There are three empirical models for PSF: Gaussian, ABC, and fractal.^{7} If double-scattering processes are present, these models expand to double Gaussian, double ABC, and double-fractal functions, as shown in Table 1. The single coordinate $r$ is used because PSF is assumed to have rotational symmetry. Constants $w1$, $w2$, $k1$, $k2$, and $A$ are scale factors. Spectral indexes $n1$ and $n2$ usually range from 1 to 3. $B$, $rt$, $\sigma 1$, and $\sigma 2$ are also constants. When $w2$ or $k2=0$, the double PSF model is simplified to a single PSF model. The ABC model is very similar to the fractal model, except in the small $r$ region. Their expressions will be the same when $Br2\u226b1$. Therefore, the fractal model can be regarded as a simplified version of the ABC model and is used as the second PSF term in the ABC model in Table 1. The PSF usually has a long tail extending to the millimeter dimension. This tail in the Gaussian model decays quickly. Thus, the Gaussian model is usually not as accurate as the fractal model.^{5} For these reasons, the fractal model, which is simple and accurate, is preferred in most cases.