NIRSpec IFU prism instrumental FWHM (left) as a function of wavelength. The purple curve shows the pre-flight estimates. The black points are measurements of emission lines from the planetary nebula SMP LMC 58, which have negligible intrinsic line width. The blue curve is the optimal scaling of the pre-flight expectation to match the observed measurements of the planetary nebula, with the shaded region showing the ±1σ uncertainty. The right panel shows the corresponding spectral resolving power R = λ/FWHM.