2. Various contributors to the night-time illuminations

On the Earth's surface, at night and in the open air, there is no such thing as total darkness. Several contributors are superimposed, giving the different illumination conditions encountered at night. Knowledge of these contributors and their variability is essential to assess the ultimate signal-to-noise ratios that can be achieved. This analysis need not be limited to the visible spectral range, as technological capabilities and spectral illumination conditions may give an advantage to the exploitation of infrared spectral bands between 1 and 2.5 μm (IR1 band or SWIR: Short Wave Infra-Red).

The various contributors reviewed are as follows:

• atmospheric diffusion ;

• the Moon;

• stellar illumination;

• the Milky Way ;...