The position angle of the slit (and spectrograph) is controlled by the rotator, whose control box is located above Side 1 of the spectrograph, when at its default position angle (E--W). Turn on the power switch on the end of the control box, a second one on the front of the box, and lastly, the LED display. The position angle is changed by setting the forward/reverse switch, and turning up the motor's rheostat knob. Do not rotate the spectrograph too quickly --- it can easily run away to a very awkward and potentially dangerous position.
Unfortunately, the displayed position angle is actually its supplement
plus 
. In other words, the rotator should be set to 183
- PA for positive PA, and -PA + 3 for negative PA.
This 
offset can vary somewhat from night to night, and
by as much as a few degrees between observing runs. A precision fiducial
for the normal E--W (PA = 90
) orientation
is provided in the form of a reference mark inscribed on
the instrument rotator hub, which should be aligned with the mechanical
pointer. Do not set the rotator to angles whose displayed
values are beyond the limits written on the rotator control box or cables,
etc. will be pulled loose!
After setting the rotator, you may wish to turn off the LED display while taking exposures.