In terms of signal alone (counts/s/Å), FOS-2 is more efficient than ISIS in the red. This is partially offset by the greater sensitivity of the Tektronix CCDs used on ISIS. Detailed differences can be seen by comparing the Oke `AB' magnitudes which give 1count/s/Å for each, from Tables and in Appendix . However, the signal--to--noise ratio obtained in an astronomical spectrum often depends also on the spectral resolution achieved. For example, even with the lowest--dispersion ISIS grating in the red (R158R), the spectral resolution obtained is about a factor 4 better than that of FOS, and sky--subtraction longwards of 7000Å can often be much more accurate with the better resolution. As FOS is optimised for red light, ISIS probably has as good a throughput at blue and violet wavelengths. If this is then coupled with the higher quantum efficiency of the Tektronix CCD then it is likely that the most efficient way to proceed is to use FOS-2 for first order in the red, a dichroic with a crossover wavelength of around 5000 Å, and a low dispersion grating on the ISIS blue arm with the Tektronix CCD. Alternatively Tektronix CCDs can be used on both arms of ISIS, although these suffer from fringing at red wavelengths.