Data analysis of the NISP instrument for the Euclid mission: from the first validation tests on ground up to the verification phase after launch

The Euclid telescope, launched from Cape Canaveral on July 1st, 2023, is dedicated to studying dark matter and dark energy from its orbit at the Sun-Earth Lagrangian point L2. It is equipped with two instruments: the visual imager (VIS) and the Near-Infrared Spectrometer and Photometer (NISP). The Euclid Wide Survey (Scaramella et al. 2022) will cover approximately 14 500 deg2 of the extragalactic sky, and will by complemented by the Euclid Deep Survey that will cover about 40 deg2 and reach two magnitudes fainter. Several ground-test campaigns were performed to assess NISP instrument basic functionalities and performances, some highlights will be reviewed in this thesis. The analysis comprises two key aspects: the evaluation of NISP image data acquired during ground-tests and in-flight commissioning, and the verification of instrument performance in spectroscopic mode through the use of pixel-level simulated images. The analysis includes the characterisation of NISP detectors, from instrumental background to point-spread function (PSF) evaluations, confirming that NISP performance is well within the Euclid requirement. We introduce a calibration methodology to estimate the filter wheel assembly position from PSF distortions that have been successfully applied during the commissioning and will continue to be used during the mission for calibration purpose. Additionally, we investigated noise events that occur during image acquisition, with a particular focus on ’snowballs’ and cosmic ray tracks. Finally, we present a spectroscopic pixel-level simulation campaign of the NISP instrument, where spectral energy distributions were generated and processed by the Euclid spectroscopic channel simulator. The analysis revealed a 3.5 sigma NISP red grism spectroscopic detection limit for the continuum, with measurements of H = 19.5 ± 0.2 AB mag for the Euclid Wide Survey and H = 20.8 ± 0.6 AB mag for the Euclid Deep Survey, in agreement with Euclid requirements. Furthermore, an investigation into the impact of galaxy shape on NISP detection capability was carried out, revealing a degradation in the quality of extracted spectra as the size of galaxies’ disks increases.