Index: /fact/tools/pyscripts/pyfact/extractor.py
===================================================================
--- /fact/tools/pyscripts/pyfact/extractor.py	(revision 13366)
+++ /fact/tools/pyscripts/pyfact/extractor.py	(revision 13367)
@@ -104,20 +104,22 @@
 
 
-    def __call__(self, data):
+    def __call__(self, data, zero_level = 0):
         all_hits = []
         for pix_data in data:
             hits = []
             for i in range( data.shape[1]-1 ):
+                dat = pix_data[i] - zero_level
+                next_dat = pix_data[i+1] - zero_level
                 if ( self.slope > 0 ):
-                    if ( pix_data[i] > 0 ):
+                    if ( dat > 0 ):
                         continue
                 else:
-                    if ( pix_data[i] < 0):
+                    if ( dat < 0):
                         continue
-                if ( pix_data[i] * pix_data[i+1] <= 0 ):
+                if ( dat * next_dat <= 0 ):
                     # interpolate time of zero crossing with 
                     # linear polynomial: y = ax + b
-                    a = (pix_data[i+1] - pix_data[i]) / ((i+1) - i)
-                    time = -1.0/a * pix_data[i] + i
+                    a = (next_dat - dat) / ((i+1) - i)
+                    time = -1.0/a * dat + i
                     hits.append(time)
             all_hits.append(hits)