Re: [ATM] How dark is dark

["Don Clement" <don_clement@charter.net>]

The baffling issue is more about geometry than a specific coating. Even the
"blackest" surfaces will reflect some light. With the proper geometry even a
mirror surface can be made to absorb almost all light. A great example of
this is to stack of mirror finish razor edges. (The old  SS double edge
safety razor blades) Taken individually, each razor edge has a mirror like
finish. But when stacked and viewed edge on, the collective mirrored edges
appear to be perfectly black. The light is bounced back and forth between
the almost mirrored surfaces. Each time a very small percentage of light is
absorbed until after many reflections almost all the light is absorbed.

I use geometry when possible to trap light or reflect light out of the
detector entrance. An example is the flexible bellows used in my focusers.
The internal surface of the flexible bellows has 3/8" thick deep pleats,
which trap and absorb light in the same way that light is absorbed by the
stack of razor edges. I make a black anodized 3" to 2" adapter with a
45-degree cone that fits inside the flexible bellows. Black anodized
surfaces are partially reflective, especially at longer wavelengths where
silicon ccds are most sensitive. The view looking into the 3" to 2" adapter
inside the flexible bellows is like looking into pitch black. The reflected
light from the anodized cone is trapped in the deep pleats of the flexible
bellows. I also thread the internal black anodized surfaces with a sharp "V"
thread for the same reason. The sharp "V" threads behave as a helical stack
of razor edges to trap and absorb light. The sharp "V" threaded internal
surface appears much blacker (less reflected light) than a surface that is
smooth black anodized.



Don Clement

Running Springs, California
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