Pre-launch optical verification of the Euclid NISP instrument and comparison with simulated images

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)$\le 0.3$ 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.