Re: ATM 3-Vane or 4-Vane Spiders

[Myosin@aol.com]

Some time inthe 1950s there was a super article in J Opt Soc Amer analysing 
empirically the problems you bring up. Somewhere I have a copy of it and if I 
can find it will post the reference, but in a nutshell, they found that TOTAL 
diffraction is the problem, and nince a 3-vane has 1/4 fewer vanes, it has 
correspondingly less diffraction. The thinner the vanes the better, but best 
are no vanes at all. The mass of the vanes radiates to the sky and is a bit 
cooler than ambient air and consequently the vanes are optically much 
"thicker" than their physical thickness. a thin wire is best of all - very 
small mass. 

In the late 50's, based on the data in this article, I built a three vane 
spider for a 12.5 long focus reflector. It was quite stable. I used phosphor 
bronze wire pulled very taut. Unfortunately, for a number of reasons the 
scope never saw first light so I cannot tell you how the spider behaved in 
use, but mechanically it was fine.

I also seem to recall that that article was summarized in S&T but am not 
certain

Frank