From Moustakas et al. 2011, http://adsabs.harvard.edu/abs/2011arXiv1112.3300M
Our baseline optical observations consist of deep
B_WRI imaging available as part of the NDWFS third public data release.
The imaging was carried out using the MOSAIC-I camera at the
KPNO/Mayall 4 m telescope, reaching a 5-sigma depth of ~26.5,
~25.5, and ~25.3 mag in a 2" diameter circular aperture in B_W,
R, and I, respectively. Note that although K-band imaging from the
NDWFS is also available for ~60% of the Bootes survey region, we
do not use these older data in favor of the more recent near-infared
(near-IR) observations described below.