The Blanco Cosmology Survey: Data Acquisition, Processing, Calibration,
  Quality Diagnostics and Data Release
  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
$\sim45$ 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 $\sim2$% 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 {\tt 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 $z\sim1$. We highlight some selected science results to date and provide a
full description of the released data products.
