The UCLES focal modifiers reduce the f/ratio of the
collimator and thereby increase the beam diameter from 170 mm to 220 mm, i.e.
an increase of 37%. This results in an overfilling of the optics, but also
changes the slit projection factors, allowing
the use of a 37% wider and longer slit without reducing the spectral
resolution. This results in a net gain in throughput when the seeing FWHM
exceeds 
.
The extra slit length can make the difference between having well sampled sky
rows or not, especially for the 31.6 lines mm
grating.
UCLES has two focal modifiers, optimized for light bluer and redder than
5500Å. The control program will automatically select the correct one to use,
unless overridden by a specified colour.
UHRF also has a focal modifier; it is fundamental to the UHRF optical path and is automatically selected when UHRF is configured.
Main commands: FM IN/OUT