? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders & Diffraction Spikes

["Rod Shea" <RodShea@comcast.net>]

Art,

What problems do you see in making a carbon fiber spider?  I am getting set 
up to do the same, and am building a curing oven and vacuum setup.  I had 
planned on 3 straight vanes made of several thin unidirectional CF plies, 
and one thin ply of bidirectional Kevlar (to ?prevent catastrophic 
fracture), leading to a total thickness of 0.025" - 0.030".  I had thought I 
would angle the outer plies of CF diagonally, similar to the crossing wires 
in a wire spider, and include some doll house flat wire in one of the vanes, 
insulated by Kevlar.
I have not yet figured out the details of the center, but plan on molding in 
some hollow plastic rod to pass the secondary holder bolt through.  Any 
thoughts?

Thanks,

Rod

----- Original Message ----- 
From: <artbianconi@blast.net>
To: <atm@atmlist.net>
Sent: Sunday, November 14, 2004 10:50 PM
Subject: [ATM] Thin Titanium Spiders & Diffraction Spikes


> 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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