Photographing the Apollo LMs

No need to take images of the LM descent stages, the astronauts left
retroreflectors on the surface for the purpose of being able to measure the
true distance of the moon from Earth. They still work, simple fire a strong
enough laser beam at them and enough is reflected to measure the earth-moon
distance.

I think this is still done every few years or so.

Adam



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John Young, while in the confined space of the lunar module with Charlie
Duke on the moon:

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"adam bootle" wrote in message ...
No need to take images of the LM descent stages, the astronauts left retroreflectors on the surface for the purpose of being
able to measure the true distance of the moon from Earth. They still work, simple fire a strong enough laser beam at them and
enough is reflected to measure the earth-moon distance.

I think this is still done every few years or so.

Adam

On average, less than one photon is detected in the return from each pulse.

The amount of expensive equipment required to perform the measurement
precludes your average moon-hoax bonehead from believing it's what you're
telling him it is.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

On average, less than one photon is detected in the return from each
pulse.

The amount of expensive equipment required to perform the measurement
precludes your average moon-hoax bonehead from believing it's what you're
telling him it is.


I was rather hoping to encounter the above average bonehead rather than the
common variety ! You know the kind, they can actually sit and listen and
reason for themselves other than depend on arguments they have seen on the
television !

Amazes me when you ask if it was a fake why didn't the USSR blow the whistle
on it, and they actually turn around and say that the USSR was in on it, lol
!

Anyway, time for me to back outside and cloudwatch !

Adam

In message , Fleetie
writes
"adam bootle" wrote in message
...
No need to take images of the LM descent stages, the astronauts left
retroreflectors on the surface for the purpose of being
able to measure the true distance of the moon from Earth. They still
work, simple fire a strong enough laser beam at them and
enough is reflected to measure the earth-moon distance.

I think this is still done every few years or so.

Adam

On average, less than one photon is detected in the return from each pulse.

The amount of expensive equipment required to perform the measurement
precludes your average moon-hoax bonehead from believing it's what you're
telling him it is.

The boneheads and the people making money claim that unmanned probes
could have landed the reflectors, and that the pictures taken of
reflectors on the moon by the astronauts show they were set up wrongly.
But is it really impossible for a well-equipped amateur to get a signal
from a reflector? Given the availability of large telescopes, lasers on
the surplus market, narrow-band filters, and CCDs to isolate the signal
I'm surprised no-one's tried.
--
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Remove spam and invalid from address to reply.

"Jonathan Silverlight"
The boneheads and the people making money claim that unmanned probes could have landed the reflectors, and that the pictures
taken of reflectors on the moon by the astronauts show they were set up wrongly.
But is it really impossible for a well-equipped amateur to get a signal from a reflector? Given the availability of large
telescopes, lasers on the surplus market, narrow-band filters, and CCDs to isolate the signal I'm surprised no-one's tried.

I believe they use a ONE METRE telescope both for transmission and reception
of the pulse. (The same telescope, IIRC.) And they still get less than one
photon per pulse.

Actually, I just found this link:

http://home.austin.rr.com/broadb/chr...eflectors.html

This link says the scope they use is ~0.75m diameter. It also says that
each laser pulse has a power of 7.5 GIGAWATTS. You don't get that kind
of laser cheap, especially with such a short pulse (only ~200ps).

Unless your amateur has won the lottery, I doubt they'd see much.

Apparently they are going to upgrade their equipment:

http://optics.org/articles/news/8/1/23

They hope to get 5 to 10 photons per pulse with a 3.5 METRE telescope.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

JRS: In article , dated Tue, 14
Dec 2004 00:15:37, seen in news:uk.sci.astronomy, Fleetie
posted :
"Jonathan Silverlight"
The boneheads and the people making money claim that unmanned probes could
have landed the reflectors, and that the pictures
taken of reflectors on the moon by the astronauts show they were set up
wrongly.
But is it really impossible for a well-equipped amateur to get a signal from a
reflector? Given the availability of large
telescopes, lasers on the surplus market, narrow-band filters, and CCDs to
isolate the signal I'm surprised no-one's tried.

I believe they use a ONE METRE telescope both for transmission and reception
of the pulse. (The same telescope, IIRC.) And they still get less than one
photon per pulse.

Actually, I just found this link:

http://home.austin.rr.com/broadb/chr...eflectors.html

This link says the scope they use is ~0.75m diameter. It also says that
each laser pulse has a power of 7.5 GIGAWATTS. You don't get that kind
of laser cheap, especially with such a short pulse (only ~200ps).

Agreed. But if it were one pulse per second, it would only be 1.5 watts
average, which sounds much more reasonable. High peak powers are easier
than high continuous powers.

One might modulate the 1.5W laser with a pseudo-random data stream, and
correlate what the telescope thinks it sees with a 2.5-second delayed
version of the modulation, thus rejecting on average most noise, and
getting a range resolution of a bit-length or so.

The return 'beam' would be reasonably wide, so sender and receiver could
be well-separated; or transmit for 2 seconds, wait half a second, watch
for 2 seconds, wait, ... .

Best not done in the visible, to avoid alarming the neighbours.

In order to detect the reflectors convincingly, one must also look
elsewhere on the Moon, to show that nothing is found there.

