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Old 01-24-2017, 01:57 AM #1
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Default Blue laser and the night sky ?

Why does the beam seem to stop abruptly at some point ?



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Old 01-24-2017, 02:00 AM #2
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Default Re: Blue laser and the night sky ?

Could you try and resize your picture? It is awfully big

-Alex
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Old 01-24-2017, 02:01 AM #3
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Default Re: Blue laser and the night sky ?

It has to appear to end some time right? It's like an asymptotic type thing. The closer you are to the axis of the beam the end of the beam compresses closer and closer. Imagine drawing it out with your mind. Otherwise the beam would appear to bend.

Quote:
Originally Posted by Hap View Post
Could you try and resize your picture? It is awfully big

-Alex
But it's still viewable, right?
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Old 01-24-2017, 02:05 AM #4
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Default Re: Blue laser and the night sky ?

I've noticed this too. More so with red and blue lasers, and less with DPSS yellows and greens.
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Old 01-24-2017, 02:05 AM #5
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Default Re: Blue laser and the night sky ?

I feel like that might be because of the relative brightness of the nearer, narrower part of the beam being so much greater than the further, diffuse part of the beam that it just gets overpowered at a certain point.
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Old 01-24-2017, 02:21 AM #6
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Default Re: Blue laser and the night sky ?

A laser technician at a nearby navy base in Maryland explained that to me.
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Old 01-24-2017, 03:23 AM #7
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Default Re: Blue laser and the night sky ?

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Originally Posted by Crazlaser View Post
It has to appear to end some time right? It's like an asymptotic type thing. The closer you are to the axis of the beam the end of the beam compresses closer and closer. Imagine drawing it out with your mind. Otherwise the beam would appear to bend.


But it's still viewable, right?
I think I need to do a "einstein thought experiment" to get my head around this ! lol
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Old 01-24-2017, 04:47 AM #8
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Default Re: Blue laser and the night sky ?

This may sound ridiculous but, diachi, before you say that I'm crazy here me out. ......assuming you'll say I'm crazy.

Notice when you point the beam at long distances on level ground, for example at a mountain ten miles away, you don't see the dot on the mountain. There is math to back this up, of which I can't do but have seen and heard people talk about. When you look from directly behind the laser, the beam is squashed. For example hold a pencil at an angle with one eye open and it will look short. But the pencil only looks short. If you were to stand at a observe the beam at a different angle, say 500 feet from the laser but still looking at the mountain where the laser hits the mountain, you would see a longer beam than if you were standing behind the laser, until eventually the dot would be no longer obscured and you would see it on the mountain. I've actually done this experiment and found it to be true. The beam seems to get longer, the bigger the angle between the laser beam and the observer. Until basically you hit are looking at the beam perpendicular and that is as much of the beam as you can see. But as the beam goes into the distance it exponentially looks more and more compressed. Say at the beginning of the beam what looks like a half of the visible beam is 1000 feet into the air. Well the closer you get to the end, what appears to be perhaps the last 30 percent or less (I don't have the math just guessing here) in reality is closer to 100% of the beam and that first 1000 feet was a measily amount.

The beam has to appear to send somewhere. If it didn't it would basically have to be a curved beam. Even if you were holding a pole that was infinitely long it would appear to send right? Because if you pointed it into the sky slightly to the right, you wouldn't see it way over on the other horizon that far to the right, no you'd see it continue in the straight path it was taking until it looked compressed to infinity.


I hope this doesn't sound ridiculous. I'm just repeating what a laser technician and a math and physics professor told me. It makes sense though.

My guess is the thinner DPSS beams don't hide themselves quite as much and so appear to travel slightly further before the infinite compression happens.
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Old 01-24-2017, 06:23 AM #9
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Default Re: Blue laser and the night sky ?

Quote:
Originally Posted by Crazlaser View Post
This may sound ridiculous but, diachi, before you say that I'm crazy here me out. ......assuming you'll say I'm crazy.

