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In the focal plane of the X-ray mirror assembly
two redundant position sensitive proportional
counters (PSPC) are available. These are multiwire proportional
counters with a cathode strip readout scheme for position determination.
Both spectral and spatial resolution increase with increasing energy.
X-rays penetrate into the counter volume through a polypropylene entrance
window additionally coated with carbon and lexan (to decrease
UV transmission). A filter wheel with four positions is mounted in front
of the detector. The open and closed filter wheel positions are
used for ``standard'' observations and for monitoring of particle
background; the third position is used for spectral calibrations, while
the fourth position contains a boron filter.
The insertion of the boron filter
into the optical path allows an increase of spectral resolution at lower
energies. The full two degree field of view of the XMA can
be utilized in conjunction with the PSPC. Table 4.2 gives a
tabular summary of the PSPC performance characteristics.