Re: ATM 3-Vane or 4-Vane Spiders

["Jim Miller" <jim@jtmiller.com>]

Reread the ATMJ article last night and here is my summary:

1. Total diffracted energy in spikes is proportional spider vane obscuration
which is a function of vane thickness and misalignment.
2. Spike length along which energy is spread is inversely proportional to
obscuration width.
3. Number of spikes is 2x number of vanes.  In the case of a 4 vane
non-offset spider you get 8 spikes which overlay to 4 spikes of double
intensity and same length.  Offseting blurs the same energy across a larger
pattern of four proportionately reduced intensity spikes.
4. Metal vanes appear to have a optical size somewhat larger than their
physical size due to thermal effects.
5. There is no perfect solution.  Beauty is in the eye of the beholder.

So what to do:

Make the vanes as thin as possible to minimize energy in the spikes and
ensure the spikes are as long as possible.  Impact of this is that spike
intensity is inversely proportional to the vane width including misalignment
*squared*.  Thin and aligned is *very* good!  Thin and unaligned is no
better than fat.  Thin vanes must be alignable and must not curl along their
width when tensioned.  This argues for vane endpoint attachments which
enforce flatness in addition to providing tension.

If adequate (typically tensile) strength is available to support the
secondary over the length required, then the reduced obstruction of three vs
four vanes of equal size, equal alignment and material composition will
reduce the total amount of energy in the diffraction pattern by 33% and
spread it to six vs four radial locations, making each of the resulting six
spikes (.67*.67) = .44 the intensity of the previous four.  Note this is as
good as it gets...assuming the vanes stay aligned.  Is it worth it?

This leads me to consider, probably in priority order:
1.  Minimize secondary assembly suspended mass to reduce total strength
required and therefore permit thinner vanes.  Aluminum is 1/3 the density of
steel, nylon or delrin is about half the density of aluminum.  CF materials
can be even lighter.  Select size and material according to strength needed.
2.  Use wires to limit misalignment to two wire diameters if sufficient
tensile strength is available.  Total width needs to be less than
vane+misalignment to provide benefit.
3.  Use three vanes (six wires) vs. four vanes (eight wires) if sufficient
tensile strength.
4.  Consider non-metallic "wires" such as aramid fiber (kevlar, spectra,
...) fishing line.

have fun
jtm