Image quality and seeing measurements for long horizontal overwater propagation

The effect of atmospheric turbulence on the imaging of scenes, for horizontal 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 telescope. Point-source images were recorded during the night, and the Fried parameter r(0) was derived using several methods, leading to r(0) values ranging from 2 to 4 cm depending on observing conditions. A very high level of scintillation was observed. Studies of correlations between close-by sources lead to a very small domain of isoplanatism. Daytime observations of an extended source are also performed; an image motion of small spatial coherent length seems to be drawn by a horizontal wind producing a wave-like distortion of horizontal lines and a boiling-like distortion of vertical ones.