On the scientific performance of the Euclid’s Near Infrared Spectrometer and Photometer (NISP) instrument: from launch to early flight operations.

The European Space Agency’s Euclid mission, launched on July 1, 2023, investigates the dark universe by mapping mass distribution across cosmic time through weak gravitational lensing and galaxy clustering. The Near Infrared Spectrometer and Photometer (NISP) provides near-infrared photometric and spectroscopic observations essential for redshift-based cosmological distance measurements. This thesis assesses the scientific performance of the NISP signal detection chain through theoretical modelling, instrument calibration and optimisation, and analysis of early in-flight data. The work is structured into two main research branches. The first branch addresses commissioning and performance verification of the NISP detectors. Commissioning established optimal operating conditions and confirmed consistency with pre-launch reference parameters. The main contribution concerned optimisation of the detectors’ dynamic range to prevent saturation and mitigate non-linearity effects, thereby preserving flux calibration accuracy and reliable redshift estimation. Performance verification involved in-flight assessment of detector stability, optical alignment, throughput, and flux and wavelength calibrations. A detailed evaluation of detector baseline levels and noise components was performed using dedicated calibration sequences under nominal conditions. Results show close agreement with pre-launch characterisation, confirming nominal operation and signal-to-noise ratios consistent with mission requirements. The second branch analyses bias sources affecting NISP signal estimation. Two components were identified: one intrinsic to the mathematical formulation of the estimator, and one due to onboard processing simplifications. Analytical modelling, Monte Carlo simulations, and early in-flight data demonstrate that the combined effect produces a measurable but negligible bias (<1%) under typical zodiacal background levels (1 e⁻ s⁻¹). The resulting systematic uncertainties remain below mission requirements: <0.01 mag in photometric zero-points and <4 × 10⁻¹⁹ erg cm⁻² s⁻¹ Å⁻¹ in spectroscopic sensitivity. Additional studies reveal a correlation between detector response variations and solar proton flux. Overall, this work provides a comprehensive assessment of NISP performance and establishes a framework for continuous in-flight monitoring.