perturbed, the spot diagram of the perturbed system is compared with the nominal position. The maximum to the minimum range of the pixel shift is obtained across the detector. This range is displayed in the last column of the table.

Modelling Th-Ar observations in Zemax {#sect:model}
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Procedure for model matching with observations
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The sensitivity analysis was done for individual components with a perturbation in one of the rotational degrees of freedom. However, in the actual working system, the perturbations could arise from other degrees of freedoms. This cumulative effect would result in shifting the ThAr line positions from the nominal location. We use ThAr observations taken at the beginning of the observational run on each day to estimate the perturbations in the optical components. The initial data reduction and wavelength calibration of Th-Ar data are done in IRAF. The latter part of the analysis is done using a dedicated pipeline developed in Python.

In this context, a *merit function* was constructed using RV sensitive parameters (that can be perturbed in real time), to achieve the target position of the wavelength from Th-Ar observations. The merit function constructed in such a way that the positions of the optical components 