Notice when you point the beam at long distances on level ground, for example at a mountain ten miles away, you don't see the dot on the mountain. There is math to back this up, of which I can't do but have seen and heard people talk about. When you look from directly behind the laser, the beam is squashed. For example hold a pencil at an angle with one eye open and it will look short. But the pencil only looks short. If you were to stand at a observe the beam at a different angle, say 500 feet from the laser but still looking at the mountain where the laser hits the mountain, you would see a longer beam than if you were standing behind the laser, until eventually the dot would be no longer obscured and you would see it on the mountain. I've actually done this experiment and found it to be true. The beam seems to get longer, the bigger the angle between the laser beam and the observer. Until basically you hit are looking at the beam perpendicular and that is as much of the beam as you can see. But as the beam goes into the distance it exponentially looks more and more compressed. Say at the beginning of the beam what looks like a half of the visible beam is 1000 feet into the air. Well the closer you get to the end, what appears to be perhaps the last 30 percent or less (I don't have the math just guessing here) in reality is closer to 100% of the beam and that first 1000 feet was a measily amount.

The beam has to appear to send somewhere. If it didn't it would basically have to be a curved beam. Even if you were holding a pole that was infinitely long it would appear to send right? Because if you pointed it into the sky slightly to the right, you wouldn't see it way over on the other horizon that far to the right, no you'd see it continue in the straight path it was taking until it looked compressed to infinity.


I hope this doesn't sound ridiculous. I'm just repeating what a laser technician and a math and physics professor told me. It makes sense though.

My guess is the thinner DPSS beams don't hide themselves quite as much and so appear to travel slightly further before the infinite compression happens.

I find it funny that you mentioned me and I hadn't even replied to the thread yet. Won't be calling you crazy for this one, don't worry. This one is interesting and I can't provide a definite answer to the question. Just an "educated" guess/theories based on some experience and prior knowledge. Don't take my answer here as gospel. I'm just a hobbyist, the same as most folk on here.

So, I actually don't have a proper answer to this one, though if I had to say I'd say it's really just down toperspective. I'll make some assumptions here.

As a result of perspective it looks like the beam just stops, when in actual fact it's just gradually getting less dense and losing power due to divergence and interaction with the atmosphere, thus the beam is becoming less bright. Add to that the scattered light having to travel further to get back to your eye the further away the beam gets and you also get less light being picked up by your eye, thus the beam appears dimmer again as distance increases. Due to perspective it looks like it just stops, when in actual fact it gradually gets dimmer due to divergence, atmospheric absorption and a decrease in return power due to increased distance. Perspective can do funny things.

Not to mention the beam being brighter either looking towards the source along the beam or along the beam from the source, getting dimmer as you approach 90 degrees relative to the beam. That'd have an effect I'd imagine.

If you view a high power beam from a long way away, and at a large angle relative to the beam (or even, a small angle, it's just not as apparent at a small angle, as a result of the phenomenon mentioned in the above sentence.), you'll see what I mean about it "fading out" as it propagates, until you can no longer see it. The change in perspective makes a large difference.

Some good pictures that show this come to mind but I can't see to find those at the moment.

Apologies for any grammatical mistakes or anything like that, these beers are strong...
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Old 01-24-2017, 07:17 AM #10
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Default Re: Blue laser and the night sky ?

Light waves and photons of light can behave in some very unintuitive ways such as the double slit experiment which is affected by whether or not anyone is looking at it !

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Old 01-24-2017, 10:22 AM #11
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Default Re: Blue laser and the night sky ?

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Why does the beam seem to stop abruptly at some point ?
The real question is/should be why do you see a beam at all ever-- there is no visible beam from a laser of any power in a vacuum--then you are on the way to why a beam appears to stop.

Beam visability to an observer is complicated real world phenomenon of light, eye sensitivity, characterists of human visiual perception/visual perspective, and reflection from particle/aerosols in the air the details of which are not intuitively obvious.

Whether or not you can see the beam or the extent to which you can see a beam depends upon many things -- among them the position of the observer in relation to the source of the beam , amount of aerosols in the air of the Planetary Boundary Layer, an individuals eye sensitivity to any given wavelength.

The reason is atmospheric conditions within the planetary boundary layer of the atmosphere allow you to see a beam in the first place--really all you see is reflections from particles hit by the beam--you don't actually see the beam.

"In a vacuum, the laser beam itself isinvisible - regardless of colour. However, as the beam passes through Earth's Atmosphere some of the photons encounter large airborne particles which reflect some of the light back to an observer."
"t is extremely small airborne particles called aerosols having a diameter significantly less than the wavelength of the light that causes the beam to become visible. The effect of minute particles scattering light is called Rayleigh scattering and it's most noticeable effect is to turn the daytime sky blue. Rayleigh scattering causes photons to be scattered in a roughly spherical manner around these particles. Some of the light is scattered forward (in the direction of the beam), a lesser amount is scattered to the sides and about the same amount that is scattered forward is scattered backwards towards the light source. This backwards scattering is why the beam is more visible to people standing near the astronomer using it, than people standing some distance to the side. The more of these minute particles there are in the atmosphere, the more Rayleigh scattering there is."

