down to $J$=20.0), false detection rates of even 1 per 10$^4$ real sources would greatly outnumber true brown dwarfs. We therefore need a very robust analysis of the discovery images to tie down the number of artefacts before we finally weed them out during the photometric follow-up of the candidates.

Image reduction
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The WIRCam observations targeted 335 existing MegaCam fields from CFBDS, that have already been observed in $i'$ and $z'$. The reduction of MegaCam data is described by @Delorme.2008b.

Images were run through the ‘I‘iwi Interpretor of the WIRCam Images (‘I‘iwi - [www.cfht.hawaii.edu/Instruments/Imaging/WIRCam]{}) at CFHT, which does non-linearity correction, dark subtraction, flat-fielding, sky subtraction, bad pixel masking, photometric calibration, and rough astrometric calibration. Cross-talk subtraction was also performed on the 06B and 07A data, affected by amplifier to amplifier cross-talk.

Each set of 2 times 9 WIRCam images covering one $z'$ MegaCam image was then co-aligned, using the $z'$ image used as the astrometric reference. This astrometric calibration was performed with *Scamp* [@Bertin.2006], and the images were stacked using *Swarp*[@Bertin.2002]. This produced $\sim$1 square degree WIRCam images aligned with each targeted MegaCam field.

Image analysis {#data_analysis}
--------------

Since our stacks are only 2 exposures deep, the final frame 