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Figure 6.1 shows a contour plot on an Aitoff projection of the celestial sphere representing the (logarithmic) exposure time (in seconds) of the ROSAT all-sky survey. The spring equinox is located at the center of Figure 6.1 , with increasing right ascensions plotted to the left. The lowest contour indicates an (unvignetted) exposure of 250 s, and each new contour represents an exposure increase by a factor of two. Unshaded areas indicate regions with exposures of less than 250 s. As is obvious from Figure 6.1 , such regions are of extremely limited extent. The exposure time increases sharply towards the poles of the ecliptic with relatively large latitudinal exposure variations present, especially at southern latitudes.