This paper presents the mechanical slit mask designed for the Near Infrared Spectrograph of the James Webb Space Telescope. This mechanism is designed to function at a cryogenic temperature of 30K, in vacuum. The reconfigurable mask allows to form 24 optical slits in a 137 x 137 mm 2 field of view. The slit length is fixed (4.8 mm) and their width can range from 50 µm to 137 mm. The slit positioning accuracy is ± 5 µm and the slit width accuracy is ± 8 µm. The paper concentrates on the working principle of the mechanism which is based on an improved "inch-worm" stepping motion of 48 masking bars forming the optical curtain. Voice coil actuators are used to drive the various clutches and the principal mobile stage. Ratchets which engage in the teeth of a rack machined on the bars allow to cancel the accumulation of motion errors as steps succeed one another. The design makes significant use flexure structures. Cryogenic performance and life tests have been performed successfully on subassemblies of the mechanism.

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An innovative design of a Large Angle Flexure Pivot (LAFP) is described. It combines the advantages of flexure mechanisms while surpassing one of their few flaws, small displacement strokes. The LAFP design exceeds these angular limitations to reach a deflection of 180° (±90°). The centre shifts laterally by less than ±35 μm throughout the full rotation range. The LAFP is meant to be mounted in pairs, coaxially and with the payload between them. The intended application of the LAFP is to angularly guide an optical component in a space environment for future science missions operating in a cryogenic environment. A dedicated performance test bench was developed and manufactured to test the pivot characteristics notably the lateral shift using Eddy current sensors. The test bench incorporates a representative dummy payload for mass and inertia. Extensive FEM analysis has been performed to validate the design at component level and further analysis with the pivots mounted with a representative payload on a test bench for random vibration, shock and thermal cycling environment. The second test bench for the vibration and shock tests has been manufactured incorporating a simplified launch locking device. The performance tests have confirmed a lateral shift of less than ±35 μm over an angular range of ±90°. The pivots have been successfully tested and survived vibration loads for high level sine at 24 g and random vibration at 12 grms in all three directions.

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In this paper, a validated procedure to replace the protective front-window of a commercial digital micro-mirror device (DMD) from Texas Instruments (TI) to allow its use over a large spectral range is presented. This reworking process was required since the original window employed for encapsulation is made from glass with an anti-reflection coating designed for a specific spectral range, incompatible with the required large spectral range of the demonstrator under development.In addition, a characterisation of the DMD performance in terms of spectral transmission, as well as switching time and pointing stability is presented.The motivation behind this study lies within the development of a novel instrument in the frame of the EU H2020 funded SURPRISE project. The project aims at developing a super-resolved compressive imager operating in the visible-near infrared (VNIR) and mid-wave infrared (MWIR) spectral ranges for space applications, especially targeting Earth Observation. The instrument concept is based on the use of a spatial light modulator (SLM), in this case a digital micromirror device (DMD), as a core element of its architecture to enable data acquisition and compression in single step based on the compressive sensing principle. Even though one of the long-term objectives is to develop a European-based SLM solution, a commercial SLM component has been selected for the demonstrator This allows reducing the development cost and initiating the development of the demonstrator in parallel to the development of a European-based solution.