| therefore the cooling of the
primary, undertaken to prevent the generation of local seeing which
is currently observed in conditions of very low wind (See Figure 1).
The system produces a flow of temperature controlled air across the
primary, sufficient both to maintain the mirror temperature at night-time
temperature through the day and controllable with respect to ambient
temperature through the night. Figure 2 shows the flow outlets, set
into the primary mirror plug.
Initial tests indicate that the cooling will have a highly beneficial
effect: Figure 3 shows the results from a series of tests carried
out in November 2000. In this test, the air temperature was controlled
by altering chiller parameters, and seeing was measured by monitor
|
ing the “z-rms”;
this is the fluctuation of the visual wavefront in the z (along telescope
tube) direction and is a pure function of seeing; UKIRT employs an
empirical calibration of this to predict the infrared seeing (e.g.
for flexing observing runs on this basis). The actual temperature
control was achieved by a highly-sophisticated carbon-based control
unit (Erik Starman!). While the data show that the experiment was
clearly successful at controlling the mirror seeing, the released
system will revert to more conventional computer control, servoing
the flow temperature to that of the ambient dome air. A number of
unknowns remain, both mechanical and physical, but the system will
be completed this year. The next set of tests will take place in April.
|
Figure 2.
Cooling air outlets in the centre of the primary mirror.
|
