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S.H. Moseley, C.W. Bowers (NASA/GSFC), R.K. Fettig (Raytheon ITSS/NASA/GSFC), J.P. Gardner, R.A. Kimble, J. Kuhn (NASA/GSFC), A.S. Kutyrev, E.M. Malumuth (Raytheon ITSS/NASA/GSFC), E.E. Mentzell, D.B. Mott, D.S. Schwinger (NASA/GSFC), H.I. Teplitz (NOAO), R.P. Wesenberg, B.E. Woodgate (NASA/GSFC)
We present a concept for a high-efficiency, wide-band, multi-object near-infrared spectrograph for the NGST. The spectrograph will simultaneously cover the wavelength range of 0.6-5 microns with a 4K by 4K mosaic array of InSb detectors and will offer low resolution spectroscopy (R=25 - 100) of thousands of objects simultaneously, higher resolution spectroscopy (R~1500) of about a thousand objects simultaneously, and an imaging mode for initial target acquisition and configuration of a high transmission entrance aperture mask. The key component of the spectrometer is the entrance aperture mask, a programmable microshutter array. We have demonstrated basic mechanical and electrostatic cell operation of a 3 by 3 array. We are currently in the process of scaling it up to large arrays. The first working large size arrays are expected to be tested by the end of this year.
Our spectrograph conceptual design includes two principal features to address the demanding scientific requirements of the NGST, whose primary targets will typically be highly redshifted and extremely faint: 1) our lowest resolution spectrograph design is based on refractive dispersing elements (prisms) that permit us to obtain spectra over the entire near-IR range of 0.6-5 microns ( 3 octaves) simultaneously for all objects observed, rather than the 1 octave of coverage (to avoid order confusion) available with a typical grating spectrograph. The use of prisms as the only transmissive or dispersing elements, provides both substantially higher throughput than grating/order sorter combinations as well as the significant multiplexing advantage of complete wavelength coverage in one exposure, 2) for higher resolutions than those available to prisms, we evaluate the trade-off between using 3 or first order gratings for R~1500 and a prism cross-dispersed echelle for R~5000.
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