Iridium flare diffuse compo

5 Nov 1997 17:08:34 -0800

Hi Mark,

You are correct that the atmosphere is partially responsible for "smearing"
out a flare so that it can be seen over a wider area.  However, the solar
angular size and the optical properties of the MMA's are the primary drivers
of the flare "cone of visibility".

If the MMAs were *perfect* mirrors, then they would produce a conical
reflection with the same angular subtense as the sun (~ 0.5 degrees).
However, every reflecting surface, no matter how smooth, redirects
some fraction of the incident radiation in a direction other than the
geometric specular direction.  Thus, outside the 0.5-degree cone,
the energy is not zero.  But it does drop off rapidly as you move off
axis.  The _slope_ of this drop off is directly related to the MMA's
surface properties.

The calculation of diffuse scattering from metallic surfaces is
extremely involved.  Suffice to say that you can be more than
0.25 degrees off-axis and still see a flare, but the brightness
of that flare drops rapidly with angle.  I'd guess about one
visual magnitude every 0.2 degrees.  --Rob