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# Distance can a laser beam be visible? At NIGHT

#### Sak0ya

##### New member
If I focus my 2w 445nm laser to infinity, how many km or miles can the beam be visble or the dot?

#### steve001

##### Well-known member
If I focus my 2w 445nm laser to infinity, how many km or miles can the beam be visble or the dot?
Your question has too many variables to answer. Ask again an be precise this time.

#### diachi

##### Well-known member
If I focus my 2w 445nm laser to infinity, how many km or miles can the beam be visble or the dot?
Beam divergence (both axes)? Beam diameter at aperture? Atmospheric conditions?

Not enough info to even attempt to make an answer, which would be hard to give accurately anyway.

#### CE5

##### Active member
ISS Laser Flash

In the demo, the laser has been observed with the naked eye up to the orbital distance of the ISS. - 205 mile minimum orbital distance.

So the answer to your question is = Pretty darn far!

#### diachi

##### Well-known member
ISS Laser Flash

In the demo, the laser has been observed with the naked eye up to the orbital distance of the ISS. - 205 mile minimum orbital distance.

So the answer to your question is = Pretty darn far!
You likely wouldn't actually see the dot or beam at that distance though, it'd only be visible if the beam hit your eye directly.

#### CE5

##### Active member
Well that is correct in that a nicely defined beam or defined dot is not discernible to the naked eye at great distances. However, the flash of the laser is visible- as demonstrated in the link I provided. There is also another youtube clip that is floating around out there that better illustrated what a laser flash looks like from the ISS. That video used a green laser, and the flash was clearly visible in the streaming video clip. I did a brief youtube search for that clip but no joy.

The flash was similar to the flash observed in the 200mw Green Laser 25+ KM youtube clip that @steve001 posted above. Of course the flash observed from the viewports on the ISS were not quite as prominent as the 25+ KM clip, but it was still easily observable.

And since the flash is technically part of the dot that has diverged, in broad terms (about as broad as the beam spread (600 meters across) at those distances LOL!) the photons being observed are technically still involving part of the dot. Correct?

#### RedCowboy

##### Well-known member
I expect the ISS sees a lot of ground flashes from lasers but they likely wish to deter such behavior, especially with the laser/aircraft problem there's no way they could condone such activity, likely the reason night view from the project feed is always unavailable at night or at least has been every time I look at it.

At my sisters house 50 miles from the city I can see the ISS pass by, it's tiny but you can see it as it comes by in such a straight path, there's several tracking apps/sites such as here > https://spotthestation.nasa.gov/sightings/

Every time we visit my brother in law takes us outside to watch it when it's passing over head ( NO I don't shine any lasers at it )but there's no doubt they get hit with a lot of lasers and likely don't want to encourage it, not that it's dangerous at that distance but just annoying I expect, I suggest people should not target the ISS as it could only work against our hobby.

https://www.universetoday.com/93987/amateur-astronomers-flash-the-space-station/

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#### CE5

##### Active member
@RedCowboy thanks for chiming in and to be clear the link I posted ISS Laser Flash is from Laser Pointer Safety. And at the bottom of the article it also clearly states "LaserPointerSafety.com does not recommend that the general public try to replicate this experiment, by aiming lasers into the sky at the space station or any moving dot in the sky. In fact, for aiming at stars, low-powered lasers should be used (ideally 5 mW or less) and you should circle a star instead of pointing right at it, in case the “star” is really a slow-moving airplane. More info is on our Tips for outdoor use page. "

As there are other links about this experiment out there- such as the one you posted, that do not have the same disclaimer as the one mentioned in the Laser Pointer Safety link that I posted, it is still worth while to point this aspect out.

#### RedCowboy

##### Well-known member
@RedCowboy thanks for chiming in and to be clear the link I posted ISS Laser Flash is from Laser Pointer Safety. And at the bottom of the article it also clearly states "LaserPointerSafety.com does not recommend that the general public try to replicate this experiment, by aiming lasers into the sky at the space station or any moving dot in the sky. In fact, for aiming at stars, low-powered lasers should be used (ideally 5 mW or less) and you should circle a star instead of pointing right at it, in case the “star” is really a slow-moving airplane. More info is on our Tips for outdoor use page. "

As there are other links about this experiment out there- such as the one you posted, that do not have the same disclaimer as the one mentioned in the Laser Pointer Safety link that I posted, it is still worth while to point this aspect out.
Yes, if/when ever I am thinking of pointing a laser at a star I 1st look and listen for aircraft and I don't take any chances, if an airplane is off to the side and flying away from my star I still wait until it's out of my sight, also everyone should know that helicopters can pop up over a tree line pretty quickly without much notice depending on the level of background noise so I suggest people always look and listen 1st before pointing to the sky.