[ATM] Curved vane Vs Wire spider

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

My favorite chapter (so far) in Dick Suiter's Star Testing book is Chapter
9, Obstruction and Shading. His conclusion re spiders is that for most dark
sky observing, their effect is merely cosmetic, but for planetary
observing, "spider diffraction causes a much subtler degradation" by
reducing contrast. The thicker the vane, the brighter, wider, and shorter
the diffraction spike. He suggests that the use of extremely thick curved
vane spider is a mistake. I think it was Dr. Paul who said that curved
secondary holders should more properly be called "diffraction spreaders".
Where you once had spikes, now the smeared light is spread all over the
image, reducing contrast evenly and greatly.  Mel Bartel''s said somewhere
that with his wire spider on his 20", he directly observed one or both of
the moons of Jupiter. 
	I built a secondary holder from a couple wooden circles and some 1/4"
doweling and a wire spider based on the design in Paul's Telescopes for
Stargazing in about 1984. I used .020 guitar string, perhaps a little thin
as the spider is a bit fragile, but no diffraction spikes at all. Even my
Protostar spider seems horrible by comparison. On my 20" I've used carbon
tubing from a model shop instead of wooden dowels, and .5mm guitar string,
which may be overkill on the thickness. It is super strong. Even so, no
diffraction spikes. 
	Check out Suiter's chapter and try a simple wire spider. Mel Bartel's
spider seems very complicated to me - I have yet to figure out how the
other end ends up where it does. And if you use 2 or 4 strands of wire
instead on one, you can run electricity through them to heat your diagonal.
But I swear, once you go wire, you'll never be happy looking through a
telescope that uses a regular spider again. 
	Jay (cloudy out still, getting on for 3 months now, except for nights when
the moon is high and bright...)


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