IMAGE QUALITY AND SEEING MEASUREMENTS FOR LONG HORIZONTAL OVERWATER PROPAGATION

The effect of atmospheric turbulence on the imaging of scenes, for hor izontal propagation of the light over a distance of 20 km, 15 m above the sea surface, was analysed at visible wavelengths using a 20 cm tel escope. Point-source images were recorded during the night, and the Fr ied parameter r(0) was derived using several methods, leading to r(0) values ranging from 2 to 4 cm depending on observing conditions. A ver y high level of scintillation was observed. Studies of correlations be tween close-by sources lead to a very small domain of isoplanatism. Da ytime observations of an extended source are also performed; an image motion of small spatial coherent length seems to be drawn by a horizon tal wind producing a wave-like distortion of horizontal lines and a bo iling-like distortion of vertical ones.