Forward Global Photometric Calibration of the Dark Energy Survey

Abstract

Many scientific goals for the Dark Energy Survey (DES) require 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 broad-band survey imaging itself and models of the instrument and atmosphere to estimate the spatial-and time-dependence of the passbands of individual DES survey exposures. "Standard" passbands are chosen that are typical of the passbands encountered during the survey. The passband of any individual observation is combined with an estimate of the source spectral shape to yield a magnitude m std b in the standard system. This "chromatic correction" to the standard system is necessary to achieve sub-percent calibrations. The FGCM achieves reproducible and stable photometric calibration of standard magnitudes m std b of stellar sources over the multi-year Y3A1 data sample with residual random calibration errors of σ = 5 − 6 mmag per exposure. The accuracy of the calibration is uniform across the 5000 deg 2 DES footprint to within σ = 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 < g − i < 3.0.

Producing a BOSS CMASS sample with DES imaging

“…The Dark Energy Survey (DES; The Dark Energy Survey Collaboration 2005; Abbott et al 2018a) is an imaging survey covering 5, 000 deg 2 of the southern sky. This photometric data set has been obtained in five broadband filters, grizY , ranging from ∼ 400nm to ∼ 1, 060nm (Li et al 2016;Burke et al 2018), using the Dark Energy Camera (DECam; Flaugher et al 2015) mounted on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The main goal of DES is to improve our understanding of cosmic acceleration and the nature of dark energy using four key probes: weak lensing, large-scale structure, galaxy clusters, and Type Ia supernovae.…”

Stellar Streams Discovered in the Dark Energy Survey

“…Photometric calibration was performed via the Forward Global Calibration Method (FGCM; Burke et al 2018), which utilized ancillary information about atmospheric and environmental conditions at the time of each exposure. The FGCM photometric calibration is found to have a relative photometric uniformity of~7 mmag (Burke et al 2018) and an absolute calibration accuracy of 3 mmag (DES Collaboration 2018). Individual exposures were remapped to a consistent pixel grid and co-added to increase imaging depth (Morganson et al 2018).…”

Trans-Neptunian Objects Found in the First Four Years of the Dark Energy Survey

“…The DES catalogs are exquisitely well calibrated to a common photometric system across the focal plane (Bernstein et al 2017a) and across all the exposures of the survey (Burke et al 2017). Comparison of DES magnitudes to Gaia DR2 magnitudes show uniformity across the footprint to ≈ 6 mmag RMS (Abbott et al 2018, section 4.2).…”

“…This window will retrieve near-optimal S/N ratio for the putative TNO in typical DES seeing. The factors k µ remove the variations in the photometric zeropoints of the exposures, placing fluxes on a common scale (Burke et al 2017). The RMS noise σ µ,b in f µ,b is calculated by propagating the Poisson noise of the s µ,j through Equation (16).…”