model is used as the reference and all the optical components are brought to the reference level at the beginning of each observational run during the day time. We also propose that the Zemax model can be used to improve the calibration accuracy after the match is obtained with the observations. The primary calibration is always done with the Th-Ar source in the spectrograph. However, Th-Ar lines are not distributed uniformly and it also has some saturated and blended lines. This has an impact on the accuracy of the wavelength solution. The pixel-to-wavelength map obtained from the Zemax model could improve the wavelength solution. The dispersion solution can be extended to the region where ThAr lines are inadequate [@HESP_performance]. A wavelength calibration process assigns a unique wavelength to each pixel on the CCD. It is a crucial data reduction step for all spectroscopic measurements. The error induced in the wavelength calibration can significantly affect the precision obtained in the RV observations. The model-based approach can be extended to obtain the wavelength-pixel map for features across the spectrographs using interpolation, thus improving the RV accuracy of the spectrograph.

The approach we propose to improve the calibration accuracy is as follows: The 