detection (pc)& T9 maximal detection (pc) & Final coverage(sq deg)\
CFBDSIR & 20.0 & 235 & 145 & 35 & 335\

CFBDS initial detections are made in $z'$ and only better than $10\sigma$ detections are used. CFBDSIR makes the initial detection in $J$, using only better than $10\sigma$ detections in $J$, and is only limited in $z'$ by no detections. We define any $z'$-band object with a signal below the $5\sigma$ threshold as a no detection or ”z’-dropout".

[**Survey rationale.**]{} Ultracool brown dwarfs have $z'_{AB}-J_{vega}>3.8$ for the MegaCam and WIRCam photometric systems, with some already observed at $z'-J=4.5$. Given the relative depth of the optical and NIR exposures, most of the T-dwarfs earlier than T7 detected in the $J$-band image will have a $z'$-band counterpart. The objects detected at $J$ and not at $z'$ (hereafter $z$-dropouts) are thus good candidates for some of the coolest T-dwarfs known ($>$T7), and possibly even cooler Y dwarfs.

Since our most interesting candidates are likely to only be detected in $J$-band, we need to carefully eliminate most sources of contamination in this band. The main contaminants are detectors artefacts, variable objects and asteroids. As described in detail in section \[data\_analysis\], our point spread function 