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S. C. Barden, C. Harmer, T. Armandroff, A. Dey, B. Jannuzi (N.O.A.O.)
NOAO is currently conceptualizing a wide-field, imaging spectrograph for their 4-meter telescopes. Implementation of new technologies will enhance its performance. Design goals include: 1) 40 arc-minute diameter field of view; 2) 45% peak system efficiency (telescope, instrument, and detector); 3) resolving powers from 2000 to 10000 and possibly up to 30000; 4) spectral coverage of nearly 1 octave at R=5000 onto an 8K CCD mosaic; 5) multi-slit, long-slit, and lenslet IFU capability; 6) operational range from 365 nm to 1700 nm with either CCD or IR detectors.
The instrument is based on volume-phase holographic (VPH) gratings which provide high efficiency and simplify the spectrograph design by always working in a Littrow configuration (ie. no anamorphic magnification). We compare the performance of a VPH based spectrograph to that of a classical, surface-relief grating spectrograph. To best utilize VPH gratings, the camera-collimator angle should be adjustable. This presents a challenge in mechanical design, but provides significant versatility in the instrument's functionality.
To best optimize the spectrograph, a beam diameter of about 200 mm is desired. The design must fit it within the space constraints of the existing 4-meter telescopes. The highest performance option includes a new f/6 secondary moving the focal surface forward of the primary mirror. The grating and camera reside in the Cassegrain cage, allowing sufficient room for the entire spectrograph. A scaled back design under study decreases the beam diameter to 150 mm and uses the currently existing f/8 secondary. This option may result in loss of resolving power and field of view. Folding optics may also be needed to fit the entire instrument within the Cassegrain cage. Optical schematics for both options will be shown.
We also present an exploration of the instrument's potential performance in some key science applications related to stars in local galaxies and extragalactic studies in both the optical and non-thermal IR spectral bands.
For information on VPH gratings, refer to the presentation by Arns et al.
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