Examples of a simulated tile and its corresponding tiled exposure map. Left panel: mosaic (MOS1+MOS2+pn) raw X-ray tile, comprising ≈25 overlapping pointings. Sources in the tile include both clusters (larger sources regularly spaced by 10′) all with count rates of 0.1 count s^-1 and core radius 20′′, and resolved AGNs inserted at random positions; background is added. A wavelet smoothed image of the same area is shown in Fig. 3. Right panel: combined (MOS1+MOS2 + 3.1×pn) tiled exposure map of the same region. Red crosses show the cluster positions. A nominal XMM-LSS exposure time 10 ks for each instrument is assumed in both panels. The exposure map is in MOS units: the pn exposure map is corrected for the larger pn effective area by multiplying it by 3.1 (see text) so the combined exposure map has an exposure time ≈50 ks at the pointing centres. The area is the same as that shown in Fig. 1 and the exposure maps in the right panels of both figures are the same.