... Up: Instrument Modeling . ... The near-infrared multi-object spectrograph NIRSpec is one of the three science instruments onboard the future James Webb Space Telescope. ... In this context and as part of the EADS NIRSpec study funded by ESA, we have developed an end-to-end Fourier-optics model of the instrument, which takes into account both geometrical aberrations and diffraction. ... Using this model we have assessed the effect of diffraction and optical aberrations on NIRSpec's optical...