HOW TO
DO IT:
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Make sure you have proper ventilation in the mirror cell. This is a standard
plywood and silicon glue mirror cell, as described by Richard Berry. Note the three
ventilation holes and the back of the mirror. The important thing here is to make sure air
can flow, and there is enough room to sneak a fan between the collimation screws.
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Buy a small muffin fan at your local electronics supply store, or salvage one
from an old computer. You may wish to buy an appropriate battery or battery pack, a plug,
and wire at the same time.
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Cut a piece of stiff pasteboard to match the outside dimensions of the back
of your tube. This approach works as well with square mirror boxes as it does with round
tubes.
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Cut a hole in the pasteboard to hold the fan.
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Attach the fan to the cardboard, (with
the airflow to the outside). You may wish to seal with silicon glue.
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Connect the wiring through the back of the cardboard and out to the battery.
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Attach the cardboard and fan to the back of the
tube.
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Attach the battery to the tube assembly or rocker box.
Put it where it helps your balance best. Attach with Velcro, or as in this case, bungee
cords. This power supply is a twelve volt battery used for cell phones, portable
computers, and other applications. It cost $15 or so with a charger. You could also
combine as many "D" cells as you need to power your fan.
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INTRODUCTION
At the May 2000 Riverside Telescope Maker's Conference, Bryan Greer presented an
impressive series of videos that showed light reacting to the temperature differentials in
a mirror and tube. You may read about this in his article "Understanding Thermal
Behavior in Newtonian Reflectors" in the September 2000 Sky and Telescope magazine.
However, the video was much more impressive than the magazine text and accompanying
illustrations. It was amazing to see how much the
temperature in the scope and mirror affects the stability of the view.
BACK TO TOP
HEAT AND SEEING
Heat may affect the mirror in two ways.
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Materials
generally expand as they warm and contract as they cool. Unless the mirror is exactly the
same temperature across its surface and throughout it's depth, the mirror will have some
relatively expanded areas and some contracted. If a precision polished curve is
differentially heated, then, the curve will change, and probably not uniformly. (The
degree of this effect depends on the "coefficient of expansion" of the glass, of
course, with a simple plate glass being affected more, and pyrex and zerodur being
affected less.) So, differential heat "warps" the fine curve you
started off with, and this affects the image. |
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But even
more dramatic is the effect heat has on the air in the tube. Unless the mirror and the air
in the tube are exactly the same temperature, the mirror (which is usually warmer) acts as
a heat source and creates a disturbance in the air just above the mirror. That disturbance
propogates throughout the tube. Looking through the tube, the observer sees the same
phenomenom that one sees looking at a distant object over the hot pavement of a desert
road. Images shimmer as moving warm and cold air currents bend photons back and forth at
random through the last few feet to the eyepiece. |
A fan combats both these effects.
Fans have long been used to help a mirror reach thermal equilibrium. Even if the outside
temperature remains stable (as in a controlled indoor laboratory), it takes hours for a
mirror to match the ambient temperature. Moving the cool ambient air quickly through
the tube and across the mirror helps cool the mass of glass. But fans can also help
control the tube current, and perhaps more importantly, the thin, unstable layer of warmer
air that forms just in front of the aluminized surface of the mirror.
Among the things Greer found through his research is that the mirror really
never catches up to the outside air temperature. The mirror may have its own relatively
stable temperature that is approaching ambient temperature, but as the night's temperature
drops, the glass can't cool as quickly as the ambient temperature, so it lags behind.
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HOW TO BEAT THE HEAT
It was Greer's suggestion that telescope makers use two small muffin fans. One is
in the tube just above the mirror. This runs during "cool-down time" that we all
wait through--that first hour of the evening while we are waiting for our mirrors to
equilibrate. It pulls air straight across the mirror to help it cool down to ambient.
Then there should be another behind the primary that runs all through the observing
session. It pulls air down the tube, around the primary, and out the back end.
BACK TO TOP
RESULTS
I have tried the one behind the primary. I bought a three inch or so muffin fan, attached
it to stiff cardboard, and bungeed the assembly across the back of the tube (outside the
collimation screws). The 12 volt cord runs out the back and up the side of the tube to a
phone battery that is bungeed to the tube (to balance the fan assembly). The whole setup
cost twenty-five dollars or so.
It works great. Although not dramatic, the difference is noticeable in a star test. It is
definitely worth what little effort it takes. Seeing is better with the fan on.
If I understand correctly, the fan does two things. First, it helps the mirror
and tube cool more quickly. This is obvious--you are pulling cool air right through the
tube. Second, it smooths the airflow through the scope, creating a "laminar" air
flow. Basically, all the currents are taking the same path through the scope the
whole time. Cooler or warmer "cells" of air are not wandering around in a
semi-random convection. Whatever cells there may be are smaller. No air gets to stay next
to a warm mirror for any length of time to pick up the heat. Secondly, any cells there may
be are rapidly drawn through and out the back.
We have had several visitors comment that it doesn't seem good to have a stream of air
(and dust) being pulled through our scopes. When we are in cool-down mode, we
actually have a furnace filter over the front of the scope--it traps dust but lets the air
flow. Something else that may be an advantage as far as dust goes is that the all air
entering the tube is now entering from the front, not the lower rear end of the scope
(through a chimney effect). This helps with dust, which tends to be greater near the
ground.
Considering the ease with which a fan can be attached and used, it definitely seems
worthwhile to add one. We have run star tests several times and consistently find sharper
star tests with the fan on.
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