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 is used because PSF is assumed to have rotational symmetry. Constants , , , , and are scale factors. Spectral indexes and usually range from 1 to 3. , , , and are also constants. When or , 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 region. Their expressions will be the same when . 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.