Sky-coverage for extended sources in three XDCP fields from Monte Carlo simulations at the 50% (red curve) and 90% (green) completeness level. The black solid curve shows the sky-coverage calculated by scaling the point source curve (dashed) with an offset factor of 2.4 (see Sect. 5.2). The simple scaling is shown to be a good first order description of the extended source sensitivity function. See Appendix B and Sect. 5.2 for details.