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

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

Jay Kirkland wrote:

>  Well, I just read the
> reason in a book I just got. From Lecleires' excellent A Manual for
Amateur
> Telescope Makers, pg 272-3:
> "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.
> This phenomenon, describes by Andre Couder, causes a directional flaw and
> reinforces the diffraction rings. The metallic pieces in the tube should
be
> left shiny: brass or copper is better than aluminum."

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.
The thin wire means if such a layer is formed, it contains a very much
shorter part of the light path than is the case with a sheet metal vane. A
shiny surface does not emit or reflect much radiation.

Shiny surfaces - including glossy black paint - may be preferrable as long
as you can ensure that they do not reflect any bright surfaces into the
light path. Anyway, even if shiny wires do reflect some ligth from somewhere
into the light path, is it significant?

But visible light is one thing - whether a surface is "black" or shiny to
infrared (I think broadly around 10 micron or so) is something else, and not
so easy to inspect. Ideally the vane would be black to light, and shiny to
infrared - if you could have a layer of black material, thinner than a
micron or two, this may be too thin to affect infrared.


> They also state: "Cover truss tubes with reflective material, with the
> shiny side (reflecting the infrared radiation) turned toward the
exterior."
> Which is probably one of many reasons that John Dobson recommends using
> mylar space blankets rather than nylon for tube shrouds.

Also, if this cover goes far enough, it will shoeld the spider from
radiating to space over half a sphere or so, and materially lower the
radiation losses. At the cost of wind sensitivity, of course.

Nils Olof

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