We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope. NIRSpec is designed to be capable of carrying out low-resolution (R = 30− 330) prism spectroscopy over the wavelength range 0.6 − 5.3µm and higher resolution (R = 500− 1340 or R = 1320− 3600) grating spectroscopy over 0.7 − 5.2µm, both in singleobject mode employing any one of five fixed slits, or a 3.1×3.2 arcsec 2 integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6×3.4 arcmin 2 field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.

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The NIRSpec instrument on the James Webb Space Telescope (JWST) is a multi-object spectrograph capable of measuring the near infrared spectrum of at least 100 objects simultaneously at various spectral resolutions. It operates under cryogenic conditions (T∼ 35 K). NIRSpec is part of the JWST science instruments suite. Its main purpose is to provide low (R=100), medium (R=1000) and high resolution (R=2700) spectroscopic observations over the wavelength range 0.6 µm -5.0 µm in support of the four JWST science programs. The NIRSpec instrument is being developed by the European Space Agency with EADS Astrium Germany GmbH as the prime contractor The Optical Telescope Element of JWSTThe four JWST science instruments in the Integrated Science Instruments Module (ISIM) pick-off their input beam from the focal surface of the JWST Optical Telescope Element (OTE). The OTE is a "Korsch" type, all-reflective, on-axis, three-mirror anastigmat (TMA), with the addition of a fine steering mirror (FSM) near the exit pupil. As with most on-axis TMA systems, it is used off-axis in field to enable beam clearance after the tertiary mirror. The Field of View (FOV) offset equals -0.13° (or -7.8'). The primary, secondary and tertiary mirrors have conic contours (ellipse, hyperbola, ellipse), while the FSM is a flat. The focal surface is curved, with its radius matched to the FSM to focal surface distance. This feature ensures no defocus when the FSM is operated. The telescope has a primary mirror made out of 18 hexagonal segments with a overall, flat-to-flat diameter of ∼6.6 m. The output f-number is f/20. NIRSpec will assume a maximum Wave-Front Error (WFE) at its optical entrance field plane of 145 nm RMS for all field points, equivalent to a diffraction limited performance at 1.93 µm (Strehl ratio = 0.80). The Optical Design of NIRSpec 2.

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