Multispectral analysis of the Night Sky Brightness at Oukaimeden Observatory, Morocco

ABSTRACT We modelled night sky brightness (NSB) over the Oukaimeden region south-east of Marrakesh, where the Oukaimeden Observatory of Cadi Ayyad University (OUCA) is located.The simulations were performed using the newest version of ILLUMINA (V2.2), which is an advanced heterogeneous artificial sky brightness model designed to predict the spectral contribution of artificial lighting to the NSB. This model requires specific ground-based data, including photometric and spectroscopic properties of various light fixture types (light-emitting diodes and high-pressure Sodium) present in both the Marrakesh and Oukaimeden regions. In this paper, we report the first quantitative assessment of the impact of artificial light on astronomical observations at this high-altitude facility (OUCA), along with a comparative analysis of the current state of the NSB in the Johnson B and V bands. In addition, we present some results of this study by using the Natural Sky Unit (NSU) scale, which provides an intuitive measure of sky brightness relative to it’s natural brightness. For the OUCA, the NSU scale showed slightly variation over most of the sky, ranging from 1 NSU to 1.6 NSU over the B and V bands, indicating a minimal effect of light pollution. Whereas, we found that the city of Marrakesh and the village of Ourika, are the primary contributors to light pollution at OUCA, particularly affecting observations in both spectral bands (B and V) at low elevation angles between $0^{\circ }$ and $35^{\circ }$. The findings of this study align with global efforts to preserve dark skies and natural nocturnal environments.