A revised simplified scattering model for the moonlit sky brightness profile based on photometry at SAAO

TL;DR

This study develops a more accurate simplified scattering model for moonlit sky brightness at SAAO, validated against multi-filter photometry, and introduces a method to derive atmospheric scattering phase functions, improving predictions of sky brightness in optical astronomy.

Contribution

A revised scattering model that better reflects atmospheric effects and a novel approach to derive the atmospheric phase function from observational data.

Findings

Model matches Rayleigh and Mie scattering functions well.

Deviations explained by multiple scattering and airglow.

Effective tool for predicting sky brightness with moonlight.

Abstract

This paper presents multi-filter measurements of the night sky brightness at the South African Astronomical Observatory (SAAO) in Sutherland in the presence of a bright moon. The observations cover a wide range of sky directions, lunar phases and lunar positions. A revised simplified scattering model is developed for estimating the sky brightness due to moonlight that more accurately reflects the atmospheric extinction of the lunar beam compared to models frequently applied in astronomical studies. Contributions to night sky brightness due to sources other than moonlight are quantified and subtracted from the total sky background radiation to determine the spectral intensity and angular distribution of scattered moonlight. The atmospheric scattering phase function is then derived by comparing the sky brightening to the strength of the incoming lunar beam, estimated using a novel approach. The phase function is shown to be an excellent match to the combined theoretical Rayleigh and Mie scattering functions, the latter with a Henyey--Greenstein form instead of the exponential angular relationship often used in previous studies. Where deviations between measured and model sky brightness are evident in some bands these are explained by contributions from multiple scattering or airglow, and are quantified accordingly. The model constitutes an effective tool to predict sky brightness at SAAO in optical photometric bands, especially with a bright moon present. The methodology can also be readily be adapted for use at other astronomical sites. The paper furthermore presents $UBV(RI)_c$ and Str{\"o}mgren photometry for 49 stars, most with no prior such data.