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A friend and I want to measure the curvature of the Earth using a laser and a lake.

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ProfessorFarrel

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This involves taking the laser out to a lake and shining it across the lake at around 2-3 feet high. 2.5W 445 nm laser. The laser would have a solid backdrop on one end of the lake about 3'x6'. Would this be illegal? I'm in Denver, Colorado, if it matters.

The set up would be like so:

Friend and I go out to said lake, with a rangefinder. We'll find two spots 500 metres apart from each other. My friend will take the 3'x6' board, a walkie talkie and a tape measure and set it up the board vertically on one spot. I will take a tripod, walkie talkie, laser, level and go to the other spot. Said laser has a 10 mw, 2.5w and 5w mode. I will not be touching the 5W mode. With the 10 mw mode active, I will communicate with my friend to perfectly level the laser on my side on the 10 mw mode. Once it is on the board, I'll make the laser perfectly level, and activate the 2.5W mode. Quickly, my friend and I both will measure the height of the beam of the water. As soon as we both have it measured, the laser will be turned off. Then we'll poack up, go home, and do lots of hard math. We will never shine the laser (on any mode) at the water, and I'll attach the laser to my telescope so that I can minimize how many adjustments I need to make to get the laser on target, because the more adjustments I have to make, the more time the laser will spend off target, and the higher the potential of shining it on something I don't want to shine it on. And, yes, OF COURSE we will both be wearing safety goggles :yh: . My vision is already shitty, I do not want to make it worse.

So, is this legal?
 
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How far does the beam have to travel? Might attract some attention if there's anyone nearby, but you at least won't need to worry about planes if you're terminating on something at the other end. Not sure about the legality of it, although I doubt you'd get in any trouble as long as you don't do anything stupid! :p
 
In terms of distance, the two spots will be about 1,5000 meters apart, we're on Sloan's Lake.
 
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ProfessorFarrel said:
In terms of distance, the two spots will be about 1,5000 meters apart, we're on Sloan's Lake.
Click to expand...

Sloan Lake as in the lake right next to downtown Denver? Do you mean 1,500 meters?

I'd be leery about shining a laser so far and low in such a populated area, but I've only seen that lake in passing.

Is that going to be long enough for a significant measurement?
 
It seems to me that a 445nm laser might have divergence that is too high for something like this. Have you considered using a beam expander?
 
Rivem said:
Sloan Lake as in the lake right next to downtown Denver? Do you mean 1,500 meters?

I'd be leery about shining a laser so far and low in such a populated area, but I've only seen that lake in passing.

Is that going to be long enough for a significant measurement?
Click to expand...

Yes, that Sloan's Lake. I'm a little anxious about using a laser there, too, but I don't have many feasible alternatives, though I am looking into alternatives. It's also not that close to downtown, but it is in a populated area. Hence the crapton of safety precautions I'm planning and the post here concerning legality. Also, yes, 1500 metres, whoops.

Sta said:
It seems to me that a 445nm laser might have divergence that is too high for something like this. Have you considered using a beam expander?
Click to expand...

I'm new to lazing, and this is my first laser. However, it is focusable, so I'm anticipating I should be able to minimize divergence that way. Also, a beam expander sounds (the name, at least) like the exact opposite of a solution to divergence. What does it do?
 
ProfessorFarrel said:
I'm new to lazing, and this is my first laser. However, it is focusable, so I'm anticipating I should be able to minimize divergence that way. Also, a beam expander sounds (the name, at least) like the exact opposite of a solution to divergence. What does it do?
Click to expand...


There's a limit to what divergence you can achieve with the focusing lens. The beam expander does indeed expand the beam, but it also reduces the divergence by the same factor.

So, for example, if you have a 10x beam expander and a laser with a 1mm beam at aperture and 1 mRad divergence - after the beam expander you'll have a beam diameter of 10mm but a divergence of 0.1mRad.
 
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ProfessorFarrel said:
Yes, that Sloan's Lake. I'm a little anxious about using a laser there, too, but I don't have many feasible alternatives, though I am looking into alternatives. It's also not that close to downtown, but it is in a populated area. Hence the crapton of safety precautions I'm planning and the post here concerning legality. Also, yes, 1500 metres, whoops.



I'm new to lazing, and this is my first laser. However, it is focusable, so I'm anticipating I should be able to minimize divergence that way. Also, a beam expander sounds (the name, at least) like the exact opposite of a solution to divergence. What does it do?
Click to expand...

When you expand a laser beam, you aren't just scaling the height and width of the dot, you're also expanding the length. So, at 1500 meters, with a 3x expander, the spot size will be only slightly larger than a non-expanded beam would be at 500 meters. Without an expander, especially with a 445, it will be much larger at such a long distance. However, at short distances (indoors, for example) beam expanders are generally useless. :)

Edit: ninja'd by diachi, I hope my explanation is still useful :)
 
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Okay, thanks for the explanation on beam expanders guys! I hope divergence won't be too much of an issue, but if it is, I know what to do now. The nice folks over at reddit don't seem to think this will get me into any trouble, so I'd like some second opinions here and I'll decided whether or not to do the experiment.
 
