iOPTRON - Astronomy Technology Today

A HOT TOPIC - ACTIVE COOLING OF A PRIMARY TELESCOPE MIRROR
Figure 15: Color-coding rings were added to the connections and
hoses.
Breeze II delivered -14°C air at its output
port via the internal fan of the air-conditioner
unit. We now had a usable system!
The coldest temperature I’ve measured at
the output is -23°C. It’s not still that cold
Figure 16: Interior view of telescope with mirror cover in place and
cooling-system hoses attached.
when it reaches the mirror box, of course,
with losses in the hoses, nor will the air temperature
inside the mirror box ever get close
to that low (the scope’s not insulated for
that purpose), but the mirror-box walls
keep the mirror bathed in continuously recirculating
cold air for the duration that the
system is on and running.
So, how do you get the air conditioner
to spit out -23°C? Remove its front cover
and find the temperature sensor that’s connected
to the front display – it’s tucked in
there somewhere. Wrap an eyepiece heater
strip around the sensor and tape it securely.
The heater strip fools the sensor into reporting
that it’s 108°F outside, when it’s really
anything but. Then, by adjusting the
air conditioner’s temperature setting its lowest,
65°F in my case, it will happily run the
compressor endlessly, even in winter.
A monitoring station for Cool Breeze
II, containing meters, switches and circuit
breakers, is mounted on a Walnut-veneer
plate attached to the mirror box (Figure
12). It reports the temperatures of both the
thick and the thin sections of the mirror,
along with the delta between them, plus air
temperature in the mirror box at two locations,
as well as the humidity and dew point
inside.
Behind the plate is a relay activated by
the temperature controller. In the prototype,
the fans would turn on or off to maintain
the set-point, while the compressor
stayed running. In the new system, I still
have yet to splice into the air-conditioner
unit’s fan wiring. Once that’s done, it will
58 Astronomy TECHNOLOGY TODAY