can be carried out by the observer only minutes after the exposures allowing $J$ and possibly $H$ and $K_{\rm s}$ photometry to be derived on-the-fly. It is then possible to choose the follow-up strategy most suited to each individual candidate. The final astrometric and photometric calibration is done using 2MASS [@Skrutskie.2006] stars in the field as references.

Survey analysis
===============

Ultracool brown dwarfs are extremely red objects 25 to 60 times (3.5 to 4.5 magnitudes) brighter in $J$-band that in $z'$ band. This means that even if our $z'$ images are about 3 to 4 magnitudes deeper than the detection $J$-band images, many of our candidates with strong $J$ band detection will not appear at all in the $z'$ images. However, a non detection in $z'$ band is a very strong sign that the candidate is a very red astrophysical source with $z'-J>3.5$, exactly what we are looking for. The other alternative is that these sources are spurious, such as unrecognised artefacts (remaining detector cross-talk, unflagged hot pixel, optical ghost, etc.) on the $J$ image. Since ultracool brown dwarfs are extremely rare ( our preliminary estimate is $\sim$ 1 per 25 square degree, so fewer than 1 per $500~000$ sources 