[ATM] Thermals in Reflecting Telescopes, Adler Rx, Version-2
OPTIXWIZ at aol.com
OPTIXWIZ at aol.com
Sun Aug 28 08:11:19 JST 2005
This version is edited down in size, hopefully will work for the ATM List.
Alan Adler has compiled the endorsements and references that folllow and
given permission for distribution.
Wishing you 99% Strehls,
Scope Fan Endorsements Collected From the Internet:
The fan modification is definitely worth the effort for scopes that have to
transition even a mere 5 degrees F. The sweeping of air is of even greater
benefit than simply cooling the glass, as it gives an immediate result. I was
very nicely surprised at last night's results.
I did install the side mounted fan that Alan Adler wrote about and it was
worth the work. Before starting the images were not very good.
Turned the fan on and the image cleaned up as if the bad seeing was blown
away. Turned the fan back off and the bad seeing returns.
When a fan is properly used with a Newtonian and air is blown across the
mirror which works quicker by the way. In my scope it takes about 15 seconds for
the bad seeing to go away that was generated by the layer of turbulent air over
the mirror. If the fan is then turned back off the bad seeing returns and the
images soften fine detail is lost.
Rich McMahon (rhmc at austin.rr.com)
Thanks to Bryan Greer and Alan Adler, also Gary Seronik and others (including
Pickering and Goddard many decades ago) who have been advocating fans blowing
across the face of the primary. I finished the mods to my scope yesterday
and the weather last night allowed me to check it out. Most of the time was
spent examining Castor at 300x. Seeing was such that the image would break up
occasionally at this magnification. Basically what I saw matched the double-star
images from Alan's S&T article. Turn off the fans, the pair turns into an
oblong fuzzball. Turn on the fans, wait 30 seconds, and the image turns into 2
slightly overlapping Airy disks. After 3 hours cool down, the difference was
much reduced but still noticeable.
Steve (sdh at vczpamtrapznet.com)
Per Alan Adler's instructions in his article "Thermal Management in Newtonian
Reflectors," from the January 2002 issue of Sky & Telescope, I installed a
fan on my 12.5 inch telescope. Performance improves immediately and
dramatically when the fan is running. Contrast and resolution are both improved. There
are many examples of just what the fan can do for the image, here is one.
Without the fan running, Delta Cygnii never was
split with the telescope. The image was most often, no matter how long the
scope had been left outside in an attempt to reach thermal equilibrium, just a
boiling mess. Simply turning on the fan produces two cleanly split stars with
each unaffected by turbulence.
Martin (martinhowell at earthlink.net)
Recently, based on S&T's schlieren photography of mirror currents and a short
discussion with Alan Adler at Stellafane, I put a CPU cooling fan on the
lower side of my tube at mirror level and three 1.25" holes on the top, opposite.
I can see the distortion in a star test melt away just a few seconds after I
turn on the fan, even after prolonged cooling.
Frank Frank Bov (frankbov at rochester.rr.com)
I have added two cooling fans blowing on the face of my full thickness
primary and one large blowing on the rear for a total of 130 cfm. The results have
been dramatic. I couldn't be more pleased with my scope's imaging. I wish
I'd done it three years ago.
Alan Garcia (ngarcia at columbus.rr.com)
Before anyone thinking of making an off axis system, they should read Alan
Adler's article on thermal control in the January issue of S&T. I have seen
Alan's telescope at work. It is very effective. I've had my 6" APO refractor
along side his 8" Newt on nights when he was testing his fan system.
His scope, with the fan turned on, can give refractor like images. Adler and
I have had our telescopes out side by side several times for comparison and
testing of his fan system. I was using my AP 155EDFS and Alan had his flex
mirror 8" with the fan. With his fan running and his mirror settled to the
outside temps it can sometimes give at least as
good and sometimes better lunar and planetary images.
I live close to Alan Adler and I've observed with him numerous times. While
looking at close double stars, for example, I could see stars become sharper
and become much better resolved within seconds after turning on the fan.
Conversely, the stars would swell, closing their separation, once the fan was turned
Albert Highe, 12/13/01
JOSEPH ROTHCHILD showed us his latest modifications on his red Dob based on
the article by Alan Adler entitled "Thermal Management in Newtonian Reflectors"
from Sky & Telescope, January 2002, pg. 132-136. To cool the boundary layer
of air in front of the mirror, Joseph used a 3" fan powered by a 12-volt
battery, and opposite made four 1 1/2" exit holes (same total area as the 3" hole).
He reported that while observing, turning the fan on did provide improvement,
and after turning it off, detail disappeared.
(From club newsletter)
(2/11/02 email to Alan Adler)
I did exactly as your article described, implementing two 2.75" fans in the
bottom of my Dob flush with the front of the mirror. I cut five 2.75" vent
holes out the top slight light back towards the mirror to allow the scavenged air
With the mirror/air temp 25 degrees different I was unable to see Polaris'
companion with the fans off. Once the fans were turned on for half a minute the
companion became easily discernable and Polaris itself much sharper. I
observed for 1.5 hours and then tried the Polaris star test again. Same result;
the fans and vent holes made a TERRIFIC improvement. I and my 16" Dobsonian are
Mark Hansen (www.pathwaynet.com/~mhansen/astro)
I made the modification in three hours on my 16" reflector. My "master of
the blur" became a first class, razor sharp instrument. Unbelievable, how
simple it was.
