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

We present an algorithm to photometrically calibrate wide field opticalimaging surveys, that simultaneously solves for the calibration parameters andrelative stellar fluxes using overlapping observations. The algorithm decouplesthe problem of "relative" calibrations, from that of "absolute" calibrations;the absolute calibration is reduced to determining a few numbers for the entiresurvey. We pay special attention to the spatial structure of the calibrationerrors, allowing one to isolate particular error modes in downstream analyses.Applying this to the Sloan Digital Sky Survey imaging data, we achieve ~1%relative calibration errors across 8500 sq.deg. in griz; the errors are ~2% forthe u band. These errors are dominated by unmodelled atmospheric variations atApache Point Observatory. These calibrations, dubbed "ubercalibration", are nowpublic with SDSS Data Release 6, and will be a part of subsequent SDSS datareleases.