The Blanco Cosmology Survey ( BCS ) is a 60 night imaging survey of \sim 80 deg ^ { 2 } of the southern sky located in two fields : ( \alpha , \delta ) = ( 5 hr , -55 ^ { \circ } ) and ( 23 hr , -55 ^ { \circ } ) .
The survey was carried out between 2005 and 2008 in griz bands with the Mosaic2 imager on the Blanco 4m telescope .
The primary aim of the BCS survey is to provide the data required to optically confirm and measure photometric redshifts for Sunyaev-Zel ’ dovich effect selected galaxy clusters from the South Pole Telescope and the Atacama Cosmology Telescope .
We process and calibrate the BCS data , carrying out PSF corrected model fitting photometry for all detected objects .
The median 10 \sigma galaxy ( point source ) depths over the survey in griz are approximately 23.3 ( 23.9 ) , 23.4 ( 24.0 ) , 23.0 ( 23.6 ) and 21.3 ( 22.1 ) , respectively .
The astrometric accuracy relative to the USNO-B survey is \sim 45 milli-arcsec .
We calibrate our absolute photometry using the stellar locus in grizJ bands , and thus our absolute photometric scale derives from 2MASS which has \sim 2 % accuracy .
The scatter of stars about the stellar locus indicates a systematics floor in the relative stellar photometric scatter in griz that is \sim 1.9 % , \sim 2.2 % , \sim 2.7 % and \sim 2.7 % , respectively .
A simple cut in the AstrOmatic star-galaxy classifier spread_model produces a star sample with good spatial uniformity .
We use the resulting photometric catalogs to calibrate photometric redshifts for the survey and demonstrate scatter \delta z / ( 1 + z ) = 0.054 with an outlier fraction \eta < 5 % to z \sim 1 .
We highlight some selected science results to date and provide a full description of the released data products .