  Data from a new, wide field, coincident optical and X-ray survey, the X-ray
Dark Cluster Survey (XDCS) are presented. This survey comprises simultaneous
and independent searches for clusters of galaxies in the optical and X-ray
passbands. Optical cluster detection algorithms implemented on the data are
detailed. Two distinct optically selected catalogues are constructed, one based
on I-band overdensity, the other on overdensities of colour-selected galaxies.
The superior accuracy of the colour-selection technique over that of the single
passband method is demonstrated, via internal consistency checks and comparison
with external spectroscopic redshift information. This is compared with an
X-ray selected cluster catalogue. In terms of gross numbers, the survey yields
185 I-band selected, 290 colour selected and 15 X-ray selected systems,
residing in ~11deg^2 of optical + X-ray imaging. The relationship between
optical richness/ luminosity and X-ray luminosity is examined, by measuring
X-ray luminosities at the positions of our 290 colour-selected systems. Power
law correlations between the optical richness/ luminosity versus X-ray
luminosity are fitted, both exhibiting approximately 0.2 dex of intrinsic
scatter. Interesting outliers in these correlations are discussed in greater
detail. Spectroscopic follow up of a subsample of X-ray underluminous systems
confirms their reality.
