[ATM] Thermals in Reflecting Telescopes, Adler Rx, Version-2

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Fellow ATMs:

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,

Gene Cross
___________________________________
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.

Adler rocks!
Stephen Paul
Shirley MA



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.

Rich  Neuschaefer


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 
off.

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 
to vent.

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 
grateful!

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 
the ambient
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 
Telescope
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 
volunteer
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 
certified.
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 
of a
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 
2 scopes
being 2" thick, the 8" was 1" thick. The two telescopes were pointed towards 
Saturn, to
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 
mirrors
were reading 1 1/2ºF warmer than the surrounding air. The scopes were only 
handled to
keep them aligned on Saturn. At 7:00 pm there were still slight heat waves 
detectable in
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 
on, the
surface temperature of the mirror face was reading -11ºC. This change was due 
to the
outside air being blown across the face of the mirror. The 12.5" mirror was 
still reading
3/4ºF warmer than the ambient outside temperature. The 8" mirror was also 
reading 1/2ºF
warmer than the surrounding air.

By 8:30 pm the temperature had dropped to -12ºC, still very clear and stable. 
The relative
humidity had risen to 79%. The 12.5" scope was starting to give clear images 
of Saturn,
and the 10" scope was producing unbelievably clear and sharp images of Saturn 
and it's
moons.

We stopped the tests at 9:15 pm as the moon had cleared the horizon, and we 
were getting
cold. The temperature was now -13ºC, the relative humidity was 84%, and frost 
was
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
fan.

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 
outside air.
The views of Saturn were much clearer and crisper in the 10" telescope, again 
proving the
old adage: "the proof of the pudding is in the eating".

In conclusion, we discovered that our worst enemies are the mirrors 
themselves. No
matter how cool the surrounding air becomes, the mirror cool-down does not 
keep pace
with the temperature changes. The heat migrates inward and then upward (heat 
rises) and
the mirror becomes a massive heat sink. We also noticed that glass thickness 
displayed no
differences in the cooling down rate.

Until a new and totally stabilized substance is found to replace our current 
technologies,
we will have to continue relying on people like Bryan Greer and Alan Adler to 
keep us
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 
two velcro
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 
few dollars
required to implement this change. You will invest a minimum amount of money, 
but you
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 
light trap.
    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>