Atmospheric extinction evaluates the sky transparency. Besides the natural extinction due to the molecules of the atmosphere, other sources of sky transparency degradation are clouds and aerosols.
Atmospheric extinction is a relevant to ground-based astronomy in optical and near-infrared wavelengths since it is associated with the absorption/scattering of incoming photons from astronomical sources by the Earth’s atmosphere.
The traditional method to compute the atmospheric extinction is to calculate the reduction in brightness of a star in V and r’ bands (KV and Kr'). The following figure shows the statistical summary of 20 years of KV measured at the ORM (García-Gil et al, 2010)
The Observatorio del Roque de los Muchachos (ORM), in the Canary Islands (Spain), was one of the candidates to host the future European Extremely Large Telescope (E-ELT) and is the site of the Gran Telescopio Canarias (GTC), the largest optical infrared facility to date. Sky transparency is a key parameter as it defines the quality of the