Each arm has a slide for an optional cross-dispersing grism, although at present
only the blue cross-disperser has been purchased. The cross-disperser is a
special ruling 100 lines/mm fused silica grism blazed at 4400 Å.
This will be used in
conjunction with a prime disperser which is a Bausch and Lomb 75 lines/mm grating
blazed at 3
m, to provide
complete spectral coverage from 3000 - 5600 Å on an EEV P88300 or Tek 1024
CCD or the IPCS on the blue camera. The cross-dispersed mode will use orders 5 to 10, and
the maximum slit length to avoid order overlap will be 30 arcsec. The spectral
resolving power of the cross-dispersed mode is about 2200.
Figure 
shows the format of the cross-dispersed spectrum superimposed on the outlines
of the IPCS and a TEK 1024 CCD.
[ TIFF ]
Figure: Format
of the output of the blue cross-disperser, with IPCS (solid lines)
and EEV P88300 CCD (dashed lines) outlines superimposed. The solid lines
represent the outline of the IPCS field in a typical format, and the dashed
lines a TEK 1024 chip. The
crosses mark the points at which the grating efficiency drops to half of its
peak value in that order. The central wavelength of each order is marked.