ESI is a spectrograph and imager that is used at the cassegrain focus of the Keck II telescope. ESI has three modes: echelle, low-dispersion with potential multi-slit capability and direct imaging.

There are three filter wheels stacked one atop the next. Only the top wheel is within the focal adjustment range of the collimator, hence slits can only be placed in this wheel. Each wheel has five slots into which filters or slit plates can be inserted. Generally one position in each wheel is kept open. Multiple slits, slit plates, or filters can be placed into each slot. For example one might place three 2 x 2 inch filters in a single holder. The holder would fit into one of the five "slots," while each filter would have a separately defined "position.

The echellette slits are an example of a slot with nine positions, corresponding to the six 20 arcsec long echellette slits, a "multihole" slit, and two pinholes. A "decker" is used in the middle wheel to allow only the light from one of the slits to pass into the spectrograph. The multihole slit consists of a set of eight 0.5-arcsec pinholes separated by 2.695 +/- 0.020 arcsec along the direction of the echelle slits. The pinholes are 0.3- and 0.5-arcsec diameter, and can be used in either echelle or low-D modes.

The ESI configuration, calibration lamps, and CCD are all controlled through a powerful GUI. The display of images and quick-look capability is with a new, Tcl/Tk-based FIGDISP interface.

The default mode for ESI operation includes an open-loop correction for instrument flexure. With flexure compensation on, flexure is less than 0.8 pixels peak-to-peak over the entire range of telescope elevation and rotator position angle, and for typical exposures is only a fraction of this amount. The flexure correction has been shown to be remarkably stable over a period of at least 1-1/2 years.