[ATM] Some advice on 18pt cell design

["Olivier Biot" <olivier.biot@versateladsl.be>]

Hi list,

I've been playing with PLOP lately, and I was intrigued by the almost 
equilateral triangles in an 18 point cell. With my 14" f/5 primary I 
computed both a cell with fixed and with variable angles (the angle refers 
to the position of the 2 "outer" points of the supporting triangles, seen 
from the center of the primary).

I'm using plate glass, 356mm diameter, 1783mm focal length, 24mm edge 
thickness and 75mm obstruction diameter as parameters for PLOP. I selected 
an 18 points cell, once with fixed and once with variable angles.

The best design achieves a theoretical RMS error of 7.155E-7 (millimeter) 
and almost equilateral triangles with less than 1.5% difference in the 
distances from the center of gravity to the corners (.634mm to be precise).

Then I ran a Monte-Carlo with a considerable variation of 1% for both 
supporting radii (value: 0.01), and even a 1 degree error interval for the 
angle. These settings vastly cover the equilateral triangle "error". After 
10,000 tests I came to a maximum error of 9.975E-7 and an average of 
8.008E-7 which convinces me that it is OK to use equilateral triangles for 
the support.

There are big advantages to this approach: first, your triangles are 
balanced naturally, and second, you can even use disks instead of having to 
machine triangles! Of course, you'll end up with a heavier cell when using 
disks.

Did I miss something here?

Best regards,

Olivier 


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