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The PSPC detector entrance window, as well as some of the WFC filters, are made of polycarbonate materials; when exposed to atomic oxygen, the predominant species at ROSAT's flight altitudes, such substrates will be ablated quickly. The problem will be most severe when the viewing direction of ROSAT coincides with the instantaneous velocity vector. Therefore the angle between viewing and instantaneous velocity direction will be kept greater than . The atomic oxygen problem will be most severe at those times when the Sun reaches maximum elongation above or below the orbit plane.