An Improved Photometric Calibration of the Sloan Digital Sky Survey Imaging Data

We present an algorithm to photometrically calibrate wide fi eld optical imaging surveys, that simultaneously solves for the calibration parameters and relative stellar fluxes using overlapping observations. The algorithm decouples the problem of “relative” calibrations from that of “absolute” calibrations; the absolute calibration is reduced to determining a few numbers for the entire survey . W pay special attention to the spatial structure of the calibration errors, allowing one to isolate particul ar error modes in downstream analyses. Applying this to the Sloan Digital Sky Survey imaging data, we achieve ∼ 1% relative calibration errors across 8500 deg 2 in griz; the errors are∼ 2% for theu band. These errors are dominated by unmodelled atmospheric variations at Apache Point Observatory. These calibrations, dubbed “u bercalibration”, are now public with SDSS Data Release 6, and will be a part of subsequent SDSS data releases . Subject headings: methods: data analysis, techniques: photometric, catalog s, surveys