Starship Asterisk*

by **Nitpicker** » Wed Apr 23, 2014 11:01 pm

Chris Peterson wrote:
Nitpicker wrote:It isn't. The beam becomes invisible a little before the Apollo 15 landing site.
If we're picking nits, I'd say the beam becomes invisible about 385,000 km before the Apollo 15 landing site.

I was referring to how the beam appears in the APOD, but by all means, pick away. If you had not clipped out the rest of my comment, my meaning would have been more clear, I think.

(I also failed to see the answers by you and Anthony, to Claudio's question, before I responded.)

by **Chris Peterson** » Wed Apr 23, 2014 10:27 pm

Nitpicker wrote:It isn't. The beam becomes invisible a little before the Apollo 15 landing site.

If we're picking nits, I'd say the beam becomes invisible about 385,000 km before the Apollo 15 landing site.

by **Nitpicker** » Wed Apr 23, 2014 9:21 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.

It isn't. The beam becomes invisible a little before the Apollo 15 landing site. It looks visible for almost the whole distance, simply due to the perspective.

(BTW, I wrote earlier that the gratifying thing was it appeared the beam was pointing almost directly toward the landing site. I said this because I could imagine an image taken from a different angle, where the beam might appear less accurate.)

by **Anthony Barreiro** » Wed Apr 23, 2014 6:01 pm

Chris Peterson wrote:
Anthony Barreiro wrote:It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!
Indeed, for all the stars ever pointed out with laser pointers- thousands and thousands- the beams are still on their way towards those stars. If we could somehow see them, the night would be permanently crossed with green lines.

And if there are planets orbiting those stars, and if some of those planets have appropriately reflective surfaces at exactly the right angle, maybe someday we'll see a photon or two of 532 nm (plus redshift or minus blueshift) light reflected back at us!

by **Chris Peterson** » Wed Apr 23, 2014 5:53 pm

Anthony Barreiro wrote:It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!

Indeed, for all the stars ever pointed out with laser pointers- thousands and thousands- the beams are still on their way towards those stars. If we could somehow see them, the night would be permanently crossed with green lines.

by **Anthony Barreiro** » Wed Apr 23, 2014 5:14 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.

It's a trick of visual perspective. The Moon is the apparent vanishing point of the laser beam. We use this same trick to use laser pointers to point out particular stars in the night sky, although the stars are of course much farther away than the Moon!

by **Chris Peterson** » Wed Apr 23, 2014 4:30 pm

Claudio Sánchez wrote:How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.

You are only seeing a few thousand meters of illuminated air (mainly dust).

by **Claudio Sánchez** » Wed Apr 23, 2014 4:16 pm

How can the beam be visible its full way to the moon? Beyond earth atmosphere there's no particles where the laser can be scattered.

by **Boomer12k** » Sat Apr 19, 2014 12:01 am

Shooting the Moon???

:---[===] *

by **JohnD** » Fri Apr 18, 2014 10:09 pm

Ron,
If you have read all the stuff about the Moon distance experiment, you will appreciate that:
A/ The light is returned from specially designed reflectors, left on the Moon by the Apollo landers and a Russian probe.
and
B/ even with that help, the return after travelling a third of a million miles, twice, is about one photon.
and
C/ Curiosity's laser has a range of 0.006 miles
John

by **Ron-Astro Pharmacist** » Fri Apr 18, 2014 9:04 pm

I know this sounds like something out of Reagan's "Star Wars Defense" but this APOD makes me wonder if a powerful laser could be utilized in a way to analyze atmospheres or surfaces of some of our solar system bodies similar to how Curiosity uses its laser capabilities?

http://www.space.com/23851-mars-rover-c ... -shot.html

by **neufer** » Fri Apr 18, 2014 5:58 pm

Click to play embedded YouTube video.

by **BMAONE23** » Fri Apr 18, 2014 5:18 pm

Guest wrote:This may be a stupid question, but why does the beam appear to be narrower at its destination (the moon) rather than wider? I thought that any beam of light (even a collimated beam like a laser) would gradually widen as it travels from origin to destination.

Not a stupid question at all (there aren't really any stupid questions since you are smart enough to seek the answer to an unknown.)

