centroid position with and without vignetting is compared and the error induced by vignetting was found to be $\approx$ 56 $\textrm{ms}^{-1} $.

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Distortion
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A mathematical model of a 2D dispersion map of the spectrograph is constructed using grating and prism dispersion equations along with the spectrograph parameters (focal length of the collimator/camera, the location of the detector, etc) [@mathematical]. The isolated peaks of Th-Ar observational spectra are identified and the corresponding wavelength and the echelle orders are given as inputs to the mathematical model and Zemax model. The position of ThAr lines on the detector is estimated for both the models. Later, the models are perturbed in grating blaze angle by the same amount. The difference in which the models responds to the perturbation gives the resultant distortion in the optical design. Zemax model takes into consideration the inherent distortion present in the system. The difference in the positions of isolated ThAr line peaks, between the Zemax and the mathematical model, are estimated to account for the distortion. A residual from the comparison is a measure of distortion which was found to be 400 $\textrm{ms}^{-1}$.

Other factors
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Apart from vignetting and distortion, there is also tilt in 