Limiting Magnitude, τ, t<sub>eff</sub>, and Image Quality in DES Year 1

Abstract

The Dark Energy Survey (DES) is an astronomical imaging survey being completed with the DECam imager on the Blanco telescope at CTIO. After each night of observing, the DES data management (DM) group performs an initial processing of that night's data, and uses the results to determine which exposures are of acceptable quality, and which need to be repeated. The primary measure by which we declare an image of acceptable quality is $$\tau$$, a scaling of the exposure time. This is the scale factor that needs to be applied to the open shutter to reach the same photometric signal to noise ratio for faint point sources under a set of canonical good conditions. These conditions are defined to be seeing resulting in a PSF full width at half maximum (FWHM) of 0.9 and a pre-defined sky brightness which approximates the zenith sky brightness under fully dark conditions. Point source limiting magnitude and signal to noise should therefore vary with t in the same way they vary with exposure time. Measurements of point sources and $$\tau$$ in the first year of DES data confirm that they do. In the context of DES, the symbol $$t_{eff}$$ and the expression exposure time usually refer to the scaling factor, $$\tau$$, rather than the actual effective exposure time; the exposure time in this case refers to the effective duration of one second, rather than the effective duration of an exposure.