Cumulative number counts N(>S) for observed X-ray point-sources in the soft (left) and hard band (right). The blue dashed-dotted curve is from Kim et al. (2007, Table 3, ChaMP+CDFs) and the red dashed curve from Georgakakis et al. (2008). The vertical solid lines indicate the point-source detection sensitivity for different survey durations assuming 80% observing efficiency, and the sensitivity at the 20 ks exposure time, corresponding to the confusion limit in the soft band (left-most line marked “Poles”).