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.

• In echelle mode there are ten orders on the CCD with complete wavelength coverage from 3900 Å to 10900 Å at a dispersion ranging from 0.16 Å/pixel (order 15) to 0.30 Å/pixel (order 6). The velocity dispersion is a nearly constant 11.5 km/s/pixel in all orders. Slits are 20 arcseconds in length and are available with widths of 0.3, 0.5, 0.75, 1.0, 1.25 and 6.0 arcseconds. A one arcsecond wide slit projects to 6.5 pixels (0.154 arcsec/pixel). Some experiments have been done using echelle-mode multislit masks, but The Observatory does not have the infrastructure in place to support this as a general rule.

• Using only the prisms for dispersion, ESI is a low-resolution spectrograph with multi-slit capability and R ranging (approximately linearly) from 4800 per pixel at 3900 Å to 1050 per pixel at 8000 Å. The slit length is ~8 arcminutes and 0.3, 0.5, 0.75, 1.0, 1.25, and 6.0 arcsecond wide slits are available. As of 2001 June low-D multislit capability has not been implemented.

• With the prisms out of the beam and an imaging mirror in place, ESI delivers an 8 x 2 arcminute direct field. The facility BVRI filters originally delivered are undersized with a field of 3.5 x 2.5 arcminutes. R. Ellis has kindly donated a full-field R filter which is also available. During commissioning, images as good as 0.34" FWHM were obtained.

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.