Index: /trunk/Mars/msimreflector/MFresnelLens.cc
===================================================================
--- /trunk/Mars/msimreflector/MFresnelLens.cc	(revision 19615)
+++ /trunk/Mars/msimreflector/MFresnelLens.cc	(revision 19616)
@@ -260,13 +260,28 @@
     // Positive Fresnel Lenses ... are usually corrected for spherical aberration.
 
+    // sin(omega) = R / sqrt(R^2+f^2)
+    // tan(alpha) = sin(omega) / [ 1 - sqrt(n^2-sin(omega)^2) ]
+
+    const double so    = rc / sqrt(rc*rc + F*F);
+    const double alpha = atan(so / (1-sqrt(n0*n0 - so*so))); // alpha<0, Range [0deg; -50deg]
+
     // Tim Niggemann:
     // The surface of the lens follows the shape of a parabolic lens to compensate spherical aberration
     // Draft angle: psi(r) = 3deg + r * 0.0473deg/mm
 
-    // sin(omega) = R / sqrt(R^2+f^2)
-    // tan(alpha) = sin(omega) / [ 1 - sqrt(n^2-sin(omega)^2) ]
-
-    const double so    = rc / sqrt(rc*rc + F*F);
-    const double alpha = atan(so / (1-sqrt(n0*n0 - so*so))); // alpha<0
+    const double psi = (3 + r0*4.73e-3)*TMath::DegToRad(); // Range [0deg; 15deg]
+
+    // Find dw value of common z-value
+    //
+    // tan(90-psi)  =  z/dw
+    // tan(alpha)   = -z/(w-dw)
+    //
+    // -> dw = w/(1-tan(90-psi)/tan(alpha))
+
+    // In a simplified world, all photons which hit the draft surface get lost
+    // FIXME: This needs proper calculation in the same manner than for the main surface
+    const double dw = w/(1-tan(TMath::Pi()/2-psi)/tan(alpha));
+    if (p.R()>r1-dw)
+        return -1;
 
     //             theta                   peak_z