In its normal mode of operation, UES is used with a 1 arcsec wide entrance
slit, producing a resolution element of 43 microns at the detector. This
corresponds to a resolving power
at all wavelengths. With currently available CCDs this
resolution element is just sampled adequately with two detector pixels; the
IPCS allows better sampling of the instrumental profile with four detector
pixels. The optical quality of the spectrograph can deliver 
with a sufficiently narrow slit (0.5 arcsec). While the WHT seeing is often
well matched to such narrow slits, only the IPCS with its 10 micron pixels can
at present take advantage of the resulting resolving power.