While the spectrograph is designed to be efficient from 3000 to 10000
Å, only a fraction of the full range is passed by the camera field of 
mm. The wavelength interval encompassed by these dimensions is a
function of wavelength; representative values are 
Å at 4000
Å and 2700 Å at 6500 Å. Even these intervals will be covered
fully (i.e. with no gaps between successive echelle orders) only for some
combinations of echelle grating and detector. For example, with the 
micron pixel Tektronix CCD and the 31.6 grooves/mm echelle,
continuous spectral coverage is possible only for wavelengths less than
approximately 5750 Å; further into the red two CCD exposures are required to
cover the free spectral range of this grating. With the IPCS and the 79 echelle,
two exposures are needed to encompass the full FSR at all wavelengths where the
IPCS may be used in preference to a CCD (i.e. shortward of 
Å).