The residual aberrations of adaptive optics (AO) images can be removed by the speckle reconstruction method. To achieve accurate photometry, the knowledge of the speckle transfer function (STF) is necessary. In this Letter, a new method is proposed to evaluate the STF for speckle image reconstruction based on the partial correction AO. The STF is divided into two sections: the low-frequency section and the high-frequency section, and two different generalized Fried parameters for the partial wavefront compensation are used to calculate the STF values of those two different sections. This method is tested with the different seeing conditions and the various correction modes. The results show that the combined STFs have satisfactory agreement with the simulated AO STFs and give more accurate results than the STFs obtained from the traditional spectral ratio method. Further discussions show that the method is more reliable when the AO is operated in low-order correction mode because the approximation of the STF and the two generalized Fried parameters have better accuracy when the pupil diameter is much larger than those generalized Fried parameters.
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