"A laser of any power will appear to the observer(s) to stop in the sky due to The Planetary Boundary Layer.
The Planetary Boundary Layer is the lowest part of the Troposphere that is directly influenced by the presence of the Earth's surface. As in fluid dynamics, the air (a fluid) that is in direct contact with the Earth's surface moves very little (ignoring local winds). The air above can be in significant motion (e.g. the jet stream). When two fluids are in proximity where one is not moving and the other is moving a boundary layer must develop to accommodate the transition from one state to the other."

"Therefore, the portion of the atmosphere directly above the surface of the Earth is "capped" by a boundary layer. The thickness of the Planetary Boundary Layer changes from day-to-night and from day-to-day depending on local terrain (water, desert, earth, mountains) and local weather. This thickness varies from a few hundred to a few thousand meters as does how far out a laser beam is seen from an observers view."

The above is from: http://calgary.rasc.ca/atmosphere.htm Part 2 - "Why does the beam from a green laser appear to "end" in the sky as compared to just going on forever?"

See article in link for more detail.

Beyond the Planetary Boundary layer a laser beam appears to abruptly stop because there are less and less particles to reflect the light, even though the laser beam goes on for many miles.

Exactly how bright and how far away a beam can be seen varies at any given moment will be involves a lot more than you imagine.

There is additionally, the sensitivity of the eyes of the observer to wavelength emitted by the laser part which is again a complicated thing to go into and plays a part as well.

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Old 01-24-2017, 10:23 AM #12
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Default Re: Blue laser and the night sky ?

My understanding is due to having less particles at higher altitudes, that and the other factors already mentioned.

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Old 01-24-2017, 10:35 AM #13
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Default Re: Blue laser and the night sky ?

Another real good article that follows along with what both encap and Alaskan have said is this one from a few years ago:

RASC Calgary Center - The Atmosphere, Astronomy and Green Lasers

The relevant part regarding beam ending starts at (Part 2), but the whole paper is an interesting read.

EDIT: After reading Alaskans post, I've noticed a link to this same article.

PS - Any chance of a re-size here?
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Old 01-24-2017, 10:48 AM #14
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Default Re: Blue laser and the night sky ?

This is an excellent question. Have often thought about it myself. Very interesting read.
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Old 01-24-2017, 11:03 AM #15
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Default Re: Blue laser and the night sky ?

Quote:
Originally Posted by Alaskan View Post
My understanding is due to having less particles at higher altitudes, that and the other factors already mentioned.

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Bad reasoning because if you point the laser horizontally at low mountains the beam fades off before it hits them. However as you move away from the axis of the beam the beam appears to extend until it touches the mountain.

Everyone please consider this for a moment. If you are holding a light but infinitely long steel rod (don't ask me why is doesn't bend or how you can hold it) that doesn't bend, think about this, it would appear to end, right?
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Old 01-24-2017, 11:38 AM #16
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Default Re: Blue laser and the night sky ?

After reading this entire paper, you believe that the RASC (Royal Astronomical Society of Canada)got it all wrong?
In the paper is a quote from Dr. Edward V. Browell, the head of NASA LIDAR Applications Group, who in my understanding, agrees with this.

"We transmit high-power laser beams in the zenith on many occasions and observe the same optical effect you describe. The cause of this is the enhancement of aerosols (atmospheric particles) in the planetary boundary layer (PBL) which causes enhanced scattering of the laser beam back to your eyes. Above the PBL, which can be very low at night (<100 m), the amount of aerosols is very low compared to within the PBL, and as a result the scattering of the laser beam appears to end abruptly at the top of the PBL. More sensitive detectors, such as we use in Lidar, can continue to sense the scattering from aerosols and molecules well above the PBL demonstrating that the beam does not just stop there. If you are interested, you can see from our airborne lidar images that are posted on our web site (http://asd-www.larc.nasa.gov/lidar/lidar.html) how the aerosols (and scattering) vary across the atmosphere.

I'm no scientist, and I'm probably missing something, but this explanation sounds pretty accurate to me. I'm sure we'll be getting more opinions from others, as this is an interesting topic.
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