Photometric characterization of the Dark Energy Camera

Abstract

We characterize the variation in photometric response of the Dark Energy Camera (DECam) across its 520~Mpix science array during 4 years of operation. These variations are measured using high signal-to-noise aperture photometry of $>10^7$ stellar images in thousands of exposures of a few selected fields, with the telescope dithered to move the sources around the array. A calibration procedure based on these results brings the RMS variation in aperture magnitudes of bright stars on cloudless nights down to 2--3 mmag, with <1 mmag of correlated photometric errors for stars separated by $\ge 20$". On cloudless nights, any departures of the exposure zeropoints from a secant airmass law exceeding >1 mmag are plausibly attributable to spatial/temporal variations in aperture corrections. These variations can be inferred and corrected by measuring the fraction of stellar light in an annulus between 6" and 8" diameter. Key elements of this calibration include: correction of amplifier nonlinearities; distinguishing pixel-area variations and stray light from quantum-efficiency variations in the flat fields; field-dependent color corrections; and the use of an aperture-correction proxy. The DECam response pattern across the 2-degree field drifts over months by up to $\pm 7$ mmag, in a nearly-wavelength-independent low-order pattern. We find no fundamental barriers to pushing global photometric calibrations toward mmag accuracy.

arXiv commentSubmitted to PASP

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• DELVE-ing into the Jet: a thin stellar stream on a retrograde orbit at 30 kpc
• Optical follow-up of gravitational wave triggers with DECam during the first two LIGO/VIRGO observing runs
• The Dark Energy Survey Supernova Program: Corrections on photometry due to wavelength-dependent atmospheric effects
• DES to HSC: Detecting low surface brightness galaxies in the Abell 194 cluster using transfer learning
• RR Lyrae stars in the newly discovered ultra-faint dwarf galaxy Centaurus I
• The Multiband Imaging Survey for High-Alpha PlanetS (MISHAPS) I: Preliminary Constraints on the Occurrence Rate of Hot Jupiters in 47 Tucanae