All spectrograph mechanisms are under software control, and user access to the coudé room is neither necessary nor desirable. Pressure changes affect the refractive index of air in the room, and these will be evident in the data. The user is not permitted to enter the main spectrograph room unless prior authorization from the support staff has been obtained, and then clean room clothing must be worn. Do not switch on the fluorescent lights in the coudé room during a run, or even 12 hours before the observations are due to start, as the walls fluoresce and the dark count will be seriously increased. Use only tungsten lamps while in the room.
The coudé location of the spectrograph provides great stability.
For UCLES, random shifts over one hour are 
pixels (peak to peak),
and slow drifts are consistent with refractive index changes due to pressure
variations.
Even when grating settings are changed many times during a night, it is
possible to return to a previous setting with an RMS accuracy of 4 
m
(0.2 pixels).
Repeat exposures with UHRF at R = 1.0M using a stabilized laser have shown the
instrument to be stable to 
0.2 pixels over 30 minutes. To maintain
resolution on a star, limit integration times to around 30 minutes to avoid
smearing by changes in the heliocentric correction of the star. Although the
coudé optical bench floats on airbags to vibrationally isolate it from the
hammerhead, which is itself separate from the building, arcs should not be
taken while the telescope and/or dome are slewing. It is advisable to bracket
stellar exposures by arc calibrations.
The CCD dewars can be filled from a 25 l storage dewar by activating a switch in the coude ante--room, so it is no longer necessary to disturb the coude room to fill the CCDs. The 25 l dewar must be replaced every three days.