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10 Mile Distance Laser

wellexcel

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Hello, I am new to lasers

I've read around the forums that the divergence of the beam is more important than the actual output power for long distances, and that I may need a beam expander

So, before I spend the money I would like to ask you guys for your best recommendation for a laser setup that can travel over a 7 mile distant lake on a cold night
around ~0 Celsius

A helper will be standing on the opposite side of the shore to catch it with something like a piece of cardboard attached to a tripod to be able to measure the height from water level

I don't mind the color and it doesnt need to be able to be visible past 10 miles.

I also need a fixture that I would be able to attach to a camera tripod, because we want to test the laser at several different fixed heights

Is this possible for up to $200-300 (USA)?

Thank you
 
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Sta

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A 532nm green laser is your best bet. 532s tend to have the best divergence. However, I'm not sure which company to choose, however, as I have little experience with long-distance laser usage. You'll need to ask another user about that.
A 520nm greenie could also work, given that 532s are unstable in cold temperatures.
You are definitely going to need a beam expander, there is no way a normal beam will go that distance.
 
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vortish

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a ten mile laser is going to need to be big 532 is going to be your best bet. but your looking at some thing like 2+ watt lab style. and for a 2 + watt 532 your looking like 1 to 4 k depending on size
 

crazyspaz

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Everyone is saying 532nm, forgetting that solid state lasers are temperature sensitive. Running a 532 at 0C is not going to yield good results, if any. Your best bet is something diode based, maybe a 445 with a beam expander. Check out jetlasers, I think they sell beam expanders for their higher powered 445's. Keep in mind though, the output will be more of a rectangular bar then a dot, if that matters.
 

wellexcel

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Thank you all for your kind suggestions

a ten mile laser is going to need to be big 532 is going to be your best bet. but your looking at some thing like 2+ watt lab style. and for a 2 + watt 532 your looking like 1 to 4 k depending on size
Hmm, that's definitely too expensive for this project. Is this because the pointers are illegal in the USA?

Everyone is saying 532nm, forgetting that solid state lasers are temperature sensitive. Running a 532 at 0C is not going to yield good results, if any. Your best bet is something diode based, maybe a 445 with a beam expander. Check out jetlasers, I think they sell beam expanders for their higher powered 445's. Keep in mind though, the output will be more of a rectangular bar then a dot, if that matters.
I'm glad I mentioned that it would be cold

A rectangular bar would be fine, we only need to measure the approx height difference between the start and end points



http://jetlasers.org/en/pl-e-pro-se...color-silver/10x_beam_expander-1pc/goggles-no

How about something like this with the 10x beam expander?
 
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Shakenawake

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people responding should be sure a laser they recommend can indeed go 10 miles, that's a tall order even for a BE equipped unit. it sounds to me like the OP wants the dot still small enough after this distance to hit a cardboard target.

I can personally attest that a 445nm jetlaser with a BE will go really far, but I don't know about that far

best of luck with that. I'd say get the lowest divergence laser and then add a BE and that might work. take pictures of this 10 mile beam when you achieve it please
 
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wellexcel

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people responding should be sure a laser they recommend can indeed go 10 miles, that's a tall order even for a BE equipped unit. it sounds to me like the OP wants the dot still small enough after this distance to hit a cardboard target.

I can personally attest that a 445nm jetlaser with a BE will go really far, but I don't know about that far

best of luck with that. I'd say get the lowest divergence laser and then add a BE and that might work. take pictures of this 10 mile beam when you achieve it please
Thank you, we were actually planning on video documenting this for YouTube
 
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No $300 is on the low side I doubt it. That crazyspaz is correct, at that temp stay away from 532nm. Get a powerful 520nm in the 1W+ range or I expect a 445nm in the 4W - 5W range should work except the dot at the other end will be gigantic to say the least. There was someone here who shined a green laser at a hillside several miles away and had two of his friends there, one to stand in the dot and the other took a picture, I remember it looked something like maybe 15 feet across. Sorry I don't remember the thread it was long ago. If you need the dot to be reasonably small then you must have a high quality beam expander.

Alan
 

wellexcel

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500mW PL-E Pro 445nm blue Laser

This is a 2w 445nm w/ beam expander at $380 total

If 4-5w is needed at approx 12.1km, then I will have to decide if it is worth to drop an extra $500-700 for this one time small project

There is also the issue of legality, and whether or not I would get any attention from the authorities by using it on a public lake (with not many people on a cold winter night)

Maybe I could it for a lower price afterwards to somebody who wants a used 5w pointer, but if I can get something that would work for ~$400 then I would love to keep it
 
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I don't know for sure that 4W - 5W is required for blue but I have my doubts about 2W at that distance, 520nm is several times brighter. Also you have not added your location to your profile. If your in the US be sure your not within 10 miles of an airport. And yes it will draw attention from the authorities if they see you doing this. At the very least they will want to know what your doing and why so be prepared with a good explanation just in case.

