Fig. 1. Spectral line profile obtained by fitting of data

Fig. 2. Effect of Doppler velocity of solar autorotation on images at different points of spectral line

Fig. 3. Comparison before and after data processing (wavelengths in nm). (a) Original images; (b) grayscale change of original images; (c) images after edge dimming is removed; (d) grayscale change after edge dimming is removed

Fig. 4. Removing information outside solar circle. (a) Original image；(b) image after polar coordinate transformation；(c) solar part

Fig. 5. PCA decomposition contribution rate

Fig. 6. Image comparison before and after 20th-order reconstruction

Fig. 7. Image pre-processing process

Fig. 8. MLP regression network

Fig. 9. General process of calibration of observing wavelength points using BP neural network

Fig. 10. Correlation diagram of training set and test set for method validation experiment. (a) Training set; (b) test set

Fig. 11. Results of group 1 of grouping test experiment. (a) Correlation diagram of training set；(b) correlation diagram of test set；(c) residual of each data prediction result；(d) variation of residual standard deviation of each day with time

Fig. 12. Variation of residual standard deviation of different groups of grouping test experiment. (a) Group 2; (b) group 3; (c) group 4; (d) group 5

Fig. 13. Test results in range of [-0.006 nm, 0.006 nm]