  We present the first results from the Cal\'an-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
$f_X/f_{\rm opt}$ 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.
