Photometric characterization of the Dark Energy Camera

TL;DR

This paper thoroughly characterizes the photometric response variations of the Dark Energy Camera over four years, achieving millimagnitude calibration precision and identifying key correction methods to improve photometric accuracy.

Contribution

It introduces a detailed calibration procedure for DECam that reduces photometric errors to millimagnitude levels and analyzes the temporal and spatial variations in the camera's response.

Findings

RMS variation in aperture magnitudes reduced to 2-3 mmag

Photometric zeropoint deviations attributable to spatial/temporal aperture correction variations

DECam response pattern drifts by up to ±7 mmag over months

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 $\ge20$". 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 $\pm7$ mmag, in a nearly-wavelength-independent low-order pattern. We find no fundamental barriers to pushing global photometric calibrations toward mmag accuracy.