I
recently decided that I "needed" a Big Mak - no not the hamburger, the telescope, an
Orion 180mm Maksutov. While conducting my customary exhaustive
and obsessive pre-purchase internet research about this scope,
one theme kept
recurring in the discussions. That was the need for the scope
to be acclimated to the ambient temperature in order to deliver its
best possible performance.
The two most often mentioned strategies minimize that issue seemed to be storage of the
scope in a space that is as close to the
observing site temperature as possible, and allowing for a cool down period before
viewing. But a third approach was also out there, active cooling
using fans.
There are commercially manufactured SCT / Mak fan based cooling
devices, but prices that exceeded $125 makes them more than a casual purchase.
So I decided to see what I could put together at the Ol' HBA
workbench.
Scouting around on the net turned some examples. Most involved a fan
with a duct pipe attached that was inserted into the scope to exchange
the air in the tube with the ambient air bringing the scope into
equilibrium. Many of the home built coolers I saw use a standard square
"muffin"
type fan. Using that form of fan required the maker fit a plenum to
the fan to funnel the air into the duct pipe that was then inserted
into the
scope. Making that plenum seemed to be overly involved and
difficult, so I went searching for a more friendly form of fan.
A Big Fan of This Fan
And, I found it. This fan, commonly referred to as a "5028b
blower" looked like just the ticket. As you can see this "squirrel cage" type fan's output
is directed out the port at the end of the "arm" . This form
would make it easy to attach the duct pipe of PVC that I had planned .
It was reasonably priced too, about $10 delivered via EBay (Amazon
carries them as well).
When it arrived it looked to be very promising.
Connecting the fan to a 12v power source the output stream it produced was great. It really moved some air!
See the video demonstration below
The good luck continuedwhen testing the fit of the duct pipe fitting. It
turns out that the fan's outlet was good fit for a 1/2" pvc fitting, a
bit loose, but nothing a generous application of JB Weld
wouldn't firm up nicely.
It would be helpful to be able to remove the pipe from the fan for
transport and storage, or to accommodate multiple duct pipes of various lengths in
the event that you wanted to use the fan with different sized scopes. With a thumbscrew and
a slightly
undersized hole drilled in the fitting, I was able to use the
thumbscrew to self-tap threads in the fitting to make a clamp to hold
the pipe while in use, but permit disconnection as needed.
Next, exercising a great deal of care, I measured the length that
I needed the duct pipe to be by very gently inserting a tape
measure into the rear port of the scope, up through the
baffle tube until it emerged as I watched from the front end of the
scope. The duct pipe should extend beyond the end of the baffle tube,
but must NOT be so long as to hit the secondary. With that measurement,
and a allowance for the cap
that would be glued onto the end, the pipe was cut to length.
After attaching the cap with the JB Weld, I drilled two good sized
holes completely through the pipe at a right angles, making four
outlets
for the air.
Dust In The Wind
One potential drawback of this type of cooling is the possibility of
the introduction of foreign matter into the telescope. Before we
go blowing a whole bunch of air into our
cherished optics, we need to make sure that it is reasonably clean air.
We don't want to suck a bunch of dust, pollen, chopped up bugs, or gosh
only knows what else in there!
The easiest way to address that is by adding a filter at the fan's intake. A filter placed on the open side of the
fan's squirrel cage blower
wheel should do the job. To secure the filter material, a mask was made that
would be clamped to the the fan, pinning the filter material to the
face of the fan over the air intake. The material used for
the mask was
bit of a
stiff plastic report cover. To get the proper shape, I used the
old trick of shading a piece of paper held over the face with the side
of a pencil. It provided the perfect template that was
transfered to the report cover and cut out with a hobby knife.
Looking around the house for a filter material to test the fan's
output pulling through the filter, I found some used fabric softener dryer sheets
that seemed a bit like a filter material. I folded it over four
times and put it in place for some testing.
To get some idea of the output, a simple test using a one gallon
zip lock bag was set up. I would simply measure how long it took
the filtered fan to fill up the bag.
See the video below
The results were very good indeed! It took just 8 seconds
to fill the one gallon bag!
So how does that translate to the
performance we can expect in the scope? Let's do a little HBA
"Redneck Engineering" math....
