Geometrical losses, mainly caused by focal ratio degradation, give a mean
efficiency of around 25% of the value when observing directly through the
slit. Transmission losses in the 2.5 metre fibre length are small, except
shortward of 4200 
; between 7200 and 7500 
; and
longward of 8700 
. However acquisition is critical, and much
larger losses can be produced if the astrometry is not better than
0.5 arcsec for any reason. Some on-sky measurements, taken in 1.3 arcsec
seeing, show that the total sensitivity of the system, is 1 photon/s/
at 4500 
for a star of B=15 with the TEK1 CCD on the blue arm;
and 1.7 photon/s/
at 7000 
for a star of R=15 with the
EEV3 CCD on the red arm. There is a fibre-to-fibre scatter of 30%
in each of these values.
The efficiency is very sensitive to the acquisition procedure, a shift of 1 arcsec from the best position loses about 55% of the light, and a shift of 11 arcmin in the Cassegrain rotator position angle loses about 50% of the light, although this latter figure depends upon the radial distribution of objects.