We retrieved the calibrated, dither-combined (drizzled) image from MAST[^1] to use as a detection image. This image has 0.5ksec exposure and is complete[^2] to at least 24mag.

Source extraction was done with SExtractor [@Bertin1996] in dual-image mode. The *HST* F140W detection image was used to detect sources, and photometry was obtained on the $i'$, $J$, 3.6and 4.5images in 2arcsec diameter apertures. We are unable to use large apertures to measure the IRAC fluxes (e.g. 4arcsec) due to the high spatial density of sources in the cluster core (galaxies are typically $\sim2$arcsec apart). So we measure the fluxes in 2arcsec diameter apertures and correct for the broader PSF of the IRAC data compared to the ground-based data using the ratio of the flux in the $J$ band image to the $J$ image convolved with a Gaussian kernel matching the IRAC PSF, following @Hartley2013. Fluxes were corrected to total fluxes using the growth curves of bright, unsaturated stars in the field-of-view. This method assumes that the blended sources have the same $J-\rm{IRAC}$ colour, so may provide inaccurate colours for some sources.

High redshift galaxy selection
------------------------------

To select sources likely to lie at high redshift, we employ two colour cuts. The well-tested 