[Laser] Lunar downlink

[Jim Moss]

Note that the Apollo website also show results of their tests.
http://physics.ucsd.edu/~tmurphy/apollo/highlights.html
There's even results from JANUARY...
This is an active site.
Jim
N9JIM

--- Glenn Thomas <glennt at charter.net> wrote:

> 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!
> 
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