The following information was extracted from the ''ESI: Echelle   
spectrograph and Imager'' user's manual available from the Keck II
web site, at this URL (as of this writing):                       
                                                                  
    http://www2.keck.hawaii.edu/inst/esi/ESI_Manual.html          
                                                                  
Sections of the content of this manual website are reproduced with
the kind permission of Keck Observatory.                          


Keck's Echelle Spectrograph and Imager (ESI) has three modes.  

In the echelle mode, there are ten orders on the CCD with complete wavelength
coverage from 3900 Angstroms to 10900  Angstroms.  The dispersion in Angstroms/pixel
varies from order to order, as does the spatial scale along the slit. The bluest
order (15) has a linear dispersion of ~0.15 Angstroms/pixel and the dispersion
increases moving towards the red (lower order number), inversely by the order number,
up to 0.30 Angstroms/pixel (order 6).  The scale in the dispersion direction is 0.154
arcsec/pixel; 1 arcsec projects to 6.5 pixels.  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).  The free spectral
range of every order listed falls completely on the detector. The bluer orders
contain significant amounts of data outside the free spectral range as well.

The low-resolution mode of ESI uses only the prisms for dispersion, and has
multi-slit capability.  The dispersion varies roughly linearly from 0.8
Angstroms/pixel (60 km/sec/pixel) at 3900  Angstroms, to 11 Angstroms/pixel (300
km/sec/pixel) at 11,000 Angstroms. The slit length is approximately 8 arcminutes, and
0.3, 0.5, 0.75, 1.0, 1.25, and 6.0 arcsecond wide slits are available.  There is no
need for order separation.  Since the dispersion is done by prisms, there are no
higher order spectra as there are with gratings or grisms.

The direct imaging mode of ESI has the prisms out of the beam and an imaging mirror
in place.  In this mode, 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.  The filters have been placed to allow the undersized facility filters to
illuminate part of the CCD which is not affected by bad columns and is also more or
less parallel to the rows and columns of the CCD. The unvignetted field is 2' x 8'
with good images over the full field.  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.