Flight model performance of the integral field unit for the James Webb Space Telescope's near-infrared spectrograph
2010
The Near Infrared Spectrograph (NIRSpec) developed by EADS Astrium GmbH for the European Space Agency (ESA)
is a spectrograph covering the 0.6-5.0 μm waveband to fly on the James Webb Space Telescope (JWST). NIRSpec will
be primarily operated as a multi-object spectrograph but also includes an integral field unit (IFU) allowing a 3×3 arcsec
field of view to be sampled continuously with 0.1 arcsec spatial resolution. The IFU, based on an advanced image slicer
concept, is a very compact athermal unit made of aluminium. It contains three 30-element monolithic mirror arrays
forming slicer, pupil and slit mirrors, and single-surface image relay and plane fold mirrors, produced using 5-axis
diamond-machining techniques. Many of the mirrors have complex surfaces like toric sections with 3rd-order corrections
in order to achieve the required performance within a small allowed volume, and could only have been fabricated with
the most advanced free-form machining. The mechanical design accommodates the differential expansion between the
aluminium IFU and its titanium parent assembly across a 250K drop to operating temperature using an isostatic
mounting system. This paper presents the development of the IFU from the design and diamond-machining techniques
to the optical and cryogenic testing of the assembled flight model unit.
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