Fig. 1. Definition of FWHM

Fig. 2. HFD of different type of images. (a) Noise image; (b) faint star; (c) star in focus

Fig. 3. Algorithm flow chart

Fig. 4. Comparison of images before and after anisotropic diffusion method processing. (a) Original image; (b) image after processing

Fig. 5. Comparison of images before and after Qtsu algorithm processing. (a) Original image; (b) image after processing

Fig. 6. Comparison of images before and after cluster algorithm processing. (a) Original image; (b) image after processing

Fig. 7. Performance comparison of algorithms. (a) Performance comparison of algorithms under different noise when FWHM is constant; (b) performance comparison of algorithms under different spot sizes when SNR is constant

Fig. 8. Partial image sequences of object 33320

Fig. 9. Outlier detection

Fig. 10. FWHM values obtained by fitting for different frames of image numbered 33320 (arrow points to direction of increase of i)

Fig. 11. Focusing curve fitted with measured FWHM

Fig. 12. HFD of different image frames in target numbered 33320

Fig. 13. V-shaped focusing curves after fitting