Sky Variability in the y Band at the LSST Site

Abstract

We have measured spatial and temporal variability in the y-band sky brightness over the course of four nights above Cerro Tololo near Cerro Pachon, Chile, the planned site for the Large Synoptic Survey Telescope (LSST). Our wide-angle camera lens provided a 41° field of view and a 145" pixel scale. We minimized potential system throughput differences by deploying a deep-depletion CCD and a filter that matches the proposed LSST (y_3) band (970 –1030 nm). Images of the sky exhibited coherent wave structure, attributable to atmospheric gravity waves at 90 km altitude, creating 3%–4% rms spatial sky flux variability on scales of about 2° and larger. Over the course of a full night, the (y_3) band additionally showed highly coherent temporal variability of up to a factor of 2 in flux. We estimate the mean absolute sky level to be approximately (y_3 = 17.8 mag(Vega)), or (y_3 = 18.3 mag(AB)). While our observations were made through a (y_3) filter, the relative sky brightness variability should hold for all proposed y bands, whereas the absolute levels should more strongly depend on spectral response. The spatial variability presents a challenge to wide-field cameras that require illumination correction strategies that make use of stacked sky flats. The temporal variability may warrant an adaptive y band imaging strategy for LSST, to take advantage of times when the sky is darkest.