  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.
