Atmospheric coherence time measurement by modified Fast Defocus method

•It is shown with a simulation that measured defocus aberration coefficient with M-FADE and FADE have less than 1 percent discrepancy.•The estimated coherent time with M-FADE has only a few percent discrepancies with respect to FADE therefore FADE model function can be used.•The measured coherence time for the Iranian national observatory (INO) by M-FADE and 4-DIMM defocus velocity methods has only a 5 percent deviation.