由于差分像运动监测法测量大气相干长度需要多帧统计, 本文应用波前结构函数法, 提出了一种大气相干长度的瞬时测量方法。该方法通过Shack-Hartmann波前探测器测量单帧短曝光畸变波前的Zernike系数;然后减去光学系统初始像差的Zernike系数, 去除倾斜项, 计算波前结构函数;最后,与满足Kolmogorov湍流理论的大气短曝光理论波前结构函数进行最小二乘拟合, 得到瞬时大气相干长度。利用湍流相位板构造了相应的测试系统, 并进行了外场实测对比。结果表明: 提出的基于波前结构函数法的测量结果与差分像运动监测法的测量结果基本吻合; 不同格林伍德频率下, 标准差与均值之比小于4.1%, 稳定性较好; 外场比对累计16个夜晚, 得到的平均偏差小于0.45 cm。该方法可以实现空间目标大气相干长度的单帧瞬时测量, 并可用于观测站点的视宁度、自适应光学系统内部大气湍流强度和地基大口径望远镜主镜视宁度的监测。

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.