Last Saturday night at about 10pm I was at Warm Springs Oregon. That is an Indian Res. My 2 grown children are 1/2 Warm Springs Indian. We were at a house on a hill overlooking a valley, more of a bowl 4miles in diameter.To the southwest is Mt.Jefferson about 35 miles away. It was a clear night with little wind and no clouds or particulates in the air. Hitting the sides of the valley "bowl" was child's play for my RPL-450 with a 3X beam expander. So we started pointing towards Mr.Jefferson with its white snowy slopes. All four of us were sure we could see the light scattering when it hit the snow but weren't sure. So we got out the Orion 6" reflector telescope and pointed it at the mountain. Sure enough that laser light was going 35miles reflecting off the snow and enough of it was left to see the return image in the telescope. That was some exciting stuff.
I know there may be some who will doubt you could hit the mountain at 35 miles but all four of us saw the same thing. I will get a decent camera soon and make photos of this in the near future.

Only 35 Miles ??? :-) Some folks using a telescope have found the reflectors on the moon. Still it's fun to see the beam at that distance.
Always look out for aircraft.

Mike

On the moon?

I've seen pics of the gear used for the lunar range finding experiments - seems to entail a lot more than a powerful green and a simple telescope.

As for seeing the reflection at 35 miles: that seems entirely feasible with some binoculars or a telescope, as long as its dark.

Hi Benm,
I would have to agree with you, it takes orders of magnitudes of both laser power and light gathering (telescope) than any hobbyist or amateur is going to have on hand.

I just visited the McDonald observatory lunar ranging system website.
The specs for the laser is 1500mj which is 1.5 joules of energy, it supplies this energy in a 200 picosecond pulse. Converting this amount of energy over this short time period your looking at 750GW, that Gigawatts or 7500000000 Watts with a beam diameter of 7mm and a divergence of 0.075urad (micro radians).
On the receiving end they use a 30 inch reflector coupled to very sensitive electronic sensors and when they send this 200 picosecond pulse of 750GW, they only get back a few photons of the light that was transmitted.

That will be interesting to see. A 10X beam expander should make it even easier to see the the spot with the naked eye

They went there on Mythbusters on the Moon landing Myth eppisode.

why don't i see the 450mw laser on the opto site? the max il 425.

I have some A4 sized fresnell lenses on their way here - they could be part of a very nice beam expander if somehow properly set up. The good thing about large beam expanders is that ridicilously large amounts of power are still essentially eye safe - 400 mW on a 20x20 cm would still limit eye exposure to under 1 mW (each, and in this case you might hit both at the same time ).

Regarding the moon bouncing, the returning light was measured in individual photons. That little light would be impossible to detect by eye. You'd definitely need a telescope, like Mike said, but even then, I think you'd have a difficult time seeing the reflection..

Wow, that is quite impressive. Counting the reflection, the total distance is actually 70 miles! What's even more impressive is that a lot of light had probably been scattered when it hit the snow.

I'm guessing if you were to look into the beam directly, you would be able to see it several hundred miles way in a low-light, low-disturbance environment (such as a desert).

I got up at around 2am that night(day)and pointed the beam around the edges of the valley "bowl" and got a 100 coyote chorus. That too was impressive.
When I talked with Jack about upgrading my RPL-225 he said he had three 450s' that he hadn't put up on the web site yet. He gave me the "homey hookup" supposedly. It is indeed a 450. I tend to agree with Jack that you get more "bang for your buck" with a 3X than a 10X beam expander. I have both.

You could, but you would need to get up very high to have a horizon that far away. If you were to construct 2 towers, each 500 feet tall, you could shine and observe a beam between them over 200 miles or so.

Also, you should probably define 'dark'. The human eye is capable of registering a single photon entering it if adapted to complete darkness for a long time. Under starlight, and especially moonlight, this would already be different. Considering urban background lighting, it's a different picture altogheter.

Hey that was a great story. I'm guessing that a 300mw greenie would be blindingly bright all the way to the horizon ~10-14 miles, looking straight into the beam.

How so ?

The 3X gives all the focus you need for playing around. The 10X does the same thing, barely noticeable in comparison, but at twice the price.

I thought price was what you were refering to.
However performance wise a 10x expander will put more energy into a given area do to the smaller beam divergence; which would translate into a brighter spot in the far field. And if it's distance you want then lowering the beam divergence is the goal. However very low divergences can be counter productive by spreading a beam to wide from the start will result in a given area in the far field with a lower over all radiance making it difficult to see with the naked eye. I know this from practical experience.

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).

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.

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.

^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 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

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..

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

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

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.