The present paper introduces a method for measuring the atmospheric coherent time which is based on Fast Defocus method (FADE). The configuration of the system is modified in which, instead of an annular aperture, a mask with 8 or 4 regular apertures is used. In FADE there is an optics behind the telescope to make an annular image of the star at focus but in the current method, the image of the star is recorded at defocus to make 8 or 4 spots depends on the number of apertures. The average radial distance of the spots is used to estimate the defocus aberration coefficient same as the ring radius in FADE. Then the measured defocus aberration coefficient at a fast frame rate is used to calculate the Fried parameter, wind speed, and subsequently the coherence time, by fitting a model function to the temporal structure-function of the defocus. This method is called “modified FADE”. In order to evaluate the proposed method, FADE and modified FADE methods are compared using simulation. It is shown that the discrepancy in estimated Fried parameter and coherent time are within a few percent for both. The configuration of the modified FADE with 4 apertures is very similar to 4- DIMM defocus velocity method therefore to evaluate the proposed method experimentally, the coherent time of the Iranian National Observatory is measured by modified FADE and 4-DIMM defocus velocity methods. The discrepancy of the measured Fried parameter, wind velocity, and coherent time were 1, 3, and 5 percent.

本文介绍了一种基于快速散焦法（FADE）的大气相干时间测量方法。修改系统的配置，其中使用具有8个或4个规则孔的掩模代替环形孔。在FADE中，望远镜后面有一个光学元件，可以使聚焦的恒星形成环形图像，但是在当前方法中，离焦时记录的恒星图像可以根据光圈的数量产生8或4个点。光斑的平均径向距离用于估计与FADE中的环半径相同的散焦像差系数。然后，通过将模型函数拟合到散焦的时间结构函数，以快速帧速率测量的散焦像差系数可用于计算Fried参数，风速以及随后的相干时间。此方法称为“修改的FADE”。为了评估所提出的方法，使用仿真比较了FADE方法和改进的FADE方法。结果表明，两者的估计Fried参数和相干时间的差异都在百分之几以内。改进的具有4个孔的FADE的配置与4-DIMM散焦速度方法非常相似，因此为了实验评估该方法，伊朗国家天文台的相干时间通过改进的FADE和4-DIMM散焦速度方法进行测量。测得的Fried参数，风速和相干时间的差异分别为1、3和5％。结果表明，两者的估计Fried参数和相干时间的差异都在百分之几以内。改进的具有4个孔的FADE的配置与4-DIMM散焦速度方法非常相似，因此为了实验评估该方法，伊朗国家天文台的相干时间通过改进的FADE和4-DIMM散焦速度方法进行测量。测得的Fried参数，风速和相干时间的差异分别为1、3和5％。结果表明，两者的估计Fried参数和相干时间的差异都在百分之几以内。改进的具有4个孔的FADE的配置与4-DIMM散焦速度方法非常相似，因此为了实验评估该方法，伊朗国家天文台的相干时间通过改进的FADE和4-DIMM散焦速度方法进行测量。测得的Fried参数，风速和相干时间的差异分别为1、3和5％。