Forward Global Photometric Calibration of the Dark Energy Survey
2017
Many scientific goals for the Dark Energy Survey (DES) require the calibration of optical/NIR broadband
b = grizY photometry that is stable in time and uniform over the celestial sky to one percent or better. It is also
necessary to limit to similar accuracy systematic uncertainty in the calibrated broadband magnitudes due to
uncertainty in the spectrum of the source. Here we present a “Forward Global Calibration Method (FGCM)” for
photometric calibration of the DES, and we present results of its application to the first three years of the survey
(Y3A1). The FGCM combines data taken with auxiliary instrumentation at the observatory with data from the
broadband survey imaging itself and models of the instrument and atmosphere to estimate the spatial and time
dependences of the passbands of individual DES survey exposures. “Standard” passbands that are typical of the
passbands encountered during the survey are chosen. The passband of any individual observation is combined with
an estimate of the source spectral shape to yield a magnitude mb
std in the standard system. This “chromatic
correction” to the standard system is necessary to achieve subpercent calibrations and in particular, to resolve
ambiguity between the broadband brightness of a source and the shape of its SED. The FGCM achieves a
reproducible and stable photometric calibration of standard magnitudes mb
std of stellar sources over the multiyear
Y3A1 data sample with residual random calibration errors of s = 6 7 mmag – per exposure. The accuracy of the
calibration is uniform across the 5000 deg2 DES footprint to within s = 7 mmag. The systematic uncertainties of magnitudes in the standard system due to the spectra of sources are less than 5 mmag for main-sequence stars
with 0.5 3.0 < -< g i .
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