to input optics (f/3 to f/5 converter) that is used to focus light on the slit. The light from the slit is forwarded to f/5 collimator of focal length 755mm. The collimated light is directed to the cross-dispersing prism as shown in Figure \[fig:zmx\_model\]. The primary dispersion happens using Echelle grating in the horizontal direction. The reflective echelle grating redirects the high dispersed beam onto the cross-dispersing prism for the dispersion in the vertical direction. In the return path, the collimator acts as a camera and focuses the beam on to a 4K$\times$4K CCD detector. The Echelle grating is operated in Littrow mode with a blaze angle of ${\theta}$[B]{}= $70\,^{\circ}$.

[c]{}

Grating position uncertainty
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The Echelle spectra (4000–10,000Å) is recorded on a 4k$\times$4k CCD detector as shown in Figure \[fig:grating\_gaps\] [@gratingmove]. At nominal setting, the spectral image overfills the detector plane. Certain wavelengths on either side of each Echelle order, fall outside the detector boundaries as shown in Figure \[fig:grating\_gaps\]. For specific science cases, the missing wavelengths at the edges are usually accessed by rotating the Echelle grating. The movement of the grating introduces uncertainties that invariably impacts the position repeatability, thus severely compromising the science that needs high 