($z\sim 1.5$), whose presence was considered unlikely just a few years ago. The detection of a sizable number of high-z massive clusters may pave the way to a substantial revision of the standard $\Lambda$CDM cosmological model [@hc12; @hoyle12; @waiz12]. Moreover, a largely unexplored population of small or medium-z groups awaits systematical study. Overall, a thorough investigation of the low-flux, extended X-ray sources is expected to provide significant progress in this field.

Robust detection and characterization of faint extended X-ray sources is very difficult. In the photon-starving regime of X-ray images [@hobart2005], Poissonian fluctuations in the sparse background can produce spurious source detections at low fluxes, which makes the firm detection of faint sources very difficult, irrespective of their extent. In addition, detecting an extended source is much more difficult than detecting an unresolved source with the same number of net photons. First, extended emission is spread over more pixels, resulting in a much lower contrast to the background, and can be more easily swamped by the sparse background. Second, faint extended sources can be confused with faint unresolved sources.

In addition, the mere detection of a source is not sufficient, since a robust characterization of its extension is also required 