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The design distance between the XMA and PSPC is such that
keV photons of an on-axis source are brought to
a focus (along a surface of a cone of half-angle 
)
at an optimum depth within the PSPC drift region,
such that the Gaussian spatial distribution of pulses on
the cathode wires is minimized.
Consequently the image of a point source is sharpest at 1 keV.
The combined contribution of focussing and the finite penetration of photons into the counter on the PSF can be modelled as an exponential function where r is the radial distance from the centroid position of the radial profile, in arc minutes.
The e-folding angle, 
, was found to be
