a Zemax based ray trace model. The flow chart of the approach is demonstrated in Figure \[fig:concept\]. Wavelengths of isolated Th-Ar peaks from observational spectra are identified; the wavelength and the corresponding Echelle order is fed to Zemax using Python interface. We have used the Spot diagram (CENX and CENY) of isolated ThAr wavelength from Zemax for evaluation of the centroid shift as described in (Grupp et al, 2009) [@tandp]. The Zemax parameters are perturbed in such a way that the centroid positions (Spot diagram) of the Zemax model are matched with the Th-Ar line centers obtained from the ThAr observations [@model_based]. In contrary to the traditional model based calibration approaches, we have used a Zemax optical model of the spectrograph instead of the physical model of the instrument. For this purpose, a merit function is constructed to allow perturbations of RV sensitive parameters, such that the Zemax model would match with the observations after optimization of the merit function. After the model is matched with the observations, the perturbations in the positions of the components are estimated.

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Since the error-free locking is not ensured for the optical components, we are using reference locking for precision RV observations. Zemax 