies of the node function
+    GModelSpatialDiffuseCube* cube = dynamic_cast<GModelSpatialDiffuseCube*>(model->spatial());
+    if (cube != NULL) {
+
+        // Set MC cone
+        cube->set_mc_cone(centre, radius);
+
+        // Allocate node function to replace the spectral component
+        GModelSpectralNodes* nodes = new GModelSpectralNodes(cube->spectrum());
+        for (int i = 0; i < nodes->nodes(); ++i) {
+            GEnergy energy    = nodes->energy(i);
+            GTime   time;                              // Dummy time
+            double  intensity = nodes->intensity(i);
+            double  norm      = spectral->eval(energy, time);
+            nodes->intensity(i, norm*intensity);
+        }
+
+        // Signal that node function needs to be de-allocated later
+        free_spectral = true;
+
+        // Set the spectral model pointer to the node function
+        spectral = nodes;
+
+    } // endif: spatial model was a diffuse cube
+    
+    // Compute flux within [emin, emax] in model from spectral
+    // component (units: ph/cm2/s)
+    double flux = spectral->flux(emin, emax);
+
+    // Free spectral model if required
+    if (free_spectral) delete spectral;
+
+    // Return model flux
+    return flux;
+}
+
+
+/***********************************************************************//**
  * @brief Simulate background events from model
  *
  * @param[in] obs Pointer on CTA observation.
diff --git a/src/ctobssim/ctobssim.hpp b/src/ctobssim/ctobssim.hpp
index 8476ca7..4c190ba 100644
--- a/src/ctobssim/ctobssim.hpp
+++ b/src/ctobssim/ctobssim.hpp
@@ -71,20 +71,25 @@ public:
 
 protected:
     // Protected methods
-    void init_members(void);
-    void copy_members(const ctobssim& app);
-    void free_members(void);
-    void get_parameters(void);
-    void simulate_source(GCTAObservation* obs,
+    void   init_members(void);
+    void   copy_members(const ctobssim& app);
+    void   free_members(void);
+    void   get_parameters(void);
+    void   simulate_source(GCTAObservation* obs,
                          const GModels&   models,
                          GRan&            ran, 
                          GLog*            wrklog = NULL);
-    void simulate_background(GCTAObservation* obs,
-                             const GModels&   models,
-                             GRan&            ran,
-                             GLog*            wrklog = NULL);
-    void save_fits(void);
-    void save_xml(void);
+    double get_model_flux(const GModelSky* model,
+                          const GEnergy&   emin,
+                          const GEnergy&   emax,
+                          const GSkyDir&   centre,
+                          const double&    radius);
+    void   simulate_background(GCTAObservation* obs,
+                               const GModels&   models,
+                               GRan&            ran,
+                               GLog*            wrklog = NULL);
+    void   save_fits(void);
+    void   save_xml(void);
 
     // User parameters
     std::string m_outevents;   //!< Output events file