The Integral Field Unit on the James Webb Space Telescope's Near-Infrared Spectrograph

The James Webb Space Telescope (JWST) mission is a collaborative project between the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA) and the Canadian Space Agency (CSA). On board JWST, the NIRSpec instrument developed by EADS Astrium for ESA is a near-infrared spectrograph covering the 0.6-5.0 μm domain at spectral resolutions of 100, 1000 and 2700. NIRSpec will be primarily operated as a multi-object spectrograph (MOS) but it also includes an integral field unit (IFU) allowing to continuously sample a 3"x3" field of view with 0.1". This IFU, based on the "advanced" image slicer concept, is a very compact athermal unit made of aluminium. The slicer, pupil and slit mirror arrays are each machined from monolithic blocks using diamond-turning techniques. This paper presents the integral-field spectroscopy (IFS) mode of NIRSpec. After a brief presentation of its main scientific objectives and expected performance, we will focus on its implementation in NIRSpec and the design of the IFU to the diamond machining techniques applied for manufacturing. We will finish with a presentation of the status of the development and of recent results from mirror machining and metrology.