  We present the first results of a wide field survey for cool brown dwarfs
with the MegaCam camera on the CFHT telescope, the Canada-France Brown Dwarf
Survey, hereafter CFBDS. Our objectives are to find ultracool brown dwarfs and
to constrain the field-brown dwarf mass function thanks to a larger sample of L
and T dwarfs. We identify candidates in CFHT/MegaCam i' and z' images using
optimised psf-fitting within Source Extractor, and follow them up with pointed
near-infrared imaging on several telescopes. We have so far analysed over 350
square degrees and found 770 brown dwarf candidates brighter than z'{AB}=22.5.
We currently have J-band photometry for 220 of these candidates, which confirms
37% as potential L or T dwarfs. Some are among the reddest and farthest brown
dwarfs currently known, including an independent identification of the recently
published ULAS J003402.77-005206.7 and the discovery of a second brown dwarf
later than T8, CFBDS J005910.83-011401.3. Infrared spectra of three T dwarf
candidates confirm their nature, and validate the selection process. The
completed survey will discover ~100 T dwarfs and ~500 L dwarfs or M dwarfs
later than M8, approximately doubling the number of currently known brown
dwarfs. The resulting sample will have a very well-defined selection function,
and will therefore produce a very clean luminosity function.
