iOPTRON - Astronomy Technology Today

A HOT TOPIC - ACTIVE COOLING OF A PRIMARY TELESCOPE MIRROR
Figure 6: 60 minutes of cooling with beginning temperature of 36-inch
mirror of 20°C but beginning and ending temperature of the air at 5°C; air
recirculated with fans; mirror temperatures measured at thicknesses of
30 mm, 45 mm and 65 mm; temperature Delta of 0°C, 1.0°C, and 2.0°C,
color-coded to 30 minutes.
Figure 5: 240 minutes of cooling with a beginning temperature of ambient-air
and 36-inch mirror of 20°C and ending ambient of 5°C; primary
cooled with fans; mirror temperatures measured at thicknesses of 30
mm, 45 mm and 65 mm; temperature Delta of 1.5°C, 2.0°C and 3.0°C.
duced yet still-massive 36-inch
mirror: No fan or system of
fans would get the mirror to
less than one-degree Celsius of
night-time air by using nighttime
air as the cooling media if
the mirror temperature started
out as warm as day-time air.
So, another project was born.
Using the calculator
again, I found that, if I had access
to air pre-cooled to 5°C, I
could cool the 36-inch mirror
much faster. I might meet my
goal of one-hour cooling by recirculating
the pre-cooled air
through the mirror box, but as
an added bonus, by using precooled
air, the mirror cooling
process could be started when
the Sun was still up – when
any other scope would still be
waiting for the as-yetunavailable
cooler air of twilight
– as demonstrated in
Figures 6-8, which show the
mirror cooled with fan-recirculated
pre-cooled 5°C air at
intervals of 30, 45 and 60
minutes respectively.
For each of the three typical
glass thicknesses that comprise
my mirror, the outer ring
(the portion not honeycombed)
would take the
longest to cool, as expected,
but it would certainly be possible
to cool the entire mass of
the mirror, including the core
of thicker parts, in about an
hour if I could find the source
of 5°C air. So, in Microsoft
Excel, my favorite CAD software,
I laid out an overview
drawing (Figure 9) to better
understand where parts reside
within the telescope, where
hose attachments could be
made, what circulation issues I
might encounter, etc.
I dug through boxes in
my garage and found meters
to monitor temperature but,
more importantly, to also
measure humidity or dew
point. What I wouldn’t want
to find when I pulled the mirror
cover off for observing was
Figure 7: 60 minutes of cooling with beginning temperature of 36-inch
mirror of 20°C but beginning and ending temperature of the air at 5°C; air
recirculated with fans; mirror temperatures measured at thicknesses of
30 mm, 45 mm and 65 mm; temperature Delta of 0°C, 0.25°C, and
1.0°C, color-coded to 45 minutes.
Figure 8: 60 minutes of cooling with beginning temperature of 36-inch
mirror of 20°C but beginning and ending temperature of the air at 5°C; air
recirculated with fans; mirror temperatures measured at thicknesses of
30 mm, 45 mm and 65 mm; temperature Delta at all glass thicknesses
are under 0.25°C.
Astronomy TECHNOLOGY TODAY 55