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Large Scale Non-linearity (Window Correction)


[Hasinger and Snowden1990]

Figure 3.16: Scale factor of the ``Golden Disk'' as a function of penetration depth (a) and energy (b). The plus signs show the data of PSPC-A through D, the solid curve is a parabolic fit to the data. The dashed line in b) shows the curve weighted by effective area, which is used by SASS\ [Hasinger and Snowden1990].

The window correction removes the large-scale distortions due to the bulging detector window. The PSPC counter gas, at a pressure of 1.5 bar, causes a bulging in the support grids between the aluminum struts to a scale height of tex2html_wrap_inline17011  mm. Since the electric field lines run perpendicular to the equipotential plane (the window), and since the primary electron cloud drifts along these field lines, the position of an X-ray event can be slightly shifted towards the centre of the detector for events within the central ring of the window-support structure. For events outside of the central ring, the direction of the shift is dependent on the event location. This shifting of positions obviously introduces spatial nonlinearities. The correction (termed ``golden disk'') is a function of detector coordinates and energy. The amplitude of the energy dependent term is assumed to be a smooth function of penetration depth (Fig. 3.16), and is normalized to 1.0 at 0.93 keV (since the shape of the ``golden disk'' was determined from measurements at this energy). For calibration file names and the equations for the coordinate correction, see Chap. 8.  

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