Airmass

The airmass is an additional parameter which influences the transmission. The ETC allows six different values for the airmass: 1.00; 1.25; 1.50; 1.75; 2.0; 2.5 The Airmass (AM) scales the number of photons from the sky background with

-2.78719 ^. 10^{-4 .} AM³ -6.53841 ^. 10^{-2 .} AM² +1.11979 ^. AM - 5.52132 ^. 10^-2 (15)

It is a polynomial fit to observed sky brightnesses from [SKYREF]. For small zenith distances ( < 40^o) it is similar to the van-Rhijn's function (formula 17).