One Gallon = 231 Cubic Inches
Volume of the Orion 180 Maksutov tube (approximate external measurements)
Height = 19"
Diameter = 8.5"
Radius = 4.25"
Volume of a cylinder = Pi * Radius squared * Height
So... (3.14*(4.25*4.25))*19 = 1077 cubic inches
1077 divided by 231 makes the volume of our tube 4.6 gallons.
We can move one gallon of air in 8 seconds, so we can completely change
out the air in the tube in about 37 seconds! Not bad at all.
This is of course a rough calculation, the measurements were
of the outside dimensions, no allowance made for the volume displaced
by
the mirror, baffle, and other mechanisms in the tube. The
actual exchange period is likely even shorter. In any case,
it should be plenty effective for our purposes.
Of
course, to accomplish that exchange, in addition to the inflow, there
must be a decent flow of air out of the tube. To check the flow of the
air out of the scope I did a simple test and it showed a good volume
escaping back down the baffle and out the rear port. This
positive air pressure will also insure that no dust or foreign matter
will enter.
See the video below
The Real Deal
For actual use, a more serious filter is needed. Thinking
about where I could find an appropriate material for filtering air down
to the very small particle size that I would like to keep out of the scope, it
occurred to me that a medical filter would probably meet that standard.
Doing a bit of scouting on EBay I turned up filters
used for CPAP machines. The CPAP machine is used to assist those with sleep apnea, the machine
provides a positive flow of air to a mask worn by the patient during
sleep. As you might guess for that application, those machines utilize filters that will remove very
fine particulates.
The
CPAP filter I found was touted by the manufacturer to remove smoke and
pollen particles. As those were among the smallest that the scope was
likely to encounter in the course of normal use, it seemed to be a good
fit for my purpose. An order was placed for a package of six.
The cost was $6.
The filters were large enough to permit me to use just half of a filter to cover the intake on the fan.
To
secure the filter in place on the face of the fan, I used a water
soluble glue stick around the perimeter of the fan face. That
also had the effect of sealing the edges of the filter to stop any air
from bypassing the filter. The glue is relatively weak,
easily removed when the time comes to change out the filter, but
in combination with the mask should hold the filter in place nicely.
So
how does the fan perform with this fine filter in place? As you
may guess, it did slow down the air flow. Performing the same
"fill the gallon bag" test, the time to fill the bag was 15 seconds,
about double the previous performance. That means the air in the
scope is fully exchanged every one minute and nine seconds or said another way, 52 times an hour. Still
a solid performance, and I am very comfortable trading some speed for
the excellent filtration and protection afforded the optics by this
very fine filter.
Power Play
Now to address powering our cooler. There are lots of options for 12v
power, batteries of all sizes, shapes and capacities. As the fan draws just .25 amps, it is not
terribly demanding of a battery. To keep the kit a nice compact size, I settled on a small 12v 1.2AH AGM sealed
battery. It should easily provide a couple of hours of cooling, more than
enough for all but the most extreme conditions. Along with it I
picked up a cigarette lighter socket, and a switched cigarette lighter
plug that even had a night-vision-friendly red illuminated button when
turned on. All of these were sourced from (where else) Ebay and all together represented a modest investment of
about $20.
The finishing touches were easy enough. I secured all of the
electrical connections. I also added a small case that I had kept
for years (because I was sure this day would come,) to hold the battery
(see
Honey, I told you all of that stuff cluttering my secret lair will come
in handy someday...)
I
added some Gorilla tape as a tension relief to secure the power leads on
the fan. The leads are a circuit board soldered connection, it is a bit fragile,
and a strong tug could cause it to come loose. The tape works well to secure
it from forces that might pull it free.
Too Cool for School
With
the scope horizontal, simply insert the duct pipe and press the switch.
The battery can rest on the mount tray or perhaps hang from somewhere by the loop on the case.
The switch glows red when turned on.The fan is pleasantly quiet when running, no annoying racket that you can't wait to shut off.
The business end
Calm, Cool and Collected
For
a total of around $40, which includes the power source, a fully capable SCT Cooler that will
help me get the best from my Big Mak when conditions demand a
more accelerated cooling than can be accommodated by unassisted
acclimation. For those times when circumstances don't permit
setting the scope out early, or when temps are changing fast, the fan
can help get things balanced.
Anything that improves the view is well worth doing, and saving a few bucks in the process is always welcome.
DIY
SCT / Mak Cooler 
Telescope
Mounted Green Laser * 
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