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation". Lasers differ from other sources of light because they emit light coherently. Spatial coherence allows a laser to be focused to a tight spot, enabling applications like laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over long distances known as collimation, enabling applications such as laser pointers.

The Laser Beam isn't exactly the same size though once it covers the 238,000 mi trip though. At the Moon's surface, the beam has spread to about 6.5 kilometers or 4 miles wide and scientists compare the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers or 2 miles away. The reflected light is too weak to be seen with the human eye: out of 10¹⁷ photons aimed at the reflector, only one will be received back on Earth every few seconds, even under good conditions. It only appears smaller to the efect of the Vanishing Point.

by **BDanielMayfield** » Fri Apr 18, 2014 5:10 pm

Here’s a fun fact that I came across in reading about laser range finding, highlighted in red:

physics.ucsd.edu/~tmurphy/apollo/basics wrote:We reference our measurements to the center of the telescope mount, where the azimuth axis and elevation axis intersect each other. As the telescope swings around to point at different parts of the sky, this point stays fixed—almost.... The position of the telescope relative to the center of the earth isn't as stationary as you might imagine. The continental plate drifts, the tides from the moon and sun make the site swell by about a foot twice a day, weather systems can push the local crust down, etc. We have to be aware of all of these influences and take them into account in order to extract the scientifically useful center-to-center distance between the earth and moon.

Slow continental drift and daily tidal motions (even of the Earth's solid crust) are facts widely known, but I had no idea that weather changes could cause changes in altitude (even at the millimeter scale) of the Earth’s land surfaces. Atmospheric pressure at sea level averages, what, about 15 pounds per square inch? (Sorry for the non SI units) How much does it vary about its mean? It’s amazing to me that barometric pressure differences could be enough to effect the position of a telescope relative to the Earth’s center.

by **Anthony Barreiro** » Fri Apr 18, 2014 4:58 pm

Chris Peterson wrote:
Anthony Barreiro wrote:My immediate reaction to today's apod was, "why would anybody want to take a picture of some idiot aiming a laser pointer at the Moon?" After reading the caption I can see that it's one hell of a laser pointer. Certain members of my astronomy club would give their left arm to get a laser pointer that powerful. Does anybody know the wattage?
The average output is about 2W, which is close to some handheld lasers. The difference is that while most lasers have a continuous output, this one pumps out very short high energy pulses 20 times per second. But what we see visually (or photographically) with this instrument is the same as we'd see with a 2W continuous laser. So quite a bit dimmer than the lasers used to generate artificial stars for adaptive optics systems on other telescopes.

Thanks Chris. A laser enthusiast had a 500 mW laser pointer at one of our star parties. Every time he turned it on people lost their night vision. We threatened to report him to the FAA if he brought it again.

by **Chris Peterson** » Fri Apr 18, 2014 4:42 pm

Anthony Barreiro wrote:My immediate reaction to today's apod was, "why would anybody want to take a picture of some idiot aiming a laser pointer at the Moon?" After reading the caption I can see that it's one hell of a laser pointer. Certain members of my astronomy club would give their left arm to get a laser pointer that powerful. Does anybody know the wattage?

The average output is about 2W, which is close to some handheld lasers. The difference is that while most lasers have a continuous output, this one pumps out very short high energy pulses 20 times per second. But what we see visually (or photographically) with this instrument is the same as we'd see with a 2W continuous laser. So quite a bit dimmer than the lasers used to generate artificial stars for adaptive optics systems on other telescopes.

by **Anthony Barreiro** » Fri Apr 18, 2014 4:14 pm

My immediate reaction to today's apod was, "why would anybody want to take a picture of some idiot aiming a laser pointer at the Moon?" After reading the caption I can see that it's one hell of a laser pointer. Certain members of my astronomy club would give their left arm to get a laser pointer that powerful. Does anybody know the wattage?

by **bystander** » Fri Apr 18, 2014 3:03 pm

Nitpicker wrote:...
To me, the most gratifying thing about this APOD is that the laser seems to be pointing pretty damn close to the Apollo 15 landing site.

APOD Robot wrote:...
The laser's target is the Apollo 15 retroreflector, left on the Moon by the astronauts in 1971.

Boomer12k wrote:...
Interesting experiment....how does this test Gravity???

