Rodrigo Gil-Merino, Sabine Schindler

• We present CHANDRA X-ray and NTT optical observations of the distant z=0.52 galaxy cluster RBS380 -- the most distant cluster of the ROSAT Bright Source (RBS) catalogue. We find diffuse, non-spherically symmetric X-ray emission with a X-ray luminosity of L_X(0.3-10 keV)=1.6 10^(44) erg/s, which is lower than expected from the RBS. The reason is a bright AGN in the centre of the cluster contributing considerably to the X-ray flux. This AGN could not be resolved with ROSAT. In optical wavelength we identify several galaxies belonging to the cluster. The galaxy density is at least 2 times higher than expected for such a X-ray faint cluster, which is another confirmation of the weak correlation between X-ray luminosity and optical richness. The example of the source confusion in this cluster shows how important high- resolution X-ray imaging is for cosmological research.