Optical performance of NISP grisms flight models for EUCLID mission

The launch of ESA EUCLID mission is foreseen in 36020. The goal of the mission is to understand the nature of the dark energy and to map the geometry of the dark matter. The EUCLID telescope will be equipped with two instruments working in the visible range (VIS) and in the IR range (NISP) to investigate the distance-redshift relationship and the evolution of cosmic structures. The NISP (Near Infrared Spectro-Photometer) will operate in the near-IR spectral range (0.9-2μm) with two observing modes: the photometric mode for the acquisition of images with broadband filters, and the spectroscopic mode for the acquisition of slitless dispersed images on the detectors. NISP is then using four low resolution grisms to acquire spectroscopic image in different orientations to better distinguish the spectra observed and to cover two spectral ranges: 1250-1850nm range, and 920-1300nm range. Since 2010, Laboratoire d’Astrophysique de Marseille is working on the development and the test of the NISP grisms, that are complex optical components. The grism combines four main optical functions: a grism function done by the grating on the prism hypotenuse, a spectral filter done by a multilayer filter deposited on the first face of the prism, a focus function done by the curved filter face of the prism and a spectral wavefront correction done by the grating which grooves paths are nor parallel, neither straight. The NISP instrument is now entering in the integration phase of the proto flight model of the instrument. Therefore, the NISP grism flight models have been manufactured and delivered to the grism wheel assembly for integration by end of 2017. In this paper, we present the optical performance and characteristics of the four EUCLID NISP grisms flight models that have been developed and manufactured by four different industrial partners then integrated and tested by LAM. We focus on the performance obtained on the optical performance of the component; wavefront error of the components, the spectral transmission and groove profiles. The test results analysis show that the grisms flight models for NISP are well within specifications with an efficiency better than 70% on the spectral bandpass and a wavefront error on surfaces better than 30nm RMS. The results on the component show a good control of the manufacturing and integration process despite the difficulties at the beginning of the project to manufacture these components.