Both spectrographs are focussed by adjusting the collimator position along the optical path.
UCLES offers a choice of two collimators, coated with aluminium and silver respectively. The software selects the aluminium one for central wavelengths shorter than 4000Å and the silver one at longer central wavelengths, unless overridden. Both are off--axis paraboloids.
The UHRF collimator is a 300 mm diameter fused--silica single element lens.
The UHRF echelle grating is mounted inside the collimator housing. Focus of
UHRF is achieved by moving the entire assembly. Positioning is to within
50 
m over its 450 mm travel.
It has two separate focus motions, one coarse (UFC) and one fine
(UFF). Both are read off the same encoder, so the requested value is the same
independently of which you use to drive the collimator carriage, but UFF has a
limited travel of approximately 150 ADU (encoder units) so you generally need
to use UFC to approach focus and then use UFF to achieve the desired encoder
value. UFF may fail to complete if it is near the end of its travel, or position
poorly if a motion less than 15 ADU is requested; in the latter case move UFF
away and back by 50 ADU or so.
Main commands: (UCLES) CF x
Main commands: (UHRF) UFC n, UFF n