Re: [ATM] Spider vane color?

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

John Sherman wrote:

> Then there's the idea that black vanes exposed to the sky will get cooler 
> than the air, and therefore have a layer of cold air adjacent to them, 
> which acts as a much thicker vane ...

Hi,

This is testable - use any existing telescope and mount a dummy vane (or 
two) at an angle to the ordinary ones, then (bright) star test by defocusing 
until you see the diffraction patterns of the vanes - you could compare 
their blackness. Then warm part of the vane (with your fingers) and see how 
much it takes until that part is noticeably affected - or try cool it with a 
piece of metal cooled in the freezer. When I tried, it took a good deal of 
warming to make a difference, and I couldn't measure more than about 1 deg C 
difference between black or white/shiny fully exposed to the sky. The 
difference may be greater with drier air, I could imagine.

Anyway, with most full-size upper tube shields, little more than the edges 
of the vanes are exposed to the sky, anyway. With a minimally baffled upper 
tube, things may be more sensitive (making a better reason for wire spiders 
here).

Nils Olof



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