KECK ECHELLE SPECTROGRAPH AND IMAGER
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  Instrument Overview                                                         
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    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.