- spatial disuniformity of the PMT gain (energy independent)
- different viewing angles of different scintillation positions (energy independent)
- curvature of the Be window
with associated distortion of the electric field (energy
*dependent*), due to the difference of pressure between the inside of the gas cell and the vacuum outside.

*
Y =
A _{4}(1+A_{6}/E)
(y-A_{2}) +
A_{8}
(y-A_{2})
^{2} +
A_{10}
(y-A_{2})
^{3}
*

In practice the above formula corresponds to a stepwise procedure in which :

- the coordinates are referred to the detector centre
(
*A*,_{1}*A*)_{2}

*x' = x-A*_{1}

y' = y-A_{2} - the distortion is described by a third-order polynomial in
*x',y'*

- the energy dependency is in the first-order terms only

- in the software a further step is added converting back from
*X,Y*positions in mm to pixels of user-selected (arbitrary) size*X',Y'* - in addition the software randomizes the original
*x,y*position within plus or minus half a pixel, in order to prevent artifacts due to integer-to-real conversion