We are proposing to implement an integral field unit in NIRSPEC the Near Infra-Red Spectrograph for the NGST. This unique IFU will cover a 2x2 arcsecboolean AND2 field of view sampled at 0.05" (or 3''x3'' @0.07"). The spectral resolution will go up to 3000. We will present the optical design of this unit and its implementation inside the NIRSPEC instrument. We will also show the results from an analysis of the effects of diffraction and stray-light inside the module, as well as the original method of integration we are using.. This module will be built using the image slicer technology, including 40 slices, each 900mum thick. We are also developing an innovative mechanical structure for this unit including the magnifying optics and the slicer itself, providing a very compact unit. The implementation will have a small impact on the NIRSPEC instrument, with only one to three additional gratings to be implemented in the grating wheel. The limiting magnitude. of this mode should reach AB=24-26 for a point source. This unit would provide, for a marginal cost, a unique opportunity to offer a very powerful 3D spectrograph on NGST. The NIRSPEC instrument is under the ESA responsibility, and this work is being conducted in the context of the Astrium-ESA phase A study to which we are participating.
