first step is to estimate the wavelength – pixel positions of isolated ThAr lines from observations. These wavelengths along with the echelle order are to be given as input to Zemax. The Zemax model determines the position of the wavelength on the CCD as per the instrumental design along with aberrations and perturbations. From the wavelength-pixel map obtained from both ThAr observations and Zemax model, in each order, one wavelength position (which is the best match between the model and observation) is taken. This location is anchored and the relative position (between Zemax and observations) of the other wavelengths in each order is obtained. Then using the relative positions, the wavelength-pixel map is interpolated for locations where ThAr lines do not exist.

Apart from that, traditional calibration techniques have the limitation of stability and lifetime issues. The RV observations of exo-planets, e.g., typically has a timeline ranging from weeks to years. In such a case, it is essential to ensure that the wavelength calibration errors are minimized. The model-based approach can be used to establish the accuracy using the real-time setup of the spectrograph and the Zemax model. Wavelength calibration is also unique to each spectrograph and the model-based approach 