To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


>From: Emery Greg 
>
>What is the preliminary mode of heat transfer in telescopes?  Is there a
>dominant mode or does it change from design to design?.

It changes from component to component...and you must take into account the
environment the component is situated in.

On a truss tube scope...the spider 'see' a great deal of night sky...and
can radiate to it...and cool down.

On a solid tube scope (especially with a light/dew shield extension)...the
spiders 'sees' a great deal of telescope tube...and can radiate to it...and
the tube radiates back to the spider...so there is much less temperature
drop of spider vanes in this case.  (Radiation is *always* going on...what
changes is the net result/transfer over time.  If you radiate to the 'cold'
sky...you don't get much radiation back, and the temperature drops.  If you
radiate to a telescope tube...the tube radiates about the same amount and
you have a net heat transfer that's very low...perhaps essentially zero.

Convection can plays a big part on the main mirror.

But on an SCT corrector plate...lots of radiation to the night sky...that
corrector plate dews up fast/early compared to other scope components. Tube
extensions help reduce heat loss by radiation.  Heaters can help too if
applied/used properly.

If thermal issues are a problem to you, I recommend making your design use:
- low thermal inertia (typically low mass)
- high thermal conductivity (where possible...tough to do this with glass)

If your design meets the criteria above your system will respond fast to
dropping air temps, and if you add fans you can usually do even better.

Fans can also cause problems such as vibration and slight thermal problems
of their own.  In some cases you will star test and find that turning the
fans off is better.  Then do so.  (If you don't have fans...you don't have
this option...and it's good to have options.)

I hope this helps,
Tom Krajci
Tashkent, Uzbekistan

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