Although the UCLES slit closes down to 10 
m, the accuracy of the encoders
does not make it possible for this slit to be used to calibrate the resolution
of the spectrograph for very narrow slits. A calibrated Hilger slit is stored in
the adjacent darkroom, and locates on the same kinematic mount as the slicer.
The Hilger slit requires slit angle SA 2000 (ADU) to be horizontal.