ATM Re: Spiders

["Mark D. Holm" <mdholm@telerama.com>]

Herb Watson wrote:
> 
> What's your opinion on warming the vanes slightly, like we would do the
> secondary? Would it be worth doing or would it be better to just polish them
> and wouldn't this add to the problem of stray light even with baffles?
> 

Warm air is undoubtably just as bad as cold.  I suspect the easiest scheme is the use of
wires or high tech strings rather than vanes.  Using small "vanes" such as wires held in
tension is the structurally most efficient way to minimize the mass and area of material
in the light path.  This of course means that the upper tube has to be stiff enough to
counter the tension.  

To avoid hot air problems, the vane temperature would have to be carefully controlled to
keep it equal to the air temp.  This is doable, but not as easy as a potentiometer in the
circuit.

If your scope has a fairly long tube extension anyhow, polishing might not be worth much. 
If the spider vanes are in radiative equilibrium with the tube and it is close to air
temp, the vanes emissivity (the parameter changed by polishing) doesn't matter as much.

On the other hand, at low angles, most flat black treatments aren't really that black. 
Polished metal might not be that much worse.  Also, a flat black surface scatters light
from a wide range of angles to a wide range of angles.  A polished surface reflects light
from specific sources at specific angles.  I haven't worked through this in a quantitative
way, but I think it may be true that polished spider vanes might end up reflecting less
light into the final image than black ones!

Mark Holm
mdholm@telerama.com