AES wrote in news:siegman-9D530E.09164329042010
@bmedcfsc-srv02.tufts.ad.tufts.edu:

In article ,
Skywise wrote:

It's all about signal-to-noise ratio. It might interest you to
know that astronomers still bounce lasers off the reflectors
that were placed on the Moon by Apollo astronauts. They use
high powered lasers and use telescopes to detect the return
signal. Even though the lasers are 10's of watts or more, the
amount of light detected is measured in number of photons.

Any favorite or recommended publications on these lunar ranging
experiments?

(Either seriously technical archival-journal-level articles, or good
quality Scientific American or Popular Mechanics level articles -- but
preferably well-illustrated and available on line.)


After posting I found some links that I had once found before
when I was curious about the lunar ranging lasers.

Wikipedia articles:
http://en.wikipedia.org/wiki/Laser_R...etro-Reflector
http://en.wikipedia.org/wiki/Apache_...y_Lunar_Laser-
ranging_Operation


Home page for "Apache Point Observatory Lunar Laser-ranging Operation"
(aka APOLLO), which also included pictures of the equipment and of
the laser in action:
http://www.physics.ucsd.edu/~tmurphy/apollo/apollo.html


On this site are some PDF documentation. A few noted facts I found
skimming the first one describing the equipment as constructed,
http://www.physics.ucsd.edu/~tmurphy...710.0890v2.pdf

One month after Apollo 11, the 2.7 meter scope at McDonald Observatory
"...used a ruby laser with 4 ns pulse width, firing at a
repetition rate of about 0.3 Hz and ~3 J/pulse. This
station routinely achieved 20 cm range precision, with
a photon return rate as high as 0.2 photons per pulse,
or 0.06 photons per second."

"In the mid 1980’s, the McDonald operation was transferred
to a dedicated 0.76 m telescope (also used for satellite
laser ranging) with a 200 ps Nd:YAG laser operating at
10 Hz and 150 mJ/pulse."

"At about the same time, a new station began operating in
France at the Observatoire de la Cˆote d’Azur (OCA). Using
a 1.5 meter telescope, a 70 ps Nd:YAG laser firing at 10 Hz
and 75 mJ/pulse, this became the premier lunar ranging
station in the world."

Regarding the current APOLLO experiment:

"...combination of a 3.5 meter aperture and 1.1 arcsecond
median image quality near zenith translates to a high
photon return rate. Using a 90 ps FWHM Nd:YAG laser
operating at 20 Hz and 115 mJ/pulse, APOLLO obtains
photon return rates approaching one photon per pulse,
so that the requisite number of photons for one-millimeter
normal points may be collected on few-minute timescales.
To date, the best performance has been approximately 2500
return photons from the Apollo 15 array in a period of
8 minutes. The average photon return rate for this period
is about 0.25 photons per shot, with peak rates of 0.6
photons per pulse. Approximately half of these photons
arrived in multi-photon bundles, the largest containing
eight photons. APOLLO brings LLR solidly into the multi-photon
regime for the first time."


Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
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