diff --git a/ess/TraceWin.py b/ess/TraceWin.py
index 6350064a47698b5cbef78a16ee6e77eec3312d64..a44a303d5adf124127fab7e4ad70fa02d433ae89 100644
--- a/ess/TraceWin.py
+++ b/ess/TraceWin.py
@@ -309,8 +309,13 @@ class plt:
         if not hasattr(self,'sigma'):
                self.calc_sigma()
 
-        self.eps = [numpy.sqrt(numpy.det(self.sigma[i:i+2][:,i:i+2])) for i in (0,2,4)]
-        self.beta = [self.sigma[i][j]]
+        self.twiss_eps=[]
+        for j in xrange(len(self.Nelp)):
+            self.twiss_eps.append([numpy.sqrt(numpy.linalg.det(self.sigma[j][i:i+2][:,i:i+2])) for i in (0,2,4)])
+        self.twiss_beta = [[self.sigma[j][i][i] for i in (0,2,4)] for j in xrange(len(self.Nelp))]
+
+        self.twiss_eps=numpy.array(self.twiss_eps)
+        self.twiss_beta=numpy.array(self.twiss_beta)