Re: (ATM) Wire Spiders

[Chuck Grant <grant@aretha.llnl.gov>]

> From: "Patrick Martin" <PAT@css.uoguelph.ca>
>
> As I 
> understand it you would need six wires to hold the diagonal assembly 
> as opposed to four legs in a traditional setup.  I would think to 
> achieve the same rigidity in mounting the diagonal, the diameter of 
> the wires would need to be greater than the thickness of the metal 
> legs.  Now, the apparent thickness of the legs for an off axis star 
> would of course be greater (given a .02" x 1" x tube radius" leg the 
> apparent width 1 degree off axis is about .04"), but I would still 
> think the diffraction effects of the wires (unless of a "very" small 
> diameter) would be greater.  Am I wrong?  


The on axis obstruction is greater for wires for the same siffness.
But the theory is that there is a layer of air close to the flat
metal spider leg which is colder than the surrounding air due to
proximity to the spider leg.  The spider leg is colder than the
ambient air due to radiative heat loss to the sky.  So this cold air
refracts the light near the spider legs causing stronger spikes in
the diffraction pattern than what is caused by the obstruction
alone.

Wire spiders have less air in contact with the cold metal and thus
give less refraction around the spider.  That's the theory anyway.

As far as rigidity, you can spread the wires apart farther (along
the optical axis) than the width of a flat leg regaining some of the
lost strength and stiffness.

Chuck