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The ROSAT PSPC detector entrance windows, as well as one of
the WFC filters, are made of polycarbonate materials.
When exposed to atomic oxygen
(the predominate atmospheric species at the orbital altitude of ROSAT)
such substrates will be ablated quickly.
The problem will be most severe when the viewing direction of the
spacecraft coincides with the instantaneous spacecraft velocity vector.
The angle between the viewing direction and the instantaneous velocity
vector is therefore kept 
.
The atomic oxygen problem is most severe at those times when the Sun
reaches maximum elongation above or below the spacecraft orbital plane.