STUDY OF THE DEGRADATION OF DAYTIME ASTRONOMICAL IMAGES DUE TO TURBULENCE IN THE LOWER ATMOSPHERE BY MEASUREMENT OF THE STANDARD-DEVIATION OF THE ANGLE OF ARRIVAL

Fluctuations in the angle-of-arrival of a luminous wavefront are investigated by placing a narrow slit over the image of the solar limb, and observing the entrance pupil through this diaphragm. Measurement of the standard deviation σAA of these fluctuations, in a direction perpendicular to the solar limb, provides sufficient information to estimate an absolute parameter characterizing the seeing, i.e. Fried's parameter r0 or the equivalent parameter ∫LCN 2(h) d h which gives the integral, over an atmosphere thickness L which depends on the spatial and angular filterings performed by the slit, of the structure constant CN 2characterizing fluctuations in the air's refractive index. Simultaneous estimations of σAA 2 and ∫LCN 2(h) d h permit a linear relationship to be established between these two quantities. The result is interpreted theoretically by integration of the two-dimensional power spectrum for the angle-of-arrival fluctuations. The influence of filtering effects is discussed. Such a calibration can be performed for a given telescope and slit, enabling the system to be used as a seeing monitor. © 1979 D. Reidel Publishing Co.