A new method for a simultaneous search for clusters of galaxies in X-ray photon maps and optical galaxy maps is described. The method appears 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 (<IX/10>) and the Early Date Release of the Sloan Digital Sky Survey (SDSS, <J/AJ/123/567>). 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*10^-13^erg/s/cm^2^ in the ROSAT energy band 0.1-2.4keV. The clusters have redshifts z<0.5.