10 Mile Distance Laser

The sensors in the retina can respond to a single photon. However, neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual "noise" in very low light.

A standard candle at 31 miles is 0.0000000004 (4x10^-10) cp. I highly doubt the eye could pick that up, even under ideal conditions. A good point of comparison would be how bright that would be relative to a magnitude 6 star, which is about the dimmest star anyone with normal vision can see. The sun is -26.8 on the magnitude scale. 5 orders of magnitude is a factor of 100 in brightness, so a 6 magnitude star is about 10^-13 as bright as the sun. The sun is roughly 100,000 cp, so a magnitude 6 star is 10^-8 cp, or about 25 times as bright as the candle. In other words, the candle is probably not visible from 31 miles, although it would be barely visible at about 6 miles.

Ok I might believe 6 miles under ideal conditions however I did plenty of hiking and camping in remote wilderness areas, sometimes up to 6 days at a time and I have lived near mountains and wilderness areas for much of my life and I can tell you for sure you can't normally even see a lantern or flashlight or even a campfire at 20 or 30 miles.

Alan

Just get a PL520 running at ~200mA and you'll be good to go. Not too pricey. Diode laser, so no crystals/temperature to worry about. Green, so it's still plenty visible. One of these in a C6 or similar would be good for this.

Marco Polo said:

Just get a PL520 running at ~200mA and you'll be good to go. Not too pricey. Diode laser, so no crystals/temperature to worry about. Green, so it's still plenty visible. One of these in a C6 or similar would be good for this.

Click to expand...

Haha, I posted last night in the buy/sell section asking about that guy's Phobos PL520 with boost at ~300ma. Hopefully he still has it...

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Did a test run on Tues. night with the cheap 532nm on a 2 mile stretch. On his side was total darkness, I was at the public marina side with heavy light pollution

http://i.imgur.com/ecHI50s.jpg

From his end the entire beam was visible, but on my side it was very dim and I could not see much. The flashlight was actually brighter than the laser. We did bring the tripods/camera and took them into the water to test the "bulge", but I'm waiting for the much brighter 650nm and until we do the other lakes to upload the final video results

So I'm just waiting for that V7 X-Drive to restock and I can assemble the 650nm

I also found a 78mW 520nm in the buy/sell section that im interested in, but the guy seems like he disappeared from the forums. If he comes back and ships it by Monday (from US), I could possibly receive it before next weekend and do some real test runs

Hoping to fully complete this project and upload by the end of the month, but it all depends on when my shipment arrives

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And here is a final example of what the laser should appear like from the other side of a lake

http://i.imgur.com/jhCVUxH.gif

The ramps are 8 inches high and the camera is moving down to 1 ft. If we pretend that this is the 2 mile lake, this is what the camera should observe as it is lowered (with a tripod). Even at approx 1 foot high (we'll go lower), it still cannot pick up the object. To further emphasize this point, at 1 ft the calculated distance to the horizon is 1.2 miles.

So you should only really be able to see half of a 2 mile lake at 1 ft high. Something placed at 2 inches will only travel half a (0.5) mile before it gets cut off by the water. Even with an 8 inch "bulge" at the center, being 1 ft high will not be able to detect a 2 inch object standing at the opposite end of the shore

Plus there are tides/swell, so the "bulge" is even bigger...

I'm stressing these points now so that when the results are uploaded (soon), you are all prepared for them...

Thanks all for patience :thanks:

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

Since we've now gone fully into surveyor
mode, the geoid is something else to take
into consideration. If you are measuring a
body of water of sufficient size,
gravitational and georotational
irregularities will begin to affect your
measurement.