|ROSAT Home Page||ROSAT
At the altitude of ROSAT ( 550km), and with the zenith-angle observation constraint (observations are made at ), the scattering is dominated by atomic oxygen with Thomson scattered X-rays in the 1/4keV band and oxygen K fluorescent scattered X-rays at 0.53keV. Field of view integrated minimum count rates of counts s ( countss arcmin , on axis) during day-side observations with excursions to 40countss ( countss arcmin ) or more in particularly bad geometries in the 0.1-1.0 keV band are found. Typical cosmic background count rates in the same band range from 3-12 counts s ( to countss arcmin , on axis), demonstrating the possible severity of the contamination.
The temporal variation of the solar scattered X-ray background is dominated by the observation-geometry induced variations of the sunlit column density of Earth's atmosphere along the observation line of sight. This variation can be adequately modeled allowing a reasonable subtraction of the background counts.
The severity of the contamination by scattered solar X-rays has been much reduced as the mission has progressed, due to the reduced solar flux as solar maximum is left behind (ROSAT was launched close to solar maximum). The reduction comes both from the lower atmosphere (which also adds to the longevity of the mission because of reduced atmospheric drag) and from a lower solar X-ray luminosity.