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

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Raphaël,


> 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...

There is also the question of whether the effects will be visible at all
under realistic conditions. One can always arrange dummy spiders on any
Newtonian by pulling wires across the aperture, creating spikes in different
directions from the "old" ones. Could you see the difference when they are
exposed to (almost) a hemi-sphere of dark sky? Could you, if the wires are
at the lower end of a diagonal cage, exposed to much less of the cold sky?
"Ajouter au tube du télescope un prolongement aussi long que le permet la
grandeur de la coupole pour diminuer l'angle solide sous lequel est vu le
ciel."

I did try a quick and dirty experiment once to locally heat dummy vanes from
hacksaw blade, using my fingers, to see if the defocused dark line from the
spider appeared thicker than the parts in thermal equilibrium, I recall I
needed to heat a good deal with my fingers to make a visible difference. My
guess is that with wires, you would have a hard time to tell indeed - anyone
ready to try?

Nils Olof

whose school French wasn't very good to start with... but trying hard, now I
find Couder had already thought of all I have on the subject and then some.


> A big part of couder article is vailable here
> http://astrosurf.com/altaz/effetthermique.htm

"Réduire la longueur du trajet optique dans cet air froid en diminuant la
hauteur des lames....en leur substituant des systèmes de fils d'acier" - "Un
fil rond, substitué à la lame s'entoure comme elle d'une couche d'air froid,
mais cette gaine est cylindrique: le trajet de la lumière selon une de ses
cordes est très court"
Isn't this exactly what the wire spider does - and if the wire spider is so
good (I haven't tried it, but others have) is this the explanation?

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