We report on the design, development and commissioning of an Integral Field Unit (IFU) that has been built for the Echellette Spectrograph and Imager (ESI) at the W.M. Keck Observatory. This image slicer--based IFU, which was commissioned in the spring of 2004 covers a contiguous field of 5.65 x 4.0 arcseconds in 5 slices that are 1.13 arcseconds wide. The IFU passes a spectral range of 0.39-1.1 um with a throughput of between 45 % and 60 % depending on wavelength and field position. The IFU head resides in an ESI slit mask holder, so that ESI may be converted to the IFU mode remotely by selecting the appropriate slit mask position. This IFU is the first of a family of designs for the spectrograph, providing a range of field-coverages and dispersions. In addition, we present the first-light science imaging and spectroscopic observations of RXJ1131-123, a low-redshift, lensed quasar. These observations show the 4 spectra of the lens and lensed-images captured in a single pointing.
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