[ATM] wire spider - not the effect you wanted

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

	As with any spider, the thickness of the vanes is crucial. The smaller the
mirror, the more surface area of the mirror is covered by the vanes and the
greater the diffraction effect. While mirrors can always get smaller, there
is a point at which the thickness of the spider vane can't decrease any
more and still hold a secondary. So at some point, no matter how thin the
vanes, you're either going to be stuck with spikes or mount your secondary
on an optical flat. On my 12 1/2" f6, my 4 vane spider was of .0125" or
.33mm guitar string. I think this approaches or is at the limit of thinness
and fragility. Only on the very brightest of objects like Venus at low
power are very faint, very short spikes barely visible. On my 20" f5.7 the
wire for my 4 vane spider is .020 or .51mm thick. This actually might be
overkill. Like Mel Bartel's tri, you can move the whole telescope by
grabbing the wire, and still not lose collimation. Because the mirror is
comparatively very large and the spider thickness very small, there are
virtually no diffraction effects. 
	On the 12 I did paint it with a super light spray of flat black. The
diameters above are from a direct measurement with a digital thickness
gauge. The paint is so light it adds virtually no thickness, and in fact is
now missing on much of the spider. I've just recently learned that paint
does not stick to stainless steel. So I wouldn't bother blackening. If the
spider catches a lot of stray light, I don't know if that will add to the
diffraction effect or not. You could experiment with a flashlight and
maybe an extended tube baffle, as long as you protect the secondary and
focuser from errant beams. 	I may be wrong about this, but I seem to
recollect that mountain climber's ropes of the same diameters have to be
stronger per inch the longer the rope is. Conversely, the longer the rope,
the less its weight bearing capacity. If that's true, (and Ellen MacKenzie
will surely know if she reads this), then shorter lengths of smaller
diameter wire will make for stronger vanes than one long length, something
to consider for a smaller diameter mirror where you really want the
thinnest possible, but still securely strong supports. 
	Also with the .0125, you have to work with it carefully. If you pull it to
test it's strength, you can really cut yourself quite effortlessly. And
once you bend it or kink it, you can't straighten it out again. Don't even
trey.If you twist it twist it/wrap it too much, it will snap. So once it's
tied down or screwed down, any changes involve a new piece. But since the
stuff it practically free, you can make 4 or 6 or 10 attempts at each vane
w/o cost. The 020" I think you could pull a Volkswagon with. 


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