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I have been having an e-mail exchange with Bryan Greer. His latest e-mail is below. Very interesting!!
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Dear John,
It's good to hear from you again.
It is easy to let two different issues to blend in one's mind. If you ask
ten telescope owners what they're trying to do by adding fans, maybe eight
of them will say they want to bring their scope into equilibrium faster.
Fans certainly do hasten this initial equilibrium (see the image john01.gif
attached), but this aspect was not the surprising part of what I discovered
ten years ago. (Amateurs have know for a hundred years about this initial
cooldown problem, and that fans help.) If this were the only thermal
problem, it could be "solved" by simply having a bit more patience, and
waiting for the scope to reach equilibrium on its own.
The more important function of fans is to keep the surface temperature of
the mirror tracking the ever-falling ambient temperature more closely, and
this must be done throughout the night. This is at the heart of the issue.
For most locations on Earth, the nighttime temperature under clear skies
falls too fast for even small Pyrex mirrors to track it close enough. See
the second graphic (john02.gif) to see what I mean. The mirror on the left
will suffer from thermal problems all night long. This is why any solution
you come up with MUST be able to operate continuously, and not just switched
off after the initial equilibrium is reached.
What delta T is "close enough"? The mirror surface needs to be within about
1 degree Celcius (~ 2 F) to keep the boundary layer from forming with any
significant refracting strength. This value is based on the color schlieren
testing I did in the 1990s (see Sept. 2000 Sky & Telescope for more
details). Remember, the strength of the boundary layer is PURELY a function
of this delta T.
Cooling by refrigeration, or other sub-ambient techniques (Peltiers), would
certainly help with the first problem (i.e., initial cooldown), but they are
problematic, and largely unnecessary, for the more important objective of
tracking the falling ambient. For one thing, you now have a new challenge to
prevent the mirror's temperature from falling below ambient. If that
happens, dew will form quickly if the relative humidity is high enough.
Also, a mirror's surface that is below the ambient by a few degrees will
form a boundary layer exactly like a warm mirror will (yes, it flows
downward instead), and will cause the same optical image damage. You would
need a temperature sensor and feedback to control the refrigeration (or
Peltier) process. (Of course, fans are blowing air that is always at ambient
temperature, so in this sense they are "self-regulated".) Finally, if you
tried to use the refrigeration device during observing, it would create very
strong thermal gradients in the optical path. This, alone, would relegate
its use only during the initial cooldown phase, and then switched off.
For my location (Ohio, U.S.A.), fans can keep a mirror that is up to about
2-1/2" thick (75 mm) tracking our ambient close enough to keep the boundary
layer at bay (i.e., within a degree of ambient). This requires airflow over
the front and back surfaces. If the mirror is around 1-1/2" thick or less,
airflow only across one surface (usually the back) will do the job. Your
day/night temperature drops are larger than mine, so your scope could
benefit from airflow across both surfaces. Don't fret if you can only get to
the front. That will still be a big improvement over no fans at all.
I am glad to hear this is being discussed online. I will try to read the
links you gave me, but also feel free to post this information (and images)
to those sites, as well. If you can get your hands on them, I recommend
reading my three "Sky & Telescope" magazine articles on this subject (Sept
2000, May 2004, and June 2004).
Sincerely,
Bryan Greer
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