  Chandra X-ray Observatory, (formerly known as AXAF), will observe down to the
flux limit of 2\times 10^{-16} erg~s^{-1} cm^{-2}. In its first year of
operation Chandra's CCD detectors will observe over 1500 quasars
serendipitously in the soft (0.5--3.5 keV) band. Over 200 quasars will be
detected in X-rays in the redshift range 3<z<4 and over 400 quasars in 2<z<3.
This will enable us to determine the high redshift X-ray luminosity function.
This is the contribution by unabsorbed sources only. The total numbers would be
larger by about 60%
