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The Echellette Spectrograph and Imager (ESI) for Keck 2 is a versatile Cassegrain instrument which will take data in 3 independent modes. In the echellette mode, it is a medium- dispersion prism cross-dispersed spectrometer featuring a 20- arcsec slit height, 11.5 km/sec/pixel average resolution and full coverage of the (0.39 to 1.09)-micron spectral range in a single exposure. In the prism-only mode, it is a low- dispersion multi-slit spectrometer which covers a 2.0-arcmin- wide field area with an 8.0-arcmin height perpendicular to dispersion. Prismatic resolution is roughly linear with wavelength, ranging from about 62 km/sec/pixel at 0.39 microns to 285 km/sec/pixel at 0.80 microns. In direct-imaging mode, the aforementioned 16.0 sq arcmin field area is reimaged directly unto the CCD detector at a resolution of 0.153 arcsec/pixel. ESI contains an on-axis reflecting collimator which accommodates an off-axis field of view. Cross dispersion is provided by an Ohara BSL7Y prism used in double-pass, followed by a second prism of the same material used in single-pass. The camera is a 10-element all-spherical Epps lens which services a single flat (2048 by 4096 by 15-micron) CCD. The same camera and detector are used for all 3 operating modes without modification. The ESI mechanical design is based upon the 'space-frame' concept which was used successfully for the Keck telescope(s) mechanical structure(s). This results in large weight reduction relative to more typical Cassegrain spectrographs, with the added expectations of very high stiffness and sub-pixel image stability during long exposures. ESI is funded by a grant from CARA and the project has been under way for about 27 months. Most of the mechanical design work is finished and construction is in progress. Electronics, data reduction and user-interface software are nearing completion. All of the optics (including coatings) have been completed and delivered. A thinned science-grade MIT/Lincoln Laboratory CCD for ESI has also been delivered. It is anticipated the ESI may be operational toward the end of 1998.
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