(to be perturbed) are declared as variables; the target is the observational $X$-centroid position and the value of the merit function is the spot diagram CENX. Coordinate breaks are applied to perturb the positions of the optical components during optimization in the Zemax model.

In the analysis, we have considered the non-uniform illumination of the pupil plane[@apod1; @apod2]. The telescope beam is picked up by a circular fiber and the starlight is fed to the spectrograph. To simulate this non-uniform illumination, we have used a Gaussian apodization factor 1 in Zemax (fall in the intensity of 13% of the peak value). Apodization factor 1 was considered based on the analysis of beam exit from the fiber (exit pupil). The optimization of the *merit function* is obtained by perturbing the three rotational degrees of freedom $(X, Y, Z)$ of the selected optical components.

To reduce the computational load/intensity, in the initial case of a matching model with the observations, the wavelength selection is done such that sampling across the detector is maintained. Six Echelle orders are selected across the detector with well-sampled six wavelengths from each order across the FSR of each order. Figure \[fig:select\] shows the wavelengths marked in red 