Color–color diagrams used to calibrate the SDSS filters to the USNO reference catalog. Stars are matched in a common field between the SDSS and USNO catalogs, and a linear fit to the color–color relation is found. The linear relation is applied to the stars in the USNO catalog that are matched to the field of PLCK G287.0+32.9 in order to find the magnitude zero-point. ﹩3\sigma ﹩ clipping is applied to remove outliers. Top: transformation between SDSS r and USNO R2 filters. Bottom: transformation between SDSS g and USNO B2 filters.