35 mile laser shoot

[dr-ebert]

You can simulate how the beam would look let's say 1 km away: simply defocus the laser so that it creates a spot about 1m in diameter (corresponding to 1mrad divergence). Then look at the laser. I did that with my 250mW red: it's very bright (not blinding, not painful, something like sunlight reflecting off a piece of chrome).

100km away, the beam diameter would be 100 times greater, the area covered would be 10,000 times as big. Thus the brightness would be 10mag less (astronomically speaking). That would most likely still make it a lot brighter than Venus (I'm too lazy currently to calculate an estimate of the visual magnitude you'd get).

Actually I have little doubt that you could clearly see one of our tiny handheld lasers on the ISS.... IF you could track it accurately (which would require a GOTO-telescope as a mount).

 

[ElektroFreak]

About the beam expander.. the higher the rating on the expander, the less concentrated the beam. Therefore, if you wanted to view a reflection from a distant object, you'd really want the lowest rated expander that gives you the desired low divergence. If you can do it with a 2X or 3X, then you wouldn't get any better results with a 10X. In fact, you'd get worse results.

 

[dr-ebert]

Assuming that a high rating means a large beam diameter at exit, that's not correct. True, you'll get less energy density at the exit aperture. However, for any two beam expanders, there would be a distance where the beam from the lower-rated expander would have diverged to the diameter of the beam from the higher-rated one... beyond that distance, the higher-rated one would be better.

 

[ElektroFreak]

^Which is why I said use the lowest diameter (rating) necessary to obtain the desired low divergence. One of the main reasons to use a beam expander is to obtain lower divergence values from a system that otherwise wouldn't allow for it. I get what you're saying about the larger being good for longer distances, and that's true to a point. If you expand the beam too much, you'll get low divergence out to a very great distance, but that's useless if you're trying to utilize a distant reflection and the beam is not concentrated enough to make the return trip. Of course, you could use detection equipment and utilize a return of just a few photons, but with the eye alone, even with binoculars or a telescope, it would be extremely difficult.

 

[Benm]

Actually I have little doubt that you could clearly see one of our tiny handheld lasers on the ISS.... IF you could track it accurately (which would require a GOTO-telescope as a mount).

That would be a very interesting experiment to perform

350 km is quite a distance - i'm sure you'd have to shoot up from a completely dark area to be noticed, but it migth be feasible.

Considering 100 mW and 1 mrad, you would put about 1 uW/m2 (or 0.1 nW/cm2, or 6.2x10E8 eV/cm2) on the space station. Counting that in 2.3 eV (532 nm) photons, it makes 2.7x10E8 a second.

I seems to be such that about 1000 (green) photons a second must make it into your (fully dark adapted) eye before you will conciously notice. Considering a 40 mm2 pupil, the result found before is about 100.000 times that limit - so it should, at least in theory, be feasible.

For it to actually work, it's important to know how much background light there is, even from the dark side of the earth. I doubt the laser will stand out brightly if pointed up from the middle of a city, though it just might

 

[ElektroFreak]

Somewhere I read about the fact that a 3-5mW HeNe has been viewed from space. It would be no trouble for any of our hobbyist lasers to reach a spacecraft in earth orbit or well beyond..

 

[dr-ebert]

Actually...

The solar constant is about 1kW/m2. The sun also has a brightness of -26.8m. The brightest stars, like Sirius, are about 25m less bright - that corresponds to a factor 10^10, i.e. 10^-7W/m2.

A 100mW laser in 1km distance illuminates about 1m2, i.e. 10^-1W/m2. In 1000km distance (typical ISS distance; passing directly overhead, it could be as low as 400km), the area would be 10^6 times larger, for an area intensity of 10^-7W/m2.

In other words, on the ISS, our tiny lasers would look like a very bright star like Sirius... the limit of naked-eye visibility would be reached at around geostationary orbit.

Assuming a city emits 10MW of light (e.g. 10,000 1kW streetlamps) in all directions (a half sphere), the area brightness in 1000km would be 10^-6W/m2. Our laser would not stand out for the ISS crew, but might be visible due to color. A rural area background should be dark enough to make it clearly visible.

