visible in Fig. \[zJspT\], brown dwarfs later than T8 populate the $z'-J>$3.8 colour range. This figure as well as Fig. \[wilkins\] make use of spectra from http://www.jach.hawaii.edu/$\sim$skl/LTdata.html [@Martin.1999; @Kirkpatrick.2000; @Geballe.2001; @Leggett.2002; @Burgasser.2003; @Knapp.2004; @Golimowski.2004; @Chiu.2006]. As described in @Delorme.2008b these colours are synthesised from the spectra using the filters, atmosphere, telescope and detector transmission and sensitivity of the instruments used in CFBDSIR. Given the large colour spread of late T-dwarfs, we set a very conservative colour selection threshold of $z'-J>$3.5, to select as many ultracool dwarfs as possible. Since the 3.5$<z'-J<$3.8 colour range is mainly populated by warmer T-dwarfs and is still very far from any densely populated colour locus, as illustrated by Fig. \[colpop\], including it in our selection does not increase the number of contaminants significantly.

![$z'-J$ colour spectral type relation. The green symbols represent synthetic MegaCam/WIRCam colours for known objects with publicly available spectra. The black dashed line is the resulting averaged colour-spectral type relation. Red points show the observational colours and spectral types of CFBDS spectroscopically confirmed T-dwarfs. \[zJspT\] ](14277fg2.ps)

In addition to this colour selection, the main filtering criterion is based on the $\chi ^2$ from PSF fitting, to distinguish point sources from galaxies and 