iOPTRON - Astronomy Technology Today

A HOT TOPIC - ACTIVE COOLING OF A PRIMARY TELESCOPE MIRROR
Figure 9: Graphic initially created for cooling the 36-
inch mirror with recirculated pre-cooled air.
a recently dew-rinsed mirror.
The air-flow diagram (Figure 9) was
color coded to show which direction the
cool air would enter the mirror box. In case
it was needed, I incorporated an old hair
dryer in the center of the new mirror box
cover (using a 2-inch 180-degree elbow
pipe, so the hair dryer would also recirculate
Figure 10: Original prototype: “Cool Breeze.”
the air it warmed) should the dew point be
expected to rise after Sunrise of a night of
observing. At an elevation of 7000 feet,
winters can get cold, so the recirculating
path for continually heating the same volume
of air to warm the glass made sense just
like the cooling flow diagram.
In this system, I’d monitor the glass
temperature of a thick and a thin spot on
the mirror, use two sensors for monitoring
the air temperature in the mirror box, plus
monitor humidity and dew point of the air
in the mirror box. I had an extra thermocouple
display, so I’d also monitor the outgoing
air of the air cooler.
Nicknamed “Cool Breeze,” the prototype
used an old mini-fridge compressor/coil
cooling system. Repurposing other
items like some old plywood, adding a couple
fans and cutting up some 1/2-inch aluminum-clad
insulation sheeting, I soon had
a multi-pass recirculating air system. Initial
tests with it showed I could easily reach 5°C
(Figure 10).
I used 4-inch insulated hose attached
to two 4-inch toilet flanges as hose connections
on the box end and two dryer vents
on the new mirror box cover. Cold air
would be ported into the top port, bathing
56 Astronomy TECHNOLOGY TODAY