Enamik maailma suurematest teleskoopidest on koondatud vaid üksikutesse kohtadesse kõrgmägedes. Neis paikades on selge kuiv taevas, pilvitu ja tolmutu. Stabiilne hea läbipaistvusega atmosfäär kindlustab teravamad kujutised ja taevakumm on nii tume, et kuuvalguseta öödel heidab maapinnale varje Linnutee kuma.
The seeing effects are caused by atmospheric turbulence through which some of the light arriving from a star is scattered by refractive inhomogeneities. As the light wave propagates through the atmosphere it experiences fluctuations in amplitude and phase. An image formed by focusing this wave exhibits fluctuations in intensity, sharpness and position which are commonly referred to as scintillation, image blurring and image motion. The seeing observed by a telescope on the ground is contributed by the whole atmosphere crossed by the light wave front and one distinguishes three main contributing causes: 1. The turbulence in the high atmosphere, which has a maximum near the tropopause at about 12 km. This layer is, in particular, the cause of the scintillation effect. 2. The turbulence of the atmospheric boundary layer (between 30 and 500 m). 3. The turbulence in the ground surface layer (up to about 30 ~ 50 m) and the one generated by the artificial structures of the observatory itself.
