the spectral output at the detector. The aberrations in the spectrograph would result in the asymmetry of the Point Spread Function (PSF), degrading the resolution at the detector [@PSFvbt]. Also, the spectrograph is not vacuum stabilized, hence the environmental variations are expected to affect the observations. The inherent stability of the spectrograph over several days observations (without the grating movement) was estimated to be about $ \pm $ 1 pixel ($ \pm $ 1000 $\textrm{ms}^{-1} $) [@stabilityVBT].

Necessity of correcting for the zero position drifts {#sect:need}
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ThAr lamp is the wavelength calibration source in the spectrograph. The calibration spectra are currently taken either before or after the stellar observations. The wavelength calibration is done as per the conventional procedure of generating a template spectrum with the spectrograph. The ThAr lines are identified and wavelength calibrated for a well-exposed spectrum taken with the spectrograph. This spectrum is used as a template for wavelength calibration. Rest of the ThAr frames obtained as part of a regular observational run are correlated with the template spectrum to obtain the dispersion solution. In the standard calibration procedure, the ThAr spectrum is correlated with the template to determine the overall shift w.r.t each order (or 