diffuse extragalactic sources, namely groups and clusters of galaxies.

The identification of extended sources consists of two parts: source detection and source characterization. For the first part, we use the same algorithm – a combination of Voronoi tessellation (VT) and friends-of-friends (FOF) – as used in the classical software [vtpdetect]{} based on the work by @ebeling93. VT is a useful tool to deal with typical X-ray images, which are largely dominated by empty pixels. A recent application has been presented by @diehl06, who used weighted VT in adaptive binning of X-ray images. This software is not designed for source detection, however.

Although it has been widely used for detecting galaxy clusters in optical images (for instance in @ramella01, @kim02, @panko05, @vanbreukelen09, and @barkhouse06) [^1], the method of VT+FOF is seldom used in X-ray cluster surveys. Presently, the only X-ray source detection software based on VT available to the community is [vtpdetect]{} as a part of the [ciao]{}[^2] software developed for the [*Chandra*]{} mission. However, so far it has been applied mainly to ROSAT data [@scharf97; @ebeling98; @ebeling00], and to our knowledge, only once to [*Chandra*]{} data [@boschin02]. The detection algorithms commonly used in X-ray surveys are based on wavelet transform 