Integral field unit for the echellette spectrograph and imager at Keck II

We present the design and status report on the development of an Integral Field Unit (IFU) for the Echellette spectrograph and imager (ESI), a recently developed R=13000, Cassegrain spectrograph at Keck II. We have designed a family of IFU’s for the spectrograph, providing a range of field-coverages and dispersions. The optical designs are based on the Advanced Image Slicer concept of Content. We describe the completely monolithic, passive, and modular implementation of this design as an IFU head. Each IFU head resides in an ESI slit mask holder, so that it is completely selectable/deselectable as an observing mode during a nights observing run.