iOPTRON - Astronomy Technology Today

A HOT TOPIC - ACTIVE COOLING OF A PRIMARY TELESCOPE MIRROR
Figure 17: The active cooling system is shown in use, pre-cooling the 36-inch primary mirror
to the desired temperature.
become a more automated system. But, for
now, a manual system is fine.
Regarding dew point, the NOAA
website has very detailed hourly forecasts
for anywhere in the U.S. (http://www.
wrh.noaa.gov/psr/). Type in your location
and press enter, then click on the map as
close as you can to your observing spot. Towards
the lower right of the page that
opens, you’ll find the “Hourly Weather
Graph” (Figure 13). Click on it, and it will
enlarge.
After using this site for the last year, I
find it rather accurate. The winds die down
about when it forecasts, and temperatures
and dew points match too. For this mirrorcooling
system, knowing dew point is critical.
You don’t want to pre-cool the mirror
to the temperature predicted for 11 p.m.
when the present dew point happens to be
above that temperature. I prefer to keep the
pre-cool set-point at least 10°F above the
dew point. Anything closer, and it may be
raining soon anyhow. At my observing site
in northern Arizona, which is considered an
“elevated desert” at 7000 feet (with cactus
too!), I’ve seen 9-percent humidity and -
20°F dew points when the actual temperature
is 70°F and expected 11-p.m.
temperature was 40°F, but during our
Monsoon Season in July and August, humidity
is a problem. As with any telescope,
“first, do no harm.”
A recent addition is the use of 4-inch
cam-lock quick-disconnects for the hoses
(Figure 14). These give the Cool Breeze II
system that “NASA” look, but really speed
things up by allowing disconnection of the
hoses from the mirror cover when removing
it from the scope. Notice the color coding
rings on connections and hoses (Figure 15).
The cold and warm return ports have
been reversed from the original configuration,
with the cold-air inlet now porting to
the “bottom” of the mirror box. This is not
a problem for the system as fans mounted
in the mirror box move the air around the
primary, but in this configuration the spot
on the mirror box wall to which the cold-air
inlet points is now below the mirror. Previously,
the cold-air inlet was on top, bathing
the mirror from top to bottom and also
forming a frost patch on the mirror box wall
right above the mirror. Um, yeah, it melted,
and during the fourth use of the system two
long drips on the mirror were the result.
With the switch of the hoses, that won’t
happen again. With the cold-air inlet on
bottom side of the mirror cover, any frost
patch forms below the mirror, not above it
Figure 16.
The system works great – better than I
expected, actually. The primary mirror cools
inside the telescope while still covered by
the roll-off observatory, waiting for Sunset,
and the mirror is at temperature, ready for
observing, when we are. In Figure 17, the
Sun has set, and the observatory building
has been rolled away from the telescope,
clearing the 32-foot diameter of the block
patio on which I roll the observing
ladder from position to position. When
not attached to the telescope, the mirror
cover is placed on the wheeled cart that
carries the air conditioner, and the hoses are
draped on top of it. Then, I simply roll
the whole assembly to a portion of patio
block that’s not within the turning radius of
the telescope.
Professional telescopes use active cooling,
why not yours?
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Astronomy TECHNOLOGY TODAY 59