data extend the previous ROSAT range by a factor of about 40 in X-ray flux and to substantially fainter optical magnitudes. While the bulk of the type-1 AGN population still follows the general correlation along a constant $f_X/f_{opt}$ line, the type-2 AGNs cluster at higher X-ray-to-optical flux ratios. There is also a population of normal galaxies emerging at low fluxes (thus discovered in the Chandra and XMM era).

To be consistent with the already published deep ROSAT catalogs [@lehmann2001], we used a modified version of the X-ray to optical flux ratios: $$log_{10}(f_x/f_o)\equiv log_{10}(f_{0.5-2{{\rm keV}}}/f_R)\equiv
log_{10}(f_{0.5-2{{\rm keV}}})+0.4 R + 5.71,$$ where the flux is measured in erg cm$^{-2}$ s$^{-1}$ units in the 0.5-2 keV band and R is in Vega magnitudes. The slight change in the normalization [@maccacaro1988] is motivated by the significantly narrower X-ray energy band used (the original definition was based on the [*Einstein*]{} medium sensitivity survey band, 0.3-3.5 keV), which introduces a factor of 1.77 decrease in the flux for objects with a spectral energy index of $-$1 (classical type-1 AGN) and the use of the R-band instead of V (here we assumed a V$-$R color of 0.22, typical value for galaxies).

To use this new X-ray to 