[Laser] Re: street light EME

[Tim Toast]

Here are some thoughts i had on reflections from the moon. 
The background solar noise from earthshine is maximum at
new moon and minimum at full moon. As seen from the moon,
the phase of the earth is opposite that of the moon. So
that at full moon almost no sunlight is being reflected
from earth onto the moon - other than what is refracted
onto it during an eclipse. So it makes sense that the best
times to try this are when the earth shine noise is the
least, somewhere between first quarter and last quarter
phase. As the moon passes first quarter, less and less dark
area is available and also the more off-center your aim.
Reflections are coming from increasingly steep angles
nearing the edge with the increased losses. I would expect
the strongest reflections to be from near the center of the
moon. I would also expect the lowest of all noise to be
during a total lunar eclipse, when there is no earth-shine
and no direct sunlight near the FOV. But the earth refracts
a sizable amount of light onto the moon during eclipses -
being brighter the dirtier the earth's atmosphere. So
anyway, it's a trade off between several things when trying
to find the best time to try this. 

Imagine if the moon were a perfect sphere with a mirrored
surface. When looking at it from earth, you would see a
reversed image of the earth maybe only 1/20 of a degree
wide (a rough guess). To see this effect take a look in
your passenger side rear view mirror - "objects are closer
than they appear". The FOV required to capture all of this
area would be quite a bit smaller than the moon itself
appears in the sky. Most of the reflected energy from earth
would be in a small area close to the center of the moon's
face, perhaps only 1/10 of the moon's diameter or about 3
minutes of arc wide. The moon is not a perfect spherical
mirror-ball of course and the streetlight radiation and
other light coming from the earth illuminates the entire
moon's face. So, the entire moon's surface (the usable dark
surface anyway) reflects the light back to some degree. 

* as a side note: At radio frequencies the moon appears
much smoother than at optical wavelengths, and this
"sphere" effect may be more pronounced. But generally at
radio frequencies, the beam widths are larger than the moon
and it may be hard to even notice this effect at all with
small-ish antennas. Some experienced moonbounce people can
probably shed some light on that.
 
Another thing about reflections from the moon (at optical
wavelengths) is that the lunar surface has a slight
retro-reflective property due to it being an "odd"
non-lambertian reflector. It can be seen in astronaut's
photographs from the lunar surface, where the camera is
pointed 180 degrees away from the sun. So the moon, as a
whole, reflects earth's light back towards earth
significantly better than you would expect from a surface
of a given albedo. It is not as cut and dry as you might
think.

http://jeff.medkeff.com/astro/lunar/obs_tech/albedo.htm

Either way, it sounds like another reason to include as
much of the usable surface as possible in the detector FOV
in order to capture the maximum energy. A wider FOV lets in
more noise though of course.

If it's possible to detect this modulated light at all,
then i can't help but wonder if one small modulated
searchlight, well focused and turned toward the moon, could
outshine a few cities worth of stray light leaking upward
from their outdoor lighting, most of which is pointing down
at the ground. 



       
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