We present the first results from the Calán-Yale Deep Extragalactic Research ( CYDER ) survey .
The main goal of this survey is to study serendipitous X-ray sources detected by Chandra in an intermediate flux range ( 10 ^ { -15 } -10 ^ { -12 } ergs s ^ { -1 } ) that comprises most of the X-ray background .
267 X-ray sources spread over 5 archived fields were detected .
The \log N - \log S distribution obtained for this sample is consistent with the results of other surveys .
Deep V and I images were taken of these fields in order to calculate X-ray-to-optical flux ratios .
Identifications and redshifts were obtained for 106 sources using optical spectroscopy from 8-m class telescopes to reach the optically faintest sources , to the same level as deeper X-ray fields like the Chandra Deep Fields , showing that the nature of sources detected depends mostly on the optical limit for spectroscopy .
In general , sources optically classified as obscured Active Galactic Nuclei ( AGNs ) have redder optical colors than unobscured AGN .
A rough correlation between f _ { X } / f _ { opt } and hard X-ray luminosity was found for obscured AGN confirming the prediction by existing models that in obscured AGN the optical light is completely dominated by the host galaxy .
The previously claimed decrease of the obscured to unobscured AGN ratio with increasing X-ray luminosity is observed .
However , this correlation can be explained as a selection effect caused by the lower optical flux of obscured AGN .
Comparison between the observed N _ { H } distribution and predictions by existing models shows that the sample appears complete up to N _ { H } < 3 \times 10 ^ { 22 } cm ^ { -2 } , while for more obscured sources incompleteness plays an important role in the observed obscured to unobscured AGN ratio .