iOPTRON - Astronomy Technology Today

A Hot Topic
Active Cooling of a Primary Telescope Mirror
By Steven Aggas
Cooling glass, specifically a
telescope mirror, has been the
topic of many conversations at
star parties and Internet forums,
covering not just the
boundary layer of warm air,
which acts like a weak lens to
deform the view before it
reaches the eyepiece, but also
concerning removing the
source of that boundary layer:
the excess heat load of the glass
mass of the primary-mirror
substrate. More telescopes than
ever before are equipped with
fans located at various points to
address both the warm-air
boundary layer and excess heat
load, and, in some instances,
fans are simply incapable of adequately
addressing these thermal
issue.
Take a look, for instance,
at the two graphs provided as
Figures 1 and 2, derived from
Robert Houdart’s telescope
mirror-cooling calculator freeware,
MirrorCooling (www.
cruxis.com/scope/mirrorcooling.htm).
The graphs differ
only in the thicknesses of the
mirror substrates for which
cooling characteristics are modeled,
30 mm versus 50.8 mm,
two typical mirror thicknesses.
The graphs demonstrate the
additional cooling challenge
faced as substrate thickness increases.
As modeled in Figure 1,
a mirror of 30-mm thickness
takes approximately 140 minutes
for surface and core temperatures
to cool within 1°C of
ambient without active cooling
measures, while Figure 2
demonstrates that, also sans active
cooling measures, a mirror
of 50.8-mm thickness has still
not achieved core and surface
temperatures within 1°C of declining
ambient, even after 240
minutes.
Now, what if your primary
mirror is a combination of
thicknesses and/or has slightly
more glass than your average
primary? My primary mirror is
36 inches in diameter and 6
inches thick but features nineteen
5 inch-deep hexagon holes
to reduce its overall mass. The
fans installed in my telescope
would be adequate for following
the gradual drop in ambient
temperature on a given
night … if it was already cooled
to the starting ambient temperature.
But, when dealing with
250 pounds of glass, there’s a
lot of initial stored heat that
would have to be gotten rid of
first!
I had an expectation, a
goal really, of wanting to spend
no more than one hour cooling
the primary mirror to less than
1°C of night-time ambient air
temperature. I’ve installed ther-
Figure 1: With an ambient temperature that steadily declines from 10°C
to 5°C over the course of 4 hours and without active cooling measures,
a mirror of 30-mm thickness reaches core and surface temperatures
that are within 1°C of ambient within approximately 140 minutes.
Figure 2: With an ambient temperature that steadily declines from 10°C
to 5°C over the course of 4 hours and without active cooling measures, a
mirror of 50.8-mm thickness reaches has not achieved core and surface
temperatures that are within 1°C of ambient even after 240 minutes.
Astronomy TECHNOLOGY TODAY 53