Re: [ATM] wire spiders

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Just FYI, I will mention how I make wire spiders.  I'm not saying mine
are the best, but maybe it will be food for thought for someone.

I use wire spiders made with 0.015" wire for moderate-sized mirrors (a
4" Cassegrain secondary) and I have used 0.010" for a 1.25" and 2"
Newtonian secondary.  Got the stainless wire from McMaster Carr.

My wire spiders look like the outline of a normal spider.  The wires 
don't cross.  I use the offset design.  There are two pieces of wire 
in the spider, so breakage of one will not cause the secondary to 
fall.  Each piece of wire makes up two "vanes".  Each vane is 
triangular, with the short side of the triangle formed by the 
secondary holder, and one vertex at an eye bolt that is mounted in the 
secondary cage (at the end of the "vane").

The wire wraps around this eye bolt once.  (It could be wrapped
several times.)  When the wire is sufficiently tentioned to hold the
mirror still, friction keeps the mirror from moving much, but it will
move (down the tube along the optical axis) if it is whacked.  The 
wire simply slides around the eye bolt.  A dab of epoxy will prevent 
this from happening, but there may be more negative things that happen 
if it is whacked.  (I have not done this to any of the three wire 
spiders I have made yet - I just avoid whacking them.)

It takes me about 10 minutes to string a wire spider.  I make the
central hubs from 1/2" thick PVC plastic.  It is non-conducting and
this allows a secondary heater to be powered.  That is the other
reason for using two pieces of wire.

Some photos of my wire spiders are seen here:

http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/12in_cass/cass_article/cass.html
See figures 13-15, in particular, just past the middle of the article.

http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/10in_F8p8/10in_F8p8.html
See the three photos of the secondary cage, also near the middle of
this article.

For wire spiders, I like the 4-vane design better than 3-vane because
less tension is required.

	Mike Lockwood


Daniel Fundo wrote:
> I've written details of my design previously here, and have sent notes
> to several individuals concerning it.  Mine is a more comventional
> design; Mel's is optimized for his tri-Dob configuration.
> 
> The one thing I disagree with is the use of a continous piece of wire. 
> In the event of breakage, you need to rewire and realign, something that
> isn't easy to do in the field or at night.  I can replace a broken wire
> in less than 5 minutes and realignment is almost guarenteed when
> complete.
> 
> While Mel's degign is a bit more compact (longitidinally - along the
> primary optical axis), it does require the wires to cross.  I can align
> the wires in my spider, so there is half the width....not that it really
> matters since the wires are all but invisible except around the
> brightest stars.
> 
> *****-----
> Dan F.
> Former Secretary/MAS
> Memphis, TN
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 




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