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The bulging detector window causes a large-scale spatial non-linearity (counter gas pressure 1.5bar, bulging 1.2mm). The primary electron cloud drifts along the electric field lines which run perpendicular to the equipotential plane (PSPC entrance window). Thus event positions within the central ring of the window support structure are slightly shifted towards the detector center while for event positions outside the central ring the shift depends on the event position. (cf. Fig. F.12 and Fig. F.13, top) This shifting introduces spatial non-linearities. The shape of the correction has been determined at a photon energy of E=0.3keV, the positional and energy-dependent effects were assumed to be separable. The energy-dependent term is a smooth function of the energy (cf. Fig. F.13, bottom). Is is normalized to unity at E=0.3keV.
In practice SASS determines the shift using a binned map with 512 512 pixels, i.e., uses a grid of bin size 16 detector pixels and has to interpolate between four neighboring grid point. The interpolation of the correction vector is performed according to Eq. (F.13) and (F.14).
Within SASS the energy dependent scale factor is taken from a table with 256 elements (channels). For the value is taken instead.
Figure F.12: Window correction tables for PSPC-B. The `Golden Disk' (arrays and ): The position correction in x and y is plotted for every 8th's array element; the energy is assumed to be 0.93keV, for which the energy dependent factor of the position correction equals 1.0. The origin of the displacement vector of element (n,m) is drawn at . The basic information was taken from files p$manpar:tabx_093_b.dat and p$manpar:taby_093_b.dat.
Figure F.13: Window correction data files according to Eqs.(F.13) and (F.14): top panel: shift of coordinates for PSPC-C: white corresponds to negative values and black to positive (top left: , top right: ). Bottom panel: energy dependent factor , normalized to unity at 0.93keV. The basic information was taken from files p$manpar:tabx_093_c.dat, p$manpar:taby_093_c.dat, and p$manpar:scal3_new.dat.
SASS source code: package PSPC, bubble CT, subroutine DCORW