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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 the particle background.
The third position contains a Boron filter, the insertion of which
allows an increase of spectral resolution at low energies.
The PSPC filter wheel carries three calibration sources in the
fourth position, which produce aluminium (1.49keV) fluorescent
K-shell photons by X-ray excitation from 
Fe.
These X-rays are used to illuminate the PSPCs in order to measure
in-orbit temporal variations in the PSPC gain
(see Sect. 3.4.1).