algorithm two occupied pixels whose distance is smaller than a chosen linking length are defined as [*friends*]{}. Starting from any occupied pixel, all its [*friends*]{} are associated with it, and then [*friends*]{} of its [*friends*]{}, until no more new [*friends*]{} can be found. All connected pixels (or the corresponding Voronoi cells) constitute a source candidate. The FOF algorithm is iteratively applied to all involved pixels, until all of them are assigned to some source candidates. We ran FOF on the occupied pixels whose photon density was above the background level. This choice is very generous in the sense that several spurious detections may survive above the background level. However, we preferred to start with a large list of candidates and to refine it later, a procedure often adopted [see @broos].

The direct application of the FOF algorithm results in a large number of blended sources, as mentioned also in @ebeling93. However, given the relatively low number density of sources in the X-ray sky [^4], most of the blended sources overlap only in their fainter outer regions, while their bright cores, corresponding to the peak of the PSF, remain isolated. In principle, the blended sources can be deblended by raising the 