I hope I haven't lost track of an order of magnitude or three somewhere

 

[steve001]

^Which is why I said use the lowest diameter (rating) necessary to obtain the desired low divergence. One of the main reasons to use a beam expander is to obtain lower divergence values from a system that otherwise wouldn't allow for it. I get what you're saying about the larger being good for longer distances, and that's true to a point. If you expand the beam too much, you'll get low divergence out to a very great distance, but that's useless if you're trying to utilize a distant reflection and the beam is not concentrated enough to make the return trip. Of course, you could use detection equipment and utilize a return of just a few photons, but with the eye alone, even with binoculars or a telescope, it would be extremely difficult.

That's darn close to what I said. With typical beams diameters of hand held lasers a beam diameter of 10mm is close to what you'll have using a 10x expander. And a reduction of divergence about 0.12 mrd. That will put more light into a given area when compared to a 3 or 2x expander.

Using commonly available lenses with focal lengths of -6mm and 60 for a 10x expander and
-6mm and 18 for a 3x expander one can see that the Rayleigh length (RL) is greater with the 10x expander.
10x expander
0.12 mrd
RL 83337.648 mm or 273.41748 feet
10mm exit beam dia

3x expander
0.40mrd
RL 7500.348 mm or 24.607441 feet
3mm Exit beam dia.
So what this means is this. The RL is the length over which the beam will expand the square root of 2 or 1.4 times it's initial diameter. After that distance the beam expands in a linear fashion. The 10 x expander will out perform a 3x expander once past the rayleigh length of the 3x expander

 

[dr-ebert]

That's what I got as well from a rough back-of-the-mind calculation. For every distance where you would seriously consider a beam expander, you get bigger=better. Even a 100x expander isn't unreasonable - this corresponds to a 10cm telescope, which using catadioptric optics (mirror/lens combinations) would still be very small.

I've once seen an ad from a telecomms company, offering lasers for data transmission. These had a 20cm telescope as a receiver/transmitter optic.

 

[Benm]

Interesting calculation on the streetlights. I suppose the visibility would depend on the (angular) size of the city as well... if you compare only the total emitted light by the entire city versus the laser, that assumes you would not see any detail in the city from orbit.

The angular acuity of a (perfect) human eye is in the order of 0.3 mrad, or 300 meters at 1000 km distance. So to be visible, the laser has to be brighter than a 300x300 meter city block, which seems very feasible if you assume such block has less than 1 MW of city lights in it.

 

[dr-ebert]

True. The laser would be (nearly) a point source, while the city would be a diffuse area. So even against a city background, it should be quite visible...

One thing not yet taken into account is air density fluctuations, which cause the "twinkling" of stars. Going in the other direction, this would cause the beam to "wander" over a larger area. However, this effect should cause deviations of much less than one arc minute, so it shouldn't seriously throw the calculations off.

 

[Benm]

Oh well, these are ballpark figures anyway, but they do show what low divergence can do even with very little power. I suppose it would be difficult to get someone to observe from space... As far as going for the furthest possible laser shoot, it's plainly up to the person that finds the longest line of sight path to use, on or off the planet

Too bad holland is perfectly flat, the best thing available here is frome one skyscraper to the next (which are all in lit cities). If we had a couple of mountains, it would be more intesting to go for a record.

 

[steve001]

This photo shows a low power <5 mw red laser expanded for communication across the English Channel. Distance is 34 miles 22 kilometers
Link http://www.lasercomms.org.uk/france.htm

This photo was taken from a distance of 47.29 miles

 

[Benm]

Looks pretty bright, but if i understand correctly, they did use beam expanders to do it. Then again, they state is only 3 mW of red, so the advantage of beam expanding could be cancelled out by more power and better wavelength (green).

 

[steve001]

Looks pretty bright, but if i understand correctly, they did use beam expanders to do it. Then again, they state is only 3 mW of red, so the advantage of beam expanding could be cancelled out by more power and better wavelength (green).

Yes they used an expander in the photos that's the point of the thread. Too demonstrate what expanding a laser beam and reducing the divergence can do for increasing visibility at great distances. As I recall the red laser was not visible from that distance without the use of an expander.

 

[laze_doctor]

Let me be the first to say, that is pretty fracking cool