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07-12-2016, 03:10 AM #1
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ProfessorFarrel
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A friend and I want to measure the curvature of the Earth using a laser and a lake.

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 . My vision is already shitty, I do not want to make it worse.

So, is this legal?

07-12-2016, 03:14 AM #2
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diachi
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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!
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07-12-2016, 03:26 AM #3
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

10 Mile Distance Laser

I live in colorado, and I'm not sure there is a lake with enough distance between the two farthest points to get a decent and noticable measurement. maybe blue mesa, I'm working near there now. possibly mcphee reservoir. maybe dillon. are you going to one of those?
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Last edited by Shakenawake; 07-12-2016 at 03:26 AM.

07-12-2016, 03:33 AM #4
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ProfessorFarrel
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

In terms of distance, the two spots will be about 1,5000 meters apart, we're on Sloan's Lake.

Last edited by ProfessorFarrel; 07-12-2016 at 03:51 AM.

07-12-2016, 03:52 AM #5
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by ProfessorFarrel In terms of distance, the two spots will be about 1,5000 meters apart, we're on Sloan's Lake.
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?
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07-12-2016, 04:08 AM #6
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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?
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07-12-2016, 04:26 AM #7
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ProfessorFarrel
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by Rivem 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?
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.

Quote:
 Originally Posted by Sta 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?
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?
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07-12-2016, 04:32 AM #8
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diachi
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by ProfessorFarrel 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?

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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Last edited by diachi; 07-12-2016 at 04:33 AM.

07-12-2016, 04:34 AM #9
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Re: GUIDE: Which company should I buy from?

Quote:
 Originally Posted by ProfessorFarrel 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?
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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Last edited by Sta; 07-12-2016 at 04:55 AM.

07-12-2016, 04:43 AM #10
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ProfessorFarrel
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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.
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07-12-2016, 04:53 AM #11
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diachi
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by ProfessorFarrel 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.
Ohh, it's a Sanwu Spiker you have? They sell a beam expander that goes specifically with their lasers: https://sites.google.com/site/hkfew5...-beam-expander
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07-12-2016, 05:00 AM #12
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by ProfessorFarrel 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.
A few other people have explored simliar

have a look in this thread: Keeping a small dot over one mile distance

and this one: 10 Mile Distance Laser

Last edited by Encap; 07-12-2016 at 05:05 AM.

07-12-2016, 11:29 AM #13
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

Quote:
 Originally Posted by diachi Ohh, it's a Sanwu Spiker you have? They sell a beam expander that goes specifically with their lasers: https://sites.google.com/site/hkfew5...-beam-expander
With the adapter you can use it on any standard lens output.
And the price is great.
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07-12-2016, 04:38 PM #14
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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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Last edited by Cyparagon; 07-12-2016 at 04:40 PM. Reason: units correction

07-12-2016, 04:59 PM #15
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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

Quote:
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.

Last edited by Encap; 07-12-2016 at 10:54 PM.

07-12-2016, 08:43 PM #16
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ProfessorFarrel
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Re: A friend and I want to measure the curvature of the Earth using a laser and a lak

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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