Simultaneous measurement of atmospheric coherence length

When the differential image motion monitor method is used to measure the atmospheric coherence length, it needs a number of frames recorded with objective motions. Therefore, this paper proposes a simultaneous measuring method of the atmospheric coherence length based on the wavefront structure function for spatial objective. The method uses a Shack-Hartmann wavefront sensor to measure the Zernike coefficients of a frame of aberrated wavefront with short exposure. Then, it subtracts the initial aberration of optical system, removes the tip and tilt items and calculates the experimental structure function of aberrated wavefront. Finally, the simultaneous atmospheric coherence length is obtained by least-square approximation fitting with the theoretical structure function of aberrated wavefront satisfying the Kolmogorov turbulence theory. A test system is constructed by a turbulence phase plate and a number of experiments are conducted in an external field. The result shows that the measured atmospheric coherence length by using the proposed method based on the wavefront structure function matches well with that of the differential image motion monitor method. The ratio of mean value to standard deviation is less than 4.1% under different Greenwood frequencies and the mean difference of two methods is less than 0.45 cm in 16 effective nights. The method implements the simultaneous measurement of atmospheric coherence lengths of spatial objectives and also could be used to evaluate the seeing of observation sites, the atmospheric turbulence strength inside adaptive system and the mirror seeing of main mirror for a ground-based large aperture telescope. © 2016, Science Press. All right reserved.