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

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

In the recent thread about wire spiders, I believe it was Mel Bartels who
said not to blacken the spider. On various web sites, I've seen many scopes
with their spiders and secondary holders left bare, and I wondered why.
Aside from paint just not adhering to stainless. 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."
	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. 


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