Final design and choices for EUCLID NISP grism

ESA Euclid mission is designed to map the geometry of the dark Universe. The NISP (Near Infrared Spectro- Photometer) is one of its two instruments dedicated to NIR with two main observing modes: the photometric mode and the spectroscopic mode, for the acquisition of slitless dispersed images using four low resolution grisms: three "red" grisms for 1250-1850nm, and one "blue" grism for 920-1300nm. The NISP grisms are complex optical components that combine four main functions: a grism done by the grating on the prism hypotenuse, a spectral filter done by a multilayer filter deposited on the first surface of the prism, a focus function done by a curved surface and a spectral wavefront correction done by the grating with curved grooves. This specific grating is made thanks to a new technic developed with SILIOS Technologies to manufacture a resin-free grating. The optical component is glued onto a mechanical ring, designed to survive to 60g DLL and to keep optical performance at 130K. The design and manufacturing of these components represent an important challenge to obtain the best performances with very constraining requirements. We will present the performance obtained on scale-1 prototypes of the filter, the grating and the mount manufactured to validate the final design choices and used to make the necessary trade-off during the development phase. All the prototypes have shown very good optical performances and have withstood vibrations and vacuum cryogenic tests that confirm the feasibility of NISP grisms and prepare the next phase for the procurement and tests of NISP grism flight models.