[ATM] wire spider

[Jay Kirkland <jaykirk2@compusmart.ab.ca>]

	My spiders are the type found in Paul's Telescopes for Stargazing, pg 118.
Because of the distance between the wire attach points on the central hub,
I think if it's made with reasonable care, the wires naturally end up
pretty well paired behind each other close to exactly. I have to admit it's
not something I paid any attention to. Given the thinness of the wire, even
if they are not lined up exactly behind each other, the vanes are still
less than 1/2 as thick as they would be for a comparable "normally" vaned
spider. 
	I hate to bring up Suiter's Star Testing book again, but I just like it so
much. In his chapter on Obstruction and Shading, he says that the
degradation in image quality for a 4 vaned spider  with vanes the thickness
of 1/128 diameter of the mirror is slight compared to a spiderless 20%
obstructed mirror. Jan van Gastel in a Curved Vane vs Wire Spider post a
couple of weeks ago linked to an article that a lot of Suiter's chapter
seems to be reduced from. 1/128 of 6" = .047" or almost 4 times as thick as
I used on my 12". So it would appear as far as actual image quality itself
goes, you're well within the bounds of vane thinness. Even if they're not
lined up behind each other. The real concern is aesthetic, which to my eye
is still a real concern. On my 6" f5, I never even considered a wire
spider. Just way too complex for such a small space, I thought (lazy). I
made a spider based on Gary Wolanski's  (see his web site for beautifully
made spiders) with vanes made of the thinnest 1" stainless sheet available
from my local model shop. It's impossible to cut metal of that thinness and
not mess up the edges, accidentally creating a thicker vane. Note if you go
with Wolanski's off centred vanes, you have to offset the spider mounts on
the tube to prevent 8 diffraction spikes. 
	But I don't know what to say: the bigger the mirror, the relative
thickness of the vanes can decrease until the diffraction effect nearly
disappears. But with smaller mirrors, perhaps there is no way of
eliminating the diffraction spikes unless you go with a flat. From what
Suiter says, the effect on the actual image of vanes (in a 4 vane
configuration) that thin should be negligible. So all I can suggest is to
experiment with various types and styles of secondary supports until you
find whatever pleases your eye best.  


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