diff --git a/python/helicalscan.py b/python/helicalscan.py
index 3c262ba..45b2623 100755
--- a/python/helicalscan.py
+++ b/python/helicalscan.py
@@ -710,6 +710,75 @@ class HelicalScan(MotionBase):
     res=(cx,cz,w,fy)
     return res
 
+  def calcParamFast(self,x=((-241.,96.),(-162.,-293.)),
+                         y=(575.,175.),
+                         z=((-1401.,-1401.),(-1802.,-1303.)),
+                         w=(1.2,1.4),
+                         mode=1):
+    '''
+    the rotation center of the stage does not change for a new cristal.
+    after the function calcParam was called once,
+    this faster coordinate transformation can be used.
+    FOR SMALL ANGLES USE MODE==0.
+    !!! THIS CODE IS NOT YET TESTED !!!
+    '''
+    #x: ((x_w0y0, x_w1y0),(x_w0y1, x_w1y1)
+    #y: lower and upper cristal point
+    #z: distance, similar to x
+    #w: start and end angle in radians
+    #mode  0:use x and z                 needs to define 1 point at start and 1 point at end
+    #      1:use x change with 2 angles  needs to define 2 point at start and 2 point at end
+
+    #mode 0 uses:
+    #x: ((x_w0y0, None  ),(None  , x_w1y1)
+    #z: ((z_w0y0, None  ),(None  , z_w1y1)
+    #w: (w0,w1)
+
+    #mode 1 uses:
+    #x: ((x_w0y0, x_w1y0),(x_w0y1, x_w1y1)
+    #z: ((None,   None  ),(None  , None  )
+    #w: (w0,w1)
+
+
+    # param[i]=(z_i, y_i, x_i, r_i,phi_i)
+    # z_i   not changed
+    # y_i   trivial
+    # x_i   not changed
+    # r_i   calculate
+    # phi_i calculate
+
+
+    try:
+      param=self.param
+    except AttributeError as e:
+      raise AttributeError('calcParam must be called first')
+
+    if mode==0:
+      for i in range(len(y)):
+        # param[i]=(z_i, y_i, x_i, r_i,phi_i)
+        r_i  =np.sqrt(x[i]**2+z[i]**2)
+        phi_i=np.arctan2(z[i],x[i])
+        param[i, 1]=y[i]
+        param[i, 3:]=(r_i,phi_i)
+    else: #mode==1:
+      for i in range(len(y)):
+        # param[i]=(z_i, y_i, x_i, r_i,phi_i)
+        r_i=np.sqrt(x[i]**2+z[i]**2)
+        x0=x[i][0]-param[i,2]
+        x1=x[i][1]-param[i,2]
+        ww=w[i]
+        if x0>x1:
+          phi_i=np.arctan2(np.cos(ww)-x1/x0,np.sin(ww))
+          r_i  =x0/np.cos(phi_i)
+        else:
+          phi_i=np.arctan2(np.cos(ww)-x0/x1,np.sin(ww))
+          r_i  =x1/np.cos(phi_i)
+        param[i, 1]=y[i]
+        param[i, 3:]=(r_i,phi_i)
+
+    pass
+
+
   def calcParam(self,x=((-241.,96.,-53.),(-162.,-293.,246.)),
                      y=(575.,175.),
                      z=((-1401.,-1401.,-1802.),(-1802.,-1303.,-1402.))):