instabilities in the spectrograph. This might lead to non-linear effects on the spectral shift across the dispersion and cross-dispersion direction. We also find that wavelength shift across the orders, due to large perturbations, do not remain uniform. The conventional correlation of ThAr spectrum with the template would not take into account these non-linearities.

The echelle spectrograph operating at VBT lacks the mechanism to revert the optical components to their original position after the movement. In such cases, it is essential to correct for the positional drifts. After the perturbations in the optics are corrected, the obtained spectra can then be correlated with the template for wavelength calibration.

We propose to use a ThAr spectrum taken at the beginning of the observational run to estimate the perturbations in the positions of the components. The centroid positions of the isolated peaks in ThAr spectra are estimated. These centroid locations depict the current positions of the optical components in the spectrograph. If there are any perturbations in the optical components, then there will be shifts in the centroid positions of the isolated wavelengths.

Using Zemax model to estimate instrumental drifts
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To estimate the optomechanical positional variations of the components, we have used 