  The Echelle Spectrograph and Imager (ESI) is a multipurpose instrument which
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 August 29, 1999. The optical performance of the instrument has been measured
using artificial calibration sources and starlight. Measurements of the average
image FWHM in echelle mode are 22 microns (0.22 arcseconds), 16 to 18 microns
(0.16 to 0.18 arcseconds) in broad band imaging mode, and comparable in the
low-dispersion prismatic mode (LDP). Images on the sky, under best seeing
conditions show FWHM sizes of 34 microns (0.34 arcseconds). Maximum
efficiencies are measured to be 30% for echelle and anticipated to be greater
than 38% for low dispersion prismatic mode including atmospheric, telescope and
detector losses. In this paper we describe the instrument and its
specifications. We discuss the testing that led to the above conclusions.
