NISP’s averaged resolving power for the BGS000 (line), RGS000 (dashed), and RGS180 (dot-dashed) grisms as a function of wavelength, assuming a source size of 0.″5 FWHM. The shaded areas illustrate the variation in the resolving power due to the variation in the PSF and the variation in the dispersion, ∆λ, across the FoV. The hatched region shows the ‘forbidden’ region below the scientific requirements.