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Temperature difference (Celsius)
5°
4°
3°
2°
1°
0
0
8˝ Pyrex Mirror
Cool-down Test
No cooling fan
With cooling fan
20 40 60 80 100 120 140
Elapsed time (minutes)
A fan greatly helps minimize the diff erence
between a primary mirror’s surface
temperature and that of the surrounding air.
The data graphed here was gathered by Ohio
ATM Bryan Greer using his own telescope and
a sensitive thermometer on a night when the
ambient air temperature was falling 2°C per
hour, a rate typical of many clear-sky locations.
the cure worse than the disease. Suspending
the fan with elastic bands, as shown in
the picture here, is a great way to do this.
As Bryan’s tests convincingly demonstrate,
any fan is better than none at
all — even a little airfl ow makes a big
diff erence. That being the case, wouldn’t a
lot of airfl ow be even better? Yes, but only
up to a point. All a fan can do is blow away
air warmed by the surface of the glass,
but heat within the glass has to get to the
surface fi rst. The limiting factor is the rate
at which heat is conducted from inside the
glass, and a more powerful fan does very
little to accelerate that process. A bigger
fan also has a potential of producing more
image-blurring vibrations.
One fi nal point. If you mostly observe
deep-sky objects and rarely push your telescope
to its magnifi cation and resolution
limits, a warm mirror might not aff ect
your observing very much. But if you like
to take in views of the Moon and planets
regularly, a little thermal management
will go a long way toward improving what
you see in the eyepiece. There is much
more to this subject, but it’s the basics that
matter most. In summary, install a fan,
don’t worry, observe happy. ✦
Contributing editor Gary Seronik is a longtime
ATM who strives to keep his scopes
calm, cool, and collimated. Some of his
eff orts are featured on his website, www.
garyseronik.com.
EASY ECLIPSE
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