But before carrying out indeed this operation it is necessary to take into account that the optical flux recorded with the telescope crossed the earth atmosphere. The operation of complete photometric calibration consists in taking into account the spectral transmission of the atmosphere so that the flux measured by the telescope is just as it would be outside the atmosphere. Atmospheric transmission for an object observed at zenith. Each curve corresponds to an altitude of observation. Of course the atmosphere is more transparent when one observes from a high place.
Absorption is far from being negligible, in particular in the blue part of the spectrum.
The atmosphere is almost opaque below 0.3 micron. However these bands are local and consequently, it is relatively easy to include their presence during the processing by carrying out judicious interpolations. The typical atmospheric absorption induced by the gas components. Ozone has effect between 4400 and 7500 A, but it is included in the curves of figure 7. To determine the effective atmospheric transmission it is necessary to take into account the zenithal distance of the observed object.