Left panel: example of particle background model subtracted from a 68′ × 68′ tile. Right panel: tiled combined exposure map (MOS1+MOS2 + 3.1×pn) of the same region. A nominal XMM-LSS exposure time 10 ks for each instrument is assumed. The 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 exposure time is even larger (≈60 ks) in areas where three pointings overlap.