dots for the modeling the observations using merit function. They are compared with the wavelengths (marked in blue dots) used in sensitivity analysis as discussed in Section \[sect:sensitivity\]. This sampling of $6 \times 6$ matrix of well-sampled wavelengths is used for estimating the perturbations. To evaluate the match of the model with the observations all the isolated wavelengths used in the sensitivity analysis are utilized.

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Results of model matching with observations
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The perturbations obtained from the match of a model with the observations are given as input to a native Zemax model. For the selected 500 Th-Ar lines (as discussed in Section \[sect:sensitivity\]), the X-centroid is computed in the perturbed Zemax model. These centroid positions are compared with the observational positions. The best match is within $\pm$ 1-pixel accuracy as shown in Figure \[fig:Model\_match\]. The model is validated by obtaining the match to a different set of ThAr frames. The positional parameters of the components might vary from the actual system parameters during the built and alignment phase. The parameters as per the design phase have a deviation of $\pm$ 24 pixels from the observations. After modeling and optimization, the match between the observation and the model is 