Experimental Results on Atmospheric Turbulence Obtained with an Infrared Speckle-interferometer

Abstract

Knowledge of the optical properties of the turbulent atmosphere is of prime importance for ground-based high angular resolution techniques, such as speckle-interferometry. The two most critical parameters are (a) the correlation time ( _c), related to the typical life-time of the speckled pattern observed in the image plane of large aperture telescopes; (b) the Fried parameter (r _o), related to the equivalent diameter of a telescope yielding the same angular resolution with long exposure time (i.e. d _o). In this paper, results related to r _o obtained from the data reduction performed on observations callected at the Kitt Peak National Observatory 3·8 m telescope in April 1981 with a one-dimensional infrared speckle-interferometer are reported. The astrophysical results of these observations are presented in a separate paper. Correlation times have been estimated in the range u = 2·2 to 4·6 w m from the average decay time of the correlation between successive images, and have been found to be in good agreement with previously published results. On bright point-like sources, accurate estimations of the Fried parameter and of its time evolution have been tracked during the total observation time of each source (between 5 and 10 min). A first insight into the statistics of the random variable r _o (t) shows a good agreement with a log-normal distribution. Comparisons of observations covering highly different conditions of turbulence show that the mean value and standard deviation of r o are probably related by a power law. Finally power spectra of r _o (t) have been computed showing in one case a quasi-periodical behaviour, interpreted in a simple wind-driven model.