Euclid. IV. The NISP Calibration Unit

The near-infrared calibration unit (NI-CU) on board NISP is the first astronomical calibration lamp based on LED to be operated in space. is a mission in ESA's Cosmic Vision 2015--2025 framework to explore the dark universe and provide a next-level characterisation of the nature of gravitation, dark matter, and dark energy. Calibrating photometric and spectrometric measurements of galaxies to better than 1.5 accuracy in a survey homogeneously mapping sim ,14,000,deg$^2$ of extragalactic sky requires a very detailed characterisation of NIR detector properties as well as constant monitoring of them in flight.

To cover two of the main contributions -- relative pixel-to-pixel sensitivity and non-linearity characteristics -- and to support other calibration activities, NI-CU was designed to provide spatially approximately homogeneous ($<$,12 variations) and temporally stable illumination (0.1--0.2 over 1200,s) over the NISP detector plane with minimal power consumption and energy dissipation. NI-CU covers the spectral range sim ,nm -- at cryo-operating temperature -- at five fixed independent wavelengths to capture wavelength-dependent behaviour of the detectors, with fluence over a dynamic range of gtrsim ,100 from sim $.

For this functionality, NI-CU is based on LED . We describe the rationale behind the decision and design process, the challenges in sourcing the right LED and the qualification process and lessons learned. We also provide a description of the completed NI-CU, its capabilities, and performance as well as its limits. NI-CU has been integrated into NISP and the satellite, and since launch in July 2023, it has started supporting survey operations.