The image scale at the coudé focus is 
mm
, or
712 
m to 1". The slit is demagnified onto the detector
by the factors given in the table; these factors are known to the control
software, and slit dimensions can be given in a variety of units: mm, arcsec, or
detector pixels.
Table: Slit projection factors
The UCLES slit is continuously adjustable from 0.1 to 10 mm in width and
from 2.5 to 40 mm in length.
A slit width of 1" projects to 54 
m on the detector, or 39 
m with
the UCLES focal modifier.
Along the slit length 1" will project to 83 
m on the
detector, or 63 
m with the UCLES focal modifier.
It is necessary to limit the slit length to prevent orders overlapping.
With the 31.6 lines mm
grating the maximum recommended slit lengths
range from 
in the blue (3100 Å)
to 
in the red (7500 Å).
The corresponding values with the 79 lines mm
grating
are 2.5 times greater. When setting the spectrograph with CONFIG,
the keyword SLAUTO will automatically set the slit length to the maximum
value which avoids order overlap and leaves one clear interorder pixel.
Although the slit may be used with UHRF, a width of 40
m = 
is required to give a resolution of 10
. UHRF usually
employs a confocal image slicer (Diego, F., 1993, Applied Optics, 32, 6284).
The entrance aperture is a square hole 
on each side.
The slicer cuts the image into 35 slices extending over 20 mm, matching
the minimum order separation provided by the cross-dispersing gratings.
To provide an interorder region on the detector, the output from the
slicer has been masked down to about 22 slices, so less than the full
entrance hole is transmitted.
The transmission of the slicer and the intensity distribution within each
slice depend both on the seeing and the exact
pointing of the telescope within the 
aperture. Because of the
illumination pattern of the image slicer, UHRF observers may find it helpful
to take flat field exposures through both a long slit and through the slicer,
using the former to determine pixel--to--pixel variations, and the latter to
identify the geometrical aspects of slicer transmission. Twilight sky
exposures are also likely to be helpful.
The UCLES slit and the UHRF slicer are located on a moving assembly which allows
the user to select either one.
Main commands: SLIT SLIT/SLICER; SW x MM/PIX/ARCSEC; SL x MM/PIX/ARCSEC; CONFIG SLAUTO