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Overview

The all-sky survey was performed in such a way that the ROSAT satellite carried out a scan along a great circle perpendicular to the current position of the Sun on the ecliptic; thus at any given time ecliptic longitudes were scanned. As the Sun moves along the ecliptic, the spin axis of the satellite follows and after a period of six months the whole sky was covered. During the survey orbital period and the rotation period were synchronized and ROSAT's viewing direction was controlled in such a way that no earth blockages occurred and that the zenith angle, i.e., the angle between the instantaneous pointing direction of the satellite and the local zenith, was always less than .