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.

The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) achieved first light on the W. M. Keck Observatory's Keck I telescope on 4 April 2012 and quickly became the most popular Keck I instrument. One of the primary reasons for the instrument's popularity is that it uses a configurable slitmask unit developed by the Centre Suisse d'Electronique et Microtechnique (CSEM SA) to isolate the light from up to 46 objects simultaneously. In collaboration with the instrument development team and CSEM engineers, the Keck observatory staff present how MOSFIRE is successfully used, and we identify what contributed to routine and trouble free nighttime operations.