Re: ATM Spider Design

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Stephen Dillinger wrote :

> *sigh* So much for communicating effectively with the written word...

My words exactly !

> Thus my question (at least I thought it was a question) as to the merits of the
> reduced number of vanes for a 3-vane spider as compared to the extra stability
> you get from a 4-vane spider.

1. less vane cross section = less diffraction
2. offset 4 vane can be _several_ times thinner than any other design,
   while still reatining superior stability
3. if you still don't believe this, ask Don Parker and the mob at ALPO,
   or read Solar System ... (Dobbins, Capen, Parker), section on 
   instruments; also see what Texerau says on the matter ...

> The part of this that I'm not sure about is how stable the boundary 
> layer is at the surface of the vanes? 

We have equilibrium, remember ? There WILL be slow movement around the
vanes, but not _turbulent_, therefore difficult to see in the eyepiece.
Also, second law of thermodynamics tells us that air in very vicinity
of cold vanes HAS to be of the same (or at least very close) temperature.

> Why should we assume that there is a stable boundary layer sitting on the
> diagonal supports?

It may not be the same air molecules, but the gradient will be stable
(again, that magic word equilibrium).

> If this effect is *significant* (I remain skeptical)

That's your right of course (to remain skeptical). But to all you skeptics
out there, instead of just sitting on your bums and doing nothing but 
criticizing others, how about YOU suggest something for a change ? Let's
compare our contributions in last two years or so. 
"If you don't make mistakes, you're doing nothing" an old saying goes.
I can't say that I'm proud of my mistakes, but I (and others I hope) have 
learned from those as well.

> You're ignoring the possibility that a solid spider vane that's not exactly
> parallel with the incoming beam will display a thicker cross-section.  A tilt
> of only 1 degree would substantially increase the apparent thickness of a
> spider vane, taking a 2" wide, 0.050" thick vane up to 0.085" apparent
> thickness. 

And you're ignoring fact that there must be TWO wires for every vane to make the
whole thing stable. The same 1 degree tilt would increase cross section to same
thickess as with solid one, as second vane starts to cut the beam too.

Well, that was enough for me, folks. I'm out of this thread, and out of discussion
for a while.

Clear skies to all,
Bratislav