Simplified schematics of the JWST/NIRSpec FS observation mode. The slit performs a spatial cut of the field of view. Because the target is not spatially resolved, this results in a 1D cut of the target light, diffracted by the instrument. Rays of light are then realigned in a thin beam by the collimator, in order to be properly dispersed. In our reference dataset, the dispersion of the light is made by a prism for observations with a low spectral resolving power and various gratings for observations with a medium spectral resolving power. The dispersed light is acquired by the detector with the spectral dimension in the abscissa and the spatial dimension in the ordinate. Finally, for each wavelength (λ,_k,ℓ)_{k∈1:K,ℓ∈1:L}, the spectral flux is the integral of the spatial PSF for this wavelength.