obtained as $\pm$ 1 pixels.

The goodness of the model also depends on the wavelength identification of Th-Ar observational spectra, as the wavelength and the corresponding echelle orders are used as inputs for the Zemax model. Th-Ar spectrum has few saturated lines [@ThAr] and the wavelength identification has an inaccuracy in the case of saturated lines. In the model matching, we have eliminated the outliers of saturated lines beyond the wavelength range of 7000 Å.

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Conclusion {#sect:conclusion}
==========

With these current settings of the spectrograph, to achieve a radial velocity precision of 10 $\textrm{ms}^{-1}$, the instrumental stability has to be at 0.01-pixel level. For instance, the grating position has ambiguity $\approx$ 25 arc sec, during the system restart for everyday observations. This results in a residual shift of 1.4 pixels across the Echelle orders. In the current configuration, the grating uncertainty could very well impede the achievable precision of the instrument. Apart from this, the design of the spectrograph suffers from vignetting (56 $ \textrm{ms}^{-1} $) and distortion effects (400 $ \textrm{ms}^{-1} $). The vignetting and distortion that could affect the RV precision were not taken into account during the design phase in the instrument as it was not 