Re: ATM 3-Vane or 4-Vane Spiders

[William Mc Hale <wmchal1@umbc.edu>]

On Thu, 4 Oct 2001, Scot Radowski wrote:

>
> When taking diffraction patterns and general light interruption into
> account, is there a good, better, best when it comes to spiders?
>
> I've seen designs where there is but a single arm for the spider, or
> 2 even.  Standards seem to be 3 and 4.  Is there a trade-off?
> Triangles are supposed to be more stable than squares, but does this
> really count when talking about vanes on a spider?
>
> Then there is the question of material and thickness.  I would
> imagine that its important to obstruct the view as little as
> possible.  Is there a good formula to consider?  Percentage of light
> loss or whatnot?
>
> Any help is appreciated.

Well ultimately it is the total area that you need to concern yourself
with.  In practice I believe it is easier to make a really good 4 vane
spider than a 3 vane spider.  The 4 vane will have more prominent
diffraction spikes (4 of them) but a 3 vane spider will have 6 spikes thus
spreading light over more of the field potentially reducing contrast more.

Bill

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