A useful figure of merit for UES is that at 4000 Å an object with
AB
= 17.0 (defined as usual above the atmosphere), observed through a
wide slit (no light losses at the slit), will produce 1
electron/s/Å when observed with the Tektronix CCD. This figure can be
converted to a more useful count rate by adopting: i) 67% transmission
for the atmosphere at ZD = 30 degrees; ii) 65% transmission through a 1
arcsec wide slit in 1 arcsec FWHM seeing; iii) a pixel size of 0.043 Å\
in the spectral direction. With these parameters, an object with AB
=
17.0 will give a count rate of 
electrons per wavelength bin per
1000 s integration, adding all the counts along the slit. From these guidelines,
approximate exposure times for objects of interest can be estimated. Obviously,
the final S/N will include contributions from sky subtraction and systematic
sources of noise such as read-out, dark current and cosmic rays.
The count rates obtained with the IPCS at 4000 Å are approximately 6 times lower than the above values.
Table 
in Appendix
lists values of AB magnitude giving 1 electron/s/Å\
for the combination WHT+UES+Tektronix CCD over the full optical range, from
3500 to 8500 Å (the estimated errors are 
mag).