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In message , Fleetie
writes
"Jonathan Silverlight"
The boneheads and the people making money claim that unmanned probes
could have landed the reflectors, and that the pictures
taken of reflectors on the moon by the astronauts show they were set
up wrongly.
But is it really impossible for a well-equipped amateur to get a
signal from a reflector? Given the availability of large
telescopes, lasers on the surplus market, narrow-band filters, and
CCDs to isolate the signal I'm surprised no-one's tried.

I believe they use a ONE METRE telescope both for transmission and reception
of the pulse. (The same telescope, IIRC.) And they still get less than one
photon per pulse.

Actually, I just found this link:

http://home.austin.rr.com/broadb/chr...eflectors.html

This link says the scope they use is ~0.75m diameter. It also says that
each laser pulse has a power of 7.5 GIGAWATTS. You don't get that kind
of laser cheap, especially with such a short pulse (only ~200ps).

The pulse power is 7.5x10^9 watts, but the average power is only 15
watts. Do you need very short pulses to get the required accuracy? And 1
meter _is_ bigger than the biggest amateur telescope, AFAIK. But I
suspect the biggest problem is safety and legislation. That sort of
power output is used in surgery, and you don't want it pointing up into
the sky without extreme precautions.

Jonathan Silverlight wrote:

In message , Fleetie
writes

"Jonathan Silverlight"

The boneheads and the people making money claim that unmanned probes
could have landed the reflectors, and that the pictures
taken of reflectors on the moon by the astronauts show they were set
up wrongly.
But is it really impossible for a well-equipped amateur to get a
signal from a reflector? Given the availability of large
telescopes, lasers on the surplus market, narrow-band filters, and
CCDs to isolate the signal I'm surprised no-one's tried.


I believe they use a ONE METRE telescope both for transmission and
reception
of the pulse. (The same telescope, IIRC.) And they still get less than
one
photon per pulse.

Actually, I just found this link:

http://home.austin.rr.com/broadb/chr...eflectors.html

This link says the scope they use is ~0.75m diameter. It also says that
each laser pulse has a power of 7.5 GIGAWATTS. You don't get that kind
of laser cheap, especially with such a short pulse (only ~200ps).

The pulse power is 7.5x10^9 watts, but the average power is only 15
watts. Do you need very short pulses to get the required accuracy? And 1
meter _is_ bigger than the biggest amateur telescope, AFAIK. But I
suspect the biggest problem is safety and legislation. That sort of
power output is used in surgery, and you don't want it pointing up into
the sky without extreme precautions.

The reason they use a 1m (or larger) aperture is to make sure the energy
density in the outgoing beam is lower than the threshold for serious eye
damage. Blink reflex doesn't help much with a sub microsecond length
pulse. The maximum peak envelope power may sound alarming but the mean
power is much more reasonable. A bit like grotty Dixons "hifi" gear.

The short pulse (or at least sharp switch on/off transition) is because
the whole object of the excercise is to measure the distance to the moon
to high accuracy. Light travels rather fast so the timing has to be
very very precise.

Regards,
Martin Brown

"Jonathan Silverlight"
The pulse power is 7.5x10^9 watts, but the average power is only 15 watts.

Irrelevant really. I said "per pulse". They could send only 1 pulse
and they'd still get the same 1 photon back per pulse.

But yes, they need such short pulses (which drives that power figure
higher) to get the temporal/spatial resolution, as you say:

Do you need very short pulses to get the required accuracy? And 1 meter _is_ bigger than the biggest amateur telescope, AFAIK.
But I suspect the biggest problem is safety and legislation. That sort of power output is used in surgery, and you don't want it
pointing up into the sky without extreme precautions.

Also true!

Naah, as I said, IMNSHO, it's out of reach of "normal amateurs" for many
reasons.

No point aiming your green laser pointer up there and expecting to see a
little twinkle from the moon a few seconds later!



Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

"Martin Brown"
The reason they use a 1m (or larger) aperture is to make sure the energy density in the outgoing beam is lower than the
threshold for serious eye damage.

Simply not true.

They use a large beam because divergence is inversely proportional to
beam diameter. They NEED as small a spot size as possible on the moon,
to increase the fraction of the laser power that hits the retroreflectors.

Blink reflex doesn't help much with a sub microsecond length pulse. The maximum peak envelope power may sound alarming but the
mean power is much more reasonable. A bit like grotty Dixons "hifi" gear.

Nuh-uhh. They arrange it so that there aren't going to be any aircraft
flying in the beam path when the measurements are done. Eye-safe is
irrelevant here. (Though even still, ISTR they STILL have extra safeguards
to cut off the pulse train if an early return is detected, but I am speaking
from memory here.)

The short pulse (or at least sharp switch on/off transition) is because the whole object of the excercise is to measure the
distance to the moon to high accuracy. Light travels rather fast so the timing has to be very very precise.

Yeah.

I noticed that the according to the URLs I cited yesterday, the new system
will have a pulse width of 100ps, while the older one uses 200ps.

Hmm, in 100ps, light travels about 100E-12 * 3E8 = 0.03 m = 3cm in vacuo.

Course, you could probably do better than that if you can detect the arrival
time of the rising edge of the pulse rather than just the pulse as a whole.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk