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   51275
   The origin of the Cosmic X-ray Background(CXB) is still now an open
   question in the X-ray astronomy. Previous observations in soft X-ray
   band have resolved more than half of the CXB flux to known classes of
   objects such as AGNs and QSOs, while there sill remains a significant
   part of the CXB flux unresolved whose constituents may be unknown X-ray
   sources. We propose short exposure observations of deep optical survey
   fields where we have well calibrated optical catalogs, and to take
   cross-correlations of measured X-ray flux distributions to various
   color-selected subsets of optical objects. Our preliminary analysis of
   SA68 field shows clear correlation between X-ray surface brightness and
   optical quasar number distribution.
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