Apache Point Observatory Lunar Laser-ranging Operation
Lunar Laser Ranging experiment

by **Chris Peterson** » Fri Apr 18, 2014 3:02 pm

JohnD wrote:Why wait until there is a total eclipse?
Why not irradiate the reflector at New Moon, when the site is in darkness?

I think they make measurements at all lunar phases, with the retroreflector array both in light and dark. The problem with reduced signal at the full moon has little or nothing to do with light interference. The problem is that when the Sun is shining directly down on the array (which only happens at the full moon) it is being structurally altered by heat into a device with poorer optical performance.

The point of making the measurement during an eclipse wasn't to actually measure the distance to the Moon, but to test the theory that the Sun shining directly into the retroreflector cavities changes the optics.

by **JohnD** » Fri Apr 18, 2014 2:46 pm

Nitpicker wrote:A new Moon rises and sets with the Sun.

Of course, NP, up to a point.
But how about this? http://www.starrynightphotos.com/moon/moon_set.htm
There must be days in the month when the crescent Moon is visible at Earthdawn (cold atmosphere) when the Apoolo 15 site is also near dawn (cold surroundings)
An eclipse is no more predictable than such an event, and I suppose less frequent.

But we argue in the vacuum of our own ignorance - mine anyway.
JOhn

PS AH! So! I've found out why they took readings at eclipse.
See: http://ucsdnews.ucsd.edu/feature/source ... 2014-02-06

The investigators wanted to know why they got so few photons returned. They postulated that the reflectors, heated in the Moon day, were the problem. So they read the reflectors at short intervals, while in sunlight and when eclipsed. They found that when the reflectors were eclipsed, as predicted they returned TEN TIMES more photons! They confirmed this finding with all the Apollo reflectors and the Russian one as the Earth's shadow swept across the Moon.
To be able to do so at such short intervals minimised any interference by changes in the Earth's atmosphere, something impossible if they waited two weeks between Moonday and Moonnight readings. J.

by **BDanielMayfield** » Fri Apr 18, 2014 2:41 pm

Bate wrote:It's amazing that any return photons can be detected above the ambient noise.

It helps to have a 3.5 meter wide light bucket waiting for the signal. It also helps to be looking for just one very specific frequency (color) of light.

by **Bate** » Fri Apr 18, 2014 2:20 pm

A minor quibble: the article says that a laser is being shot to the moon rather than light from a laser ...

The green beam in the image is light that is scattered from the beam by the atmosphere so nothing of the beam is visible to us after the atmosphere is too thin to scatter the light. I guess that that the scattering would taper off and end after 15 to 30 kilometers. It's amazing that any return photons can be detected above the ambient noise.

by **BDanielMayfield** » Fri Apr 18, 2014 1:57 pm

sinanipek wrote:Do we see the reflected beam in thi picture, too? If so, then how amazing that the beam has spreaded almost none in such a huge distance!

No, according to the Apache Point Observatory website, only about 1 out of 30 million photons fired up in the beam actually hit one of the retroreflectors that were left on the Moon, when they're on target. Even though the beam which starts out at 3.5 meters is focused (collimated) as tightly as possible it still spreads out to about 2 kilometers by the time it reaches the Moon. Then this 1/30 millionth as powerful reflected beam also spreads out on the way back, and the odds of any one photon being detected in the return signal are only about 1 in 1 quadrillion! But each laser pulse packs about 300 quadrillion photons.

The only reason we can see (or photograph) the outgoing laser beam at all is because the Earth’s atmosphere is dusty. Dust particles in the air are illuminated and scatter the green laser light, making the beam visible along a short part of its path inside our atmosphere.

by **anobium** » Fri Apr 18, 2014 1:54 pm

I can see that the laser will provide a precise, fraction of a millimeter, measurement from one device to the other, so that orbital changes are measured with great precision, but do we know to the same precision what differences occur in both instruments due to local factors such as thermal expansion? What about the distance from the devices to the centers of their respective bodies? How closely do the gravitational centers of the bodies correspond to the volumetric ("true?") centers?

by **Nitpicker** » Fri Apr 18, 2014 1:53 pm

A new Moon rises and sets with the Sun.

Starship Asterisk*

APOD: Red Moon, Green Beam (2014 Apr 18)

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