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When the spectrograph is used with the cross-dispersing prisms and the 31.6 lines/mm echelle, the minimum order separation on the detector is 0.49 mm (7.3 arcsec). This limits the slit length to less than about 6 arcsec, otherwise adjacent spectral orders will overlap. This cross-dispersing mode of operation is therefore primarily useful for objects which are close to being point sources. Even for these objects it may not be possible to obtain a good sky subtraction. As discussed in more detail in section 12, improved sky subtraction can be obtained by using the 79.0 echelle, which gives a minimum order separation of 1.24 mm (18.5 arcsec).
It is possible to use the full 5 arcmin length of the slit, in order to carry out long slit spectroscopy of extended objects. However, in this case it is no longer possible to use the cross-dispersers. The wavelength range is limited to one spectral order, and other orders must be suppressed by an appropriate narrowband blocking filter.