ProfessorFarrel said:
Okay, thanks for the explanation on beam expanders guys! I hope divergence won't be too much of an issue, but if it is, I know what to do now. The nice folks over at reddit don't seem to think this will get me into any trouble, so I'd like some second opinions here and I'll decided whether or not to do the experiment.
Click to expand...

Ohh, it's a Sanwu Spiker you have? They sell a beam expander that goes specifically with their lasers: https://sites.google.com/site/hkfew5e22/x3-beam-expander
 
ProfessorFarrel said:
Okay, thanks for the explanation on beam expanders guys! I hope divergence won't be too much of an issue, but if it is, I know what to do now. The nice folks over at reddit don't seem to think this will get me into any trouble, so I'd like some second opinions here and I'll decided whether or not to do the experiment.
Click to expand...

A few other people have explored simliar

have a look in this thread: http://laserpointerforums.com/f44/keeping-small-dot-over-one-mile-distance-95530.html

and this one: http://laserpointerforums.com/f44/10-mile-distance-laser-92273.html
 
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I can't speak to the legality of these things, but I can tell you this setup has several other problems. 500m/~40,000km is 12.5 millionths of the circumference, or 0.0045 degrees. Is your level accurate to 0.0045 degrees? I doubt it. Even if it were, how can you ensure the laser pointing angle is this accurate with respect to the case? Can you rule out (or account for) refractive problems caused by the differences in humidity/temp/pressure over the lake? After 500m, assuming 2mRad laser divergence, the beam will be 1m in diameter. How are you going to accurately find the center of this beam? There are probably 4 or 5 other variables that I'm forgetting about. This doesn't seem viable at first glance.
 
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Cyparagon said:
I can't speak to the legality of these things, but I can tell you this setup has several other problems. 500m/~40,000km is 12.5 millionths of the circumference, or 0.0045 degrees. Is your level accurate to 0.0045 degrees? I doubt it. Even if it were, how can you ensure the laser pointing angle is this accurate with respect to the case? Can you rule out (or account for) refractive problems caused by the differences in humidity/temp/pressure over the lake? After 500m, assuming 2mRad laser divergence, the beam will be 1m in diameter. How are you going to accurately find the center of this beam? There are probably 4 or 5 other variables that I'm forgetting about. This doesn't seem viable at first glance.
Click to expand...

Great comment--was thinking similar. Member LSRFAQ pointed out some of the weaknesses in a thread about doing same sort of thing:

LSRFAQ said:
If your just doing this for fun, skip reading this post.

If you need a real measurement or this is a science fair project, read on...

Save your money and hire a surveyor.
Here is why:

So at 10 miles you will have atmospheric scintillation and refraction that does not quit.
Coherent light is subject to interference at the detector for each photon's beam path.

You get horrid low and high frequency noise. This is why light bulbs or Leds are used as sources for long distance light communications.

Next Question. How will you accurately find the centroid of the LARGE blob down range? Helps if your Blob is Guassian with a brighter center, but you would still have to find the center some how. This is going to need some serious robotics.
Or at least a photodiode/photomultipler sliding on a calibrated arm and a piece of graph paper.

The traditional way, used for surveying the Pacific Missile Range Islands by a team that included my Father...
Dad was helping with the planting of early Loran "A" NAVAIDs

1. Build a Bilby Tower at each end of the transit plus one offset a known distance on shore. This gets you above the small part of the refraction from ground convection.

2. Place a taped down Jeep Headlight on a leveled platform at the top of the first tower. The light is taped down to the central portion of the collimated beam.

2. Place a Theodolite or Transit on the other two towers. Measure carefully the spot of light, then swing the instrument to the other tower. Then swing upwards, catch a few stars at a precisely known time. Then Star site reductions at night to establish a Geodetic control point. These days differential GPS provided by WAAS or similar commercial services might get you closer. Then you still have to correct for the Ellipsoid.

Farmers and Road Builders hit milestones within inches with specially assisted GPS signals broadcast from temporary "control" stations and satellites. Often a local FM radio broadcast has the corrections on a Subcarrier.
You can lease/borrow the specialized receiver.



Refs:
NOAA 200th Feature Stories: Bilby Towers

Scroll to Bottom to see what/why NOT lasers.

MODULATED LIGHT DX

"And the US Coast Guard used a 5 mW Hene with an Upcollimator as a NAV AID at 1500 yards with an eye as a detector."

http://www.dtic.mil/dtic/tr/fulltext/u2/a170823.pdf

Theodolites are all over the place. They eliminate the "guess the blob center" problem.
(insert safety warning about optically aided viewing of high power lasers here)
This is a fun project, but you need to level the source laser some how, relative to local gravity. I'd suggest starting with bubble levels.

I just did a proposal for a specialized laser Navaid of sorts. :-)

Steve
Click to expand...

Nobody who has tried such a thing ever reported back to LPF any successful results---may or may not be significant--maybe they didn't actually attempt to do their plan---who knows.
 
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Thanks Encap for that quick little writeup. In regards to my accuracy, I'm not overly concerned about accuracy. I had a friend who did this same experiment on a 3-mile stretch of water, and he got to within 500 miles of the Earth's true diameter. For that reason I believe that I can do this to a recognizable degree of accurcay (I'm hoping to be within 2,000 miles of the correct answer). Either way, it will still be a nice experiment to do and I'll definitely report back on how it went.
 
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