Grzegorz Koralewski (Poland) (eeeeeeee at interia.pl)
Kitchener Waterloo Centre
Royal Astronomical Society of Canada
Thermal Management Confirmation
Ken Saumure, KW Centre
Being a firm believer in keeping your Newtonian telescope mirror as close to
temperature as possible, I have always mounted a fan blowing air onto the
rear side of the
mirror. In the custom-built telescopes that I have built, I have always
stayed away from
metal mirror cells, and the tubes were mounted in cradles that permitted the
tube to slide
back and forth, using the mirror as a counter-balance for the telescope.
After reading Alan Adler's article in the January 2002 issue of Sky and
magazine (page 132), I immediately modified my own 10-inch Newtonian, and
remounted the 3-inch computer fan on the side of the tube (Figure 1), with
five exit holes
directly across from the fan (Figure 2). My initial trial results were
promising, and I
wanted to further investigate this procedure. I intentionally stayed away
from Mr. Adler's
computer generated data, as I did not want any outside influences to
interfere with my
testing. I had just finished a 12.5-inch, f-6, Newtonian for a customer who
lived on a
dairy farm roughly 60 km from any large light source, the new owner agreed to
his telescope and time to assist in the tests that I wanted to conduct.
All thermometers used in the tests were research grade, and were ISO
January 1, 2002, was the day selected for the tests. At 4 pm. the telescopes
were set up.
The temperature was -8 ºC, the relative humidity was 63%. There was no hint
breeze; these were ideal conditions.
The 10" and the 12.5" telescopes were set up about 10 feet apart, with
Telrads and 26
mm Plossl eyepieces installed. The 8" mirror was set up between the two
telescopes, on a
plywood mirror-cell, open to the atmosphere. All mirrors are pyrex glass, the
being 2" thick, the 8" was 1" thick. The two telescopes were pointed towards
be held as close as possible to vertical, to enable us to detect any heat
waves that would
rise in the tubes. By 6:45 pm the temperature had dropped to -10ºC, and all 3
were reading 1 1/2ºF warmer than the surrounding air. The scopes were only
keep them aligned on Saturn. At 7:00 pm there were still slight heat waves
both scopes. At 7:30 pm the top surface area of all 3 mirrors was still
registering 1 1/2ºF
warmer than the surrounding air. At 7:30 pm the outside temperature was
-11ºC, and the
fan was turned on in the 10" scope. Within 5 minutes of the fan being turned
surface temperature of the mirror face was reading -11ºC. This change was due
outside air being blown across the face of the mirror. The 12.5" mirror was
3/4ºF warmer than the ambient outside temperature. The 8" mirror was also
warmer than the surrounding air.
By 8:30 pm the temperature had dropped to -12ºC, still very clear and stable.
humidity had risen to 79%. The 12.5" scope was starting to give clear images
and the 10" scope was producing unbelievably clear and sharp images of Saturn
We stopped the tests at 9:15 pm as the moon had cleared the horizon, and we
cold. The temperature was now -13ºC, the relative humidity was 84%, and frost
starting to form on our equipment.
Our crude tests had indeed proven to us that Alan Adler's figures were
correct, and an
additional 1 1/2 to 2 ºF of front surface cooling is achievable using a
side-mounted, cross blowing
Never during the 2 1/2 hrs. of testing did the 8" or the 2.5" mirrors cool
down to the
ambient temperatures. The 10" mirror cooled down while the fan was in
operation and the
surface temperature at the face of the mirror matched the temperature of the
The views of Saturn were much clearer and crisper in the 10" telescope, again
old adage: "the proof of the pudding is in the eating".
In conclusion, we discovered that our worst enemies are the mirrors
matter how cool the surrounding air becomes, the mirror cool-down does not
with the temperature changes. The heat migrates inward and then upward (heat
the mirror becomes a massive heat sink. We also noticed that glass thickness
differences in the cooling down rate.
Until a new and totally stabilized substance is found to replace our current
we will have to continue relying on people like Bryan Greer and Alan Adler to
ahead of the game.
An extra feature I added to my telescope was the addition of a felt strip 2"
wide to cover
the exit holes when the scope is not in use. This strip is held in place with
pads. My fan is a 12 volt DC brushless motor with ball bearings, and at no
time was there
any indication of vibrations detected.
I heartily recommend that anyone who owns a Newtonian telescope invest the
required to implement this change. You will invest a minimum amount of money,
rewards will be million-fold.
"The effect is like blowing away the fog, and the detail, even with 600
diameters, was very clear. Since then, I have found the fan so far ahead of any
other method that I always use it".
A. V. Goddard - Amateur Telescope Making, Book Two (approximately 1925)
A couple years ago, I finished re-building my 12" and placed a pair
of computer cooling fans, ~2" in diameter, blowing across the face of
the primary. The fans are mounted directly onto the 5/8 baltic mirror
box. Even at top speed and high mag, there is no trace of vibration, but
the scope is a typical wooden Kriege/Berry dob, very solid. I have also
added the foam dust filters that match up with the computer fan, and
over the exit holes I use self-adhesive velcro to hold thin black
humidifier filter material in place - it makes an excellent dust and
When the fans are turned on, the effect on seeing is both immediate
and startling. I have never seen planets as clearly in 20 years of
observing. Last year Saturn looked like a cardboard cutout handing in
the fov. When the fans are turned off, the seeing immediately
degenerates, no matter how long I've been using the scope. The 12" is
full-thickness, and I don't know long it takes to reach equibrilium, or
if in fact it ever has. I don't have any fans blowing on the back of the
mirror, tho I keep thinking I should experiment with that, too.
Jay -- ATM Digest, Vol 20, Issue 29
Jay Kirkland <jaykirk2 at compusmart.ab.ca>
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