RE: ATM 3-Vane or 4-Vane Spiders

["Bell, Andrew" <AMB2@pge.com>]

FWIW - John Dobson has long recommended three-vane spiders made from wood
(e.g., cedar shingles), rather than metal.  He likes three vanes largely
because it gives you options for the largest possible unobstructed aperture
when you use an off-axis mask.  As best I recall, one of the reasons he
likes using wood is so as to minimize thermal effects.  It does seem like
the extra thickness is a down side to using wood, though. 
-- Andrew Bell
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Date: Tue, 9 Oct 2001 14:49:37 -0500
From: "Jim Fitch" <jf@arn.net>
Subject: RE: ATM 3-Vane or 4-Vane Spiders
>
> The mass of the vanes radiates to the sky and is a bit
> cooler than ambient air and consequently the vanes are
> optically much "thicker" than their physical thickness.
>
> Anyone seen this in reality?
> Nils Olof
>
I insulated two vanes of a four vane spider with Black Flocked Paper to
see if I could demonstrate any thermal effects on the diffraction spikes.
Even though the insulated vanes had a thicker profile the corresponding
spikes appeared dimmer and were thinner and longer. Unless I looked
at the wrong spikes (I don't think so) I think the temperature difference
of the vanes does affect the size of the diffraction spikes.