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The PSPC has essentially no internally produced background. The
charged particle induced background depends sensitively on the local
particle environment along the orbit as well as the rejection efficiency
of the PSPC veto electronics. The background is less than
counts s 
arcmin 
for 95 percent of the observing time with peaks of up to
counts s arcmin . The spectrum of the charged
particle background in the detector is relatively flat with a small
low pulse-height tail.
Guest investigators should realise that the charged particle
background will vary by factors of three or more during one orbit.
As far as diffuse celestial X-ray radiation is concerned,
count rates vary between 
and 
counts s arcmin depending on the exact pointing position
in the sky. The median value is anticipated
to be 
counts s arcmin . Most of this
flux appears in the C-band, i.e., at energies below 0.28 keV.
Scattered solar X-ray background is produced by atoms and molecules
in the earth's atmosphere which scatter solar X-rays. Although
the scattering cross sections are quite small, substantial amounts of
flux may be scattered into the X-ray telescope under unfavourable
viewing conditions. Clearly, this background is absent during
night-time viewing; during day-time viewing the scattered flux depends,
to zeroth order, on the Sun-lit column density of atmosphere.
Since the atmospheric density decreases exponentially with altitude
large changes in Sun-lit column density may occur quite rapidly (i.e.,
over time scales of a few minutes). Thus the expected background varies
from zero (on the night side) to more than
counts s arcmin for viewing directions
having a zenith angle of 
on the day side. The spectrum of the
scattered solar X-rays is divided equally between the C band and the
O K 
line at 0.525 keV. However, considerable hardening will
occur during solar flares.