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 3m, 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.
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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.