Re: [ATM] wire spider, and regular spider, reason not to blacken

[Raphaël GUINAMARD <rguinamard@infonie.fr>]

> > "Don't paint the spider and the secondary support black... Black paint
> > will absorb infrared light from the sky, and the metallic pieces will
heat
> > up by a fraction of a degree, which will cause a variation of the index
of
> > the air and a visible dephasing of the light passing close to these
> pieces.
 Then Nils answer:
> In normal use, heat (infrared) radiation would go from the warmer spider
to
> the cold, dark sky, losing (I believe) heat until there is an equilibrium
at
> some temperature - and a small gradient of heat within a thin layer of
air.

Nils is completely right (as usual!):
In Jean Marc Lecleire's  "A Manual for Amateur Telescope Makers, pg 272-3",
the translation of Couder's 1949 article is not correct:
Couder explain that the spider LOOSE its heat by radiation and that it is
the tiny layer of cool air around spider vanes that makes the spikes bigger
than what we would expect theoriticaly. He made the calculation and Ireally
think that he is right : painting should be avoided...
A big part of couder article is vailable here
http://astrosurf.com/altaz/effetthermique.htm
I think a translator like http://trans.voila.fr/ or other could help for the
non french speakers.

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