Alan
 

wellexcel

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I don't know for sure that 4W - 5W is required for blue but I have my doubts about 2W at that distance, 520nm is several times brighter. Also you have not added your location to your profile. If your in the US be sure your not within 10 miles of an airport. And yes it will draw attention from the authorities if they see you doing this. At the very least they will want to know what your doing and why so be prepared with a good explanation just in case.

Alan
Thanks for your advice, I could just reduce the distance down to maybe 3.5-5 miles, but the longer the distance the better for this experiment. We are testing exclusively for a convex bulge at the center of the lake (due to Earth's curvature), and the longer the distance the bigger the arc to the horizon

If the laser beam travels to the other side at a height less than what the "bulge" should be (with refraction considered), then that is a big problem. If the laser does not reach the other side, then everything is perfectly normal.

We are going to test several heights above water level, including "above" and "below" the bulge

I am pretty set on doing this experiment so it just comes down to which vendor to buy from and what distance

Regards
 
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Shakenawake

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wouldnt you need someone in a boat to measure the beam's distance from the water's surface in the middle? or a sled as the case may be? or will you just measure how much higher up the beam is from the water compared to the other side?

BTW, a laser having a 5cm diameter beam at aperture with .5mrad (pretty good divergence) will have a 810cm dot @ 10 miles
 
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wellexcel

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wouldnt you need someone in a boat to measure the beam's distance from the water's surface in the middle? or a sled as the case may be? or will you just measure how much higher up the beam is from the water compared to the other side?

BTW, a laser having a 5cm diameter beam at aperture with .5mrad (pretty good divergence) will have a 810cm dot @ 10 miles
This is what I want to do

"The Earth curves faster than most people think. On a small lake — say, 2.5 miles across — there’s a 1-foot-tall bulge in the middle of the lake due to the curvature of the Earth. On some larger bodies of water, if conditions are right, you can actually perceive the curvature of the Earth when this sort of bulge blocks your view of the opposing shore."

http://www.dummies.com/how-to/content/how-to-determine-the-distance-to-the-horizon.html

Is .5mrad with a beam expander?

It's okay if there is no way to get it that small, it still shouldn't be able to travel over the bulge
 
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Shakenawake

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no. I think a 10X expander reduces divergence by the same factor.

if the numbers are accurate, 1.8mrad would become 0.18mrad

so a 5cm @apeture beam would be about 295cm @ 10 miles

pseudonomen137's JScript mRad Calculator

note that the 2W 445 and 4W 445 use different diodes. the more powerful one also has worse divergence, so I wouldn't think it's a good solution

won't measuring distance between the apeture beam and water and the end beam and water be a problem when the end beam/dot is more than an order of magnitude larger? IDK maybe this could be compensated for by measuring from the dot's center. just trying to be helpful
 
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wellexcel

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Cool, a ~9.6 ft dot at 10 miles would be much better. I think that's good enough, and it should be slightly less at 7.5 miles

Now it's just whether or not a 2w 445nm beam could travel that distance, or if I am going to have to chop off a few miles

"piR^2" says he/she has doubts about the 2w, but "vortish" says 2w + (but with a 532nm)

At what distance do you guys think the 2w will cap? I understand that it is dependent on atmospheric conditions but let's just assume its a normal cold night with no excessive fog

I want to eliminate having to travel up and down the shore because the laser wont reach, and I need to have a precise distance measured with Google Earth

"won't measuring distance between the apeture beam and water and the end beam and water be a problem when the end beam/dot is more than an order of magnitude larger? IDK maybe this could be compensated for by measuring from the dot's center. just trying to be helpful "
Yes it would, being a 10 foot dot, so I may just scrap that idea. I only wanted to do that to measure any refraction, but now that I have a better understanding of this, that seems too difficult to accomplish.

I could measure it from the center of the dot with a 10x10 piece of cardboard, lol, that's a possibility

"measure how much higher up the beam is from the water compared to the other side?"
Yes, ex. 50 cm on one side and 60 cm on the other, or uniform
 
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Sta

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At what distance do you guys think the 2w will cap? I understand that it is dependent on atmospheric conditions but let's just assume its a normal cold night with no excessive fog
It depends on whether you choose a blue or a green. If you decide to use a green (532 or 520), then it will cap much further as green is much brighter to our eyes.
 




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