[ATM] Thin Titanium Spiders & Diffraction Spikes

[artbianconi@blast.net]

Jim, Thanks for the response. Let me tease the example you gave 
so I can project the theory some.

If a thinner spider of a given length, generates less of Huygens 
wavelets than the same spider when it's thicker, that suggests 
that the diffraction we see is a function of surface area of the 
spider exposed to the incoming image.

If that is correct, then a curved spider that has the same 
projected surface area will generate the same number of Huygens 
wavelets as a conventional one. However, because it is curved, 
the distribution of those wavelets is more uniformly distributed 
over the image field and the effect on contrast is thus not as 
apparent. The degradation is still there, but it's not obvious.

Right so Far?

Here is why I ask: I am in the process of finalizing the design 
of an open truss OTA for use with a 15" f-4.3 mirror that is 
under development. My awareness of the spiders diffraction 
"spikes" prompted me to define a semicircular spider made from Bi-
Directional Carbon fiber. I know the material having worked with 
it before.

The choice was predicated on the fact that it is remarkably 
stiff, independently of being curved and fixed at each end and 
thus ideally suited to supporting a secondary that is 
cantilevered out in the middle of space. However, a supportive 
member of the ATM list (Matt Tudor) has advised us of a resource 
(Online Metals) for 0.020" titanium in sizes that lend themselves 
to fabricating a spider. While I have a heated autoclave and a 
vacuum bagging bench, I am not certain I can fabricate a curved 
spider from CF with a wall thickness of 0.020".

Am I splitting hairs pursuing a result that will go unnoticed or 
should I continue to "go for it"?

Thanks

Art Bianconi
Milford, NJ

> >Art Bianconi wrote:
> >I am unclear how spiders cause diffraction stars so I am
> >uncertain if attempting to use super thin material like this
> >helps eliminate them.
> >
> >Can anyone shed some light (sic!) on this? Thanks

> Jim Responded: 
> You can't eliminate the diffraction spikes, but you can reduce
> the amount of light in 'em.  Here's how thin spiders help:
> 
> Imagine the light waves impinging on the spider.  The way the
> waves get to be zero amplitude (absorbed) by the spider is to
> generate Huygens wavelets of opposite phase at the spider.  
> These  wavelets spread out, interfering with the incoming
> light, and produce the spikes on the image plane.  Less light
> in the wavelets means dimmer spikes, and thinner spiders mean
> less light in the wavelets.  Also, some thought on this senario
> will explain how curved spiders spread out the spikes.



> 
>          -- Jim Burrows
>          -- mailto://burrjaw@earthlink.net
>          -- http://home.earthlink.net/~burrjaw
>          -- Seattle N47.4723 W122.3662 (WGS84) 
> 
> 
> 


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