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Gary Seronik
Telescope Workshop
GARY SERONIK
Thermal Basics
Beating the heat will get you the sharpest views from your Newtonian.
Next to collimation, But why is this a problem? Simply because air masses
the biggest barrier to
optimum optical performance with a Newtonian refl ector
is poor thermal management. And like collimation,
thermal management is a topic that invites debate and
confusion. Some even suspect the whole issue is merely
the product of hyper-obsessive perfectionism on the part
of some telescope junkies. In reality, however, neither the
problem nor its solutions are terrifi cally complex. As long
as you focus on a few basics, the results of even minimal
eff ort can pay big dividends.
The fi rst thing to understand is that a Newtonian’s primary
mirror is the root of all thermal evils. Forget about
tube currents, forget about the spider and the secondary
— that big hunk of glass sitting at the back of your scope
is where you need to direct your attention. That’s because
the primary mirror has considerable heat-storage capacity
— heat that it happily (and with agonizing slowness)
radiates whenever the air inside your scope is cooler than
the mirror itself.
By simply mounting a small computer fan with rubber bands behind a Newtonian
refl ector’s primary mirror, high-power views through the telescope
will be noticeably improved. Many of these fans are designed for 12-volt DC
power but still work eff ectively when run with a 6-volt battery pack.
70 May 2012 sky & telescope
of diff erent temperatures have diff erent densities. On its
journey from a star or planet through zones where warm
and cool air mix (typically directly in front of the primary
mirror), light wiggles and bends, producing an eff ect that
looks a lot like bad atmospheric seeing. Some telescope
users who despair that they observe from locations where
the seeing is always bad may be fi ghting telescope thermals
more often than bad seeing. The big diff erence is
that you can do something about a warm mirror!
Another basic consideration is that the amount of problem
heat that your primary mirror stores is related mainly
to its thickness. In other words, the type of glass and the
mirror’s diameter don’t make as much diff erence as its
thickness. Thick is bad, thin is good. As Bryan Greer’s
careful work (detailed in the May and June 2004 issues)
demonstrates, any mirror thicker than about ½ inch is
unlikely to cool quickly enough on its own for satisfactory
high-resolution viewing.
Third on the list of basic issues is that it’s the temperature
diff erence that matters. In other words, if you bring
your scope out of a garage on a mild summer’s evening
and the primary is sitting at 75°F (24°C) while the ambient
air is at 68°, the thermal consequences are just as
severe as in the winter when your mirror is at 39° and the
air surrounding it is at 32°. Some observers think they
don’t have to worry about thermals when it’s warm outside,
but mild climate or not, it’s a rare location where the
night temperature doesn’t drop a few degrees per hour
when the sky is clear.
What if you leave your scope outside all the time? It
helps, but it’s not usually enough by itself. The trouble is
your mirror dumps its heat more slowly than the rate at
which the night air cools. Even if your primary is at the
same temperature as the ambient air at the beginning of
an observing session, it’s unlikely to remain so for long.
So what can you do? Simple. Install a small fan to blow
air at the rear surface of your telescope’s primary mirror.
That is the easiest and most eff ective solution, and a small
DC-powered computer fan is ideal. The only “gotcha”
is that you have to mount the fan in such a way that its
vibrations (and all fans vibrate to some degree) aren’t
transferred to the telescope — there’s no point in making