ESI, a new Keck observatory echellette spectrograph and imager

The Echellette Spectrograph and Imager (ESI) is a multipurpose instrument that has been delivered by the Instrument Development Laboratory of Lick Observatory for use at the Cassegrain focus of the Keck II telescope. ESI saw first light on 1999 August 29. ESI is a multimode instrument that enables the observer to seamlessly switch between three modes during an observation. The three modes of ESI are an R = 13,000 echellette mode, a low-dispersion prismatic mode, and a direct-imaging mode. ESI contains a unique flexure compensation system that reduces the small instrument flexure to negligible proportions. Long-exposure images on the sky show FWHM spot diameters of 34 mum (0.34) averaged over the entire field of view. These are the best non-adaptive optics images taken in the visible at Keck Observatory to date. Maximum efficiencies are measured to be 28% for the echellette mode and greater than 41% for low-dispersion prismatic mode including atmospheric, telescope, and detector losses. In this paper, we describe the instrument and its development. We also discuss the performance testing and some observational results.