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.