run. Based on the Voronoi diagram, we created an area map that contains the area of Voronoi cells associated to each occupied pixel (Voronoi site). We calculated the accurate area of the cell polygons rather than counting the pixels in the discrete Voronoi cells. Finally, we computed the photon density map by dividing the original image by the area map. An example of a Voronoi tessellation and its associated photon density map is shown in Figure \[opdCorr\]. To identify source candidates, the photon density map must be compared with a background map, which is generated as described in the following section.

![image](opdCorr.ps){width="\textwidth"}

Background map generation\[bkgmap\]
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A photon in an X-ray image can be associated to a source or the background. The background is defined as the sum of all contributions not associated to astronomical sources, or associated to some astronomical component that cannot be resolved (like the Galactic diffuse emission). We estimated the average background flux adopting the same method as used in @ebeling93 [see their section D]. We assumed that the background photons in the exposure-corrected image, which is obtained by dividing the photon density image by the exposure map, are distributed according to the Poisson statistics, and 