Euclid warm testing of the near-infrared optical assembly using a unique combination of CGH interferometry and tactile precision measurements

The ESA Cosmic Vision "Euclid" mission will conduct a 6-years long survey of 15,000 square degrees of the sky to a look-back time of 10 billion years, with the aim of characterizing the matter-energy content of the Universe and to better understand the dark energy responsible for the acceleration of its expansion. The Euclid payload consists of a wide field 1.2m aperture telescope equipped with two instruments that simultaneously observe patches of < 0.5 square degree on the sky: the visible light camera (VIS, and the near-infrared spectrometer and photometer (NISP). These two instruments are separated by a dichroic plate splitting the beams around a wavelength of 920 nm. The NISP large field of view (FoV) - larger than the full moon disk - together with high demands on the optical performance and strong requirements on in-flight stability, lead to very tight and challenging specifications on the alignment and positioning of the NISP optical assembly (NI-OA). This required an extensive tolerance analysis at system level during the design phase. The hardware is now completed and went through all optical tests at assembly level. In this paper we present the strategy and results of the warm optical test. In this test, we measured the length of the optical axis behind the NI-OA - or back focal distance (BFD) - using a novel combination of computer generated hologram (CGH) and a coordinate measuring machine (CMM). The agreement between the predicted and measured BFD values is excellent, within 1 μm. In addition, we measured the system wave-front error under warm conditions in double path and found diffraction limited performance on- and off-axis all over the field of view. These warm tests validated the anticipated performance of the NI-OA and allowed us to prepare the time-consuming and risky cryogenic tests with a high level of confidence.