entire spectrum). If there is a shift in the observed spectrum from correlation result, then the spectrum is corrected for the shift and the dispersion solution is applied. The shift applied is often a constant number which does not take into account the non-linearities along and across the spectral orders. This procedure might result in a discrepancy in the wavelength solution in the following ways [@model_based; @HESP]:

-   The transient distortions to the local wavelength solutions can be corrected/tracked by wavelength calibration sources if the shifts in the calibration line positions are relatively small (at ±1 pixel level or less). In our case, the grating repeatability is uncertain up to $\pm$4.5 pixel level as shown in Figure \[fig:repeatability\]. Such large perturbations need to be properly accounted for and corrected.

-   The blends in the ThAr calibration lines are usually identified by fitting a single Gaussian to measure the centroid. However, due to a shift of ± 4.5 pixel, these lines experience different aberration effects as they pass through the optical components. This leads to inaccuracies in the centroid estimation.

-   As explained in Section \[sect:limitations\], apart from the uncertainty in zero-positioning, there are other effects like vignetting, distortion and environmental 