[Laser] Lunar downlink

[Glenn Thomas]

At 12:55 AM 2/23/2006, you wrote:
> > A good test would've been to shut the laser off and see how many
> > photons (thermal radiation from the atmosphere, the telescope or
> > even the mirror itself) they got then! I don't know if they did
> > something like this.
>
>Perhaps I'm missing something, but isn't the dominant noise caused
>by earthshine?  It seems obvious that you have to do this experiment
>at night.  Also, you would need to do it when it's night at the lunar
>target.  For example, if you did it at half moon, then the moon is
>seeing approximately half earth.  So, the target area is illuminated
>by lots of earthlight.

I suspect you mean moonshine rather than earthshine. By "earthshine" 
I take it you mean sunlight reflected from the earth (albedo) and 
then re reflected from the moon. This particular version of moonshine 
is not liquid type(*sigh*), rather sunlight reflected from the moon. 
The point of the test is to see what kind of signal they had from all 
noise sources, known and unknown.

The graph shows a cluster of lunar photons arriving as a cluster. 
This is credible. It is not consistent with a claim of receiving 0.01 
photons per pulse. Of course, on 
http://physics.ucsd.edu/~tmurphy/apollo/ they say that they are 
recovering several photons per pulse rather than the not-credible 
0.01 photons/pulse described elsewhere.

BTW, thanks for the website reference!

>You could avoid most of the noise by testing during a lunar eclipse,
>but I believe that is not necessary, because the range gate eliminates
>99.9996% of the noise (open for 100 nanoseconds every 25 milliseconds).
>http://physics.ucsd.edu/~tmurphy/apollo/timing.html

A 100 nsec window means that a priori they had to know the range to 
within 30m. Thus a 100m position error for the telescope could be 
trouble, especially when the moon is at a low elevation.

The website (http://physics.ucsd.edu/~tmurphy/apollo/basics.html in 
this case) says that they expect to receive between 1 and 5 photons 
per pulse. I don't know how reasonable this estimate is as I haven't 
(and probably won't) plow through the math. The website also says 
that they use a range gate along with a band pass filter and a 
spatial filter (a shadow mask) to reduce the noise level. 
Unfortunately, they didn't describe the observed noise level or how 
they deal with it.

Given the extra detail, what I originally thought not credible now 
looks reasonable, for the UCSD Apollo at least! I still wonder about 
the 0.01 photons/pulse claims though...

>73,
>
>Stewart KK7KA

73 de Glenn WB6W



WAR IS PEACE!
FREEDOM IS SLAVERY!
IGNORANCE IS STRENGTH!