A simulation of a 511 keV point source as seen by IBIS/ISGRI during a pointing in the early (rev. 50), left panel, and late (rev. 600), right panel, orbits of the INTEGRAL satellite. The signal level of the detected 511 keV source goes to 10σ in the first image to 7σ in the second one. These images have been obtained by adding to the real data the data generated by the IBIS Monte Carlo and then processing the result with the standard analysis software until the image level.