  A new method for a simultaneous search for clusters of galaxies in X-ray
photon maps and optical galaxy maps is described. The merging of X-ray and
optical data improves the source identification so that a large amount of
telescope time for spectroscopic follow-up can be saved. The method appears
thus ideally suited for the analysis of the recently proposed wide-angle X-ray
missions like DUO and ROSITA. As a first application, clusters are extracted
from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of
the Sloan Digital Sky Survey (SDSS). The time-consuming computations are
performed within the German Astrophysical Virtual Observatory (GAVO). On a test
area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray
flux limit of $3-5\times 10^{-13} {\rm erg} {\rm s}^{-1} {\rm cm}^{-2}$ in the
ROSAT energy band 0.1-2.4 keV. The clusters have redshifts $z\le 0.5$. The
survey thus fills the gap between traditional large-area X-ray surveys and
serendipitous X-ray cluster searches based on pointed observations, and has the
potential to yield about 4,000 X-ray clusters after completion of SDSS.
