Re: [ATM] Wire spider - er not quite the effect I want

[Dale Eason <atmpob@yahoo.com>]

The top and bottom wires must be aligned over one
another or they each become like a vane.

See
http://imaging.creol.ucf.edu/publications/Spider_Diffraction.pdf

for more info on spider diffraction

Dale Eason
--- Roger Chisholm-Batten <rogecb@tiscali.co.uk>
wrote:

> I have just finished a small 6inch f7 scope with a
> wire spider design almost 
> identical to Mel's Tridob spider.
> 
> The views of the moon at 'first light' two days ago
> (Wednesday) were 
> excellent at 215x.so ' Goodee goodee - Got a good
> one here Roger'  (The 
> finished mirror had only cost me 20GBPounds).
> 
> But swivelling round to Vega for an impromptu star
> test revealed not 4, no 
> not even 6 but 16 diffraction spikes...
> 
> Each wire crossing over kindly produced 2 definite
> spikes so it looked like 
> a firework going off - pretty but not what I had
> wanted.  Taking the 
> eyepiece out and looking at the moon through the
> focuser hole showed an 
> electrician's nightmare of wires radiating out
> against the bright moon.
> 
> In the design's defence my wires are bare stainless
> steel and not yet 
> blackened - ? use nose grease or sweaty hands ?
> 
> The 4 central points (and therefore the wires) are
> not straight above each 
> other as the secondary mirror and attached stalk had
> to be moved around to 
> collimate (which was easy once the knack of correct
> tension first had been 
> independantly discovered).
> 
> Have I forgotten or overlooked anything else?  One
> Luddite friend suggested 
> it is a mite overcomplicated (but then he likes
> curved spiders) but it looks 
> real good and has held collimation nicely in spite
> of two car journeys so I 
> am loathe to build a standard spider assembly.
> 
> Help gratefully received
> 
> Roger
> 
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> 



		
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