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Estimating the sky background is critical for ground-based astronomicalresearch. In the optical, scattered moonlight dominates the sky background,when the moon is above the horizon. The most uncertain component of a scatteredmoonlight model is the aerosol scattering. The current sky background model forCerro Paranal uses an extrapolated aerosol extinction curve. With a set ofX-Shooter sky observations, we have tested the current model as well asdetermined the aerosol extinction from the ultra-violet to near-infrared. Toour knowledge, this is the first time that a scattered moonlight model has beenused for this purpose. These observations were taken of blank sky, during threedifferent lunar phases, and at six different angular distances from the moonfor each night/lunar phase. Overall, the current model does reproduce theobservations for average conditions decently well. Using a set of skybackground models with varying aerosol distributions to compare with theobservations, we found the most likely aerosol extinction curves, phasefunctions, and volume densities for the three nights of observations andcompare them with the current model. While there were some degeneracies in theaerosol scattering properties, the extinction curves tend to flatten towardsredder wavelengths and are overall less steep compared to the extrapolatedcurve used in the current model. Also, the current model had significantly lesscoarse particles compared to the favored volume densities from the X-Shooterdata. For the three nights of sky observations, the aerosol size distributionsdiffered, most likely reflecting the changes in atmospheric conditions andaerosol content. In short, the current sky background model is in fairagreement with the observations, and we have determined better aerosolextinction curves and phase functions for Cerro Paranal.

Scattered moonlight observations with X-Shooter: Implications for the aerosol properties at Cerro Paranal and the ESO sky background model