The swift-XRT imaging performances and serendipitous survey

We are exploiting the Swift X-ray Telescope (XRT) deepest GRB follow-up observations to study the cosmic X-Ray Background (XRB) population in the 0.2-10 keV energy band. We present some preliminary results of a serendipitous survey performed on 221 fields observed with exposure longer than 10 ks. We show that the XRT is a profitable instrument for surveys and that it is particularly suitable for the search and observation of extended objects like clusters of galaxies. We used the brightest serendipitous sources and the longest observations to test the XRT optics performance and the background characteristics all over the field of view, in different energy bands during the first 2.5 years of fully operational mission.