  To characterise the NISP (Near-Infrared Spectrometer and Photometer)
instrument optical capability before the launch of the \Euclid telescope to
orbit, foreseen in 2023, data analysis of ground-based tests and Monte Carlo
simulations that mimic the expected NISP performance were carried out.
Pre-launch test data were analysed to assess the fulfilment of the mission
specifications in terms of Point Spread Function (PSF), set at EE50(PSF)$\,\leq
0.3$ arcseconds, and with a spectral resolution below 16 angstroms per pixel.
We also provide a first comparison between real images from the ground-based
tests with simulated ones. We confirm the high optical quality of the NISP
instrument, fulfilling the mission specifications in terms of PSF and spectral
dispersion with a good agreement between the different test campaigns. We
validated the PSF and spectral dispersion provided by the NISP simulator, a
crucial aspect to validate the consistency between real and simulated images.
