Starship Asterisk*

JohnD wrote: ↑Sun Feb 21, 2021 11:08 am Just for comparison, I read that the US Army is developing a TUPLARP, Tactical Ultrashort Pulsed Laser for Army Platforms, that will deliver fentosecond pulses in the Terawatt range! The average power will be only 20-50 Watts, but in such short pulses will vaporise the target. If it hits it.
See: https://www.sbir.gov/node/1654485

NGC3314 wrote: ↑Wed Feb 17, 2021 7:12 pm

Sites with visible-light AO systems use human plane spotters to avoid accidentally flashing a pilot's vision, with some work in progress on automated systems to first augment, and eventually replace, the spotters.

Chris Peterson wrote: ↑Fri Feb 12, 2021 5:33 pm

JohnD wrote: ↑Fri Feb 12, 2021 5:23 pm Chris, I'm sorry if my concern - idle, because I'm on the other side of the planet! - gave you a hard time. Thanks again for your patient education!

It was a reasonable question. Anybody who has worked around lasers at all knows that a 22 W laser is dangerous. But we're not usually accustomed to encountering them in the form of 30 cm diameter beams. That can distort our intuition.

JohnD wrote: ↑Fri Feb 12, 2021 5:23 pm Neufer,
I was concerned, and matbe started this thread down this route, but Chris' answers have reassured me!

Chris, I'm sorry if my concern - idle, because I'm on the other side of the planet! - gave you a hard time. Thanks again for your patient education!
John

Chris Peterson wrote: ↑Fri Feb 12, 2021 4:57 am
Well, as this whole long thread STARTED with questions about safety,
perhaps you can understand the confusion.

• No mind.

neufer wrote: ↑Fri Feb 12, 2021 2:13 am

Chris Peterson wrote: ↑Thu Feb 11, 2021 10:39 pm
Yes. So what? Why is that any health or safety concern? You'd be far worse off encountering a 20 mW laser beam of typical 1 mm diameter!

This long thread has NOTHING to do with any health or safety concern :!:

Rather, I was simply ADDING to your own response to John's question:

JohnD wrote: ↑Thu Feb 11, 2021 6:09 pm
Now, please explain why they are visible? So very visible!
The beam from a milliwatt hand held pointer is invisible, unless in a smoke filled room.

Chris Peterson wrote: ↑Thu Feb 11, 2021 10:39 pm
Yes. So what? Why is that any health or safety concern? You'd be far worse off encountering a 20 mW laser beam of typical 1 mm diameter!

JohnD wrote: ↑Thu Feb 11, 2021 6:09 pm
Now, please explain why they are visible? So very visible!
The beam from a milliwatt hand held pointer is invisible, unless in a smoke filled room.

neufer wrote: ↑Thu Feb 11, 2021 10:32 pm

neufer wrote: ↑Thu Feb 11, 2021 8:04 pm

neufer wrote: ↑Thu Feb 11, 2021 8:04 pm
Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:50 pm

Yes. Your point?

• 22 watts of eye sensitive yellow light (= 15,000 lumens) shouldn't be sneezed at.
  (And it is not clear to me that the ground atmospheric sodium is entirely negligible.)

Drawing of Newton dispersing light with a glass prism: Apic/Getty Images

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Chris Peterson wrote: ↑Thu Feb 11, 2021 8:08 pm
"22 watts" is meaningless without considering the density. Every square meter of ground is receiving around 1000 watts in the middle of the day. So what?

A beam with a diameter of
30 centimetres = .0707 m²

So 311 watts per square meter
in the middle of the night.

neufer wrote: ↑Thu Feb 11, 2021 8:04 pm

neufer wrote: ↑Thu Feb 11, 2021 8:04 pm
Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:50 pm

Yes. Your point?

• 22 watts of eye sensitive yellow light (= 15,000 lumens) shouldn't be sneezed at.
  (And it is not clear to me that the ground atmospheric sodium is entirely negligible.)

Chris Peterson wrote: ↑Thu Feb 11, 2021 8:08 pm
"22 watts" is meaningless without considering the density. Every square meter of ground is receiving around 1000 watts in the middle of the day. So what?

neufer wrote: ↑Thu Feb 11, 2021 8:04 pm

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:50 pm

neufer wrote: ↑Thu Feb 11, 2021 6:29 pm

Yes. Your point?

• 22 watts of eye sensitive yellow light (= 15,000 lumens) shouldn't be sneezed at.
  (And it is not clear to me that the ground atmospheric sodium is entirely negligible.)

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:50 pm

neufer wrote: ↑Thu Feb 11, 2021 6:29 pm

https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/4lgsf/ wrote:
Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

Yes. Your point?

• 22 watts of eye sensitive yellow light (= 15,000 lumens) shouldn't be sneezed at.
  (And it is not clear to me that the ground atmospheric sodium is entirely negligible.)

https://en.wikipedia.org/wiki/Tribute_in_Light wrote:

Tribute in Light set up atop Battery Parking Garage in 2018

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<<The Tribute in Light is an art installation created in remembrance of the September 11 attacks. It consists of 88 vertical searchlights arranged in two columns of light to represent the Twin Towers. Tribute in Light began as a temporary commemoration of the attacks in early 2002, but it became an annual event, currently produced on September 11 by the Municipal Art Society of New York. On clear nights, the lights can be seen from over 60 miles away, visible in all of New York City and most of suburban Northern New Jersey and Long Island.

The 88 xenon 7 kW spotlights (44 for each tower) each produce up to 350,000 lumens.

The world's most powerful searchlight today beams from the top of the pyramid-shaped Luxor Hotel in Las Vegas. The Luxor beam concentrates about 13,650,000 lumens from 39 xenon 7 kW lamps into its beam.>>

neufer wrote: ↑Thu Feb 11, 2021 6:29 pm

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:21 pm

JohnD wrote: ↑Thu Feb 11, 2021 6:09 pm
Now, please explain why they are visible? So very visible! The beam from a milliwatt hand held pointer is invisible, unless in a smoke filled room.

The shorter the wavelength, the greater the scatter in air. That's why green lasers are so much more visible than red ones. Here we have a wavelength in the middle, so we'd expect more scatter than from a red pointer.

However, the major factor is simply exposure time. In this image we see stuff in the sky that would be far less obvious visually. That tells us that the image is capturing more than the eye... as is typical for most nighttime images made with exposures of a few seconds or more. Go in a dark room, activate a 1 mW red laser pointer, and take an exposure that is several seconds long at high ISO. That beam will stand out dramatically, even if your eye sees only a few sparkles.

Visually, we see AO beams. But they don't look anywhere near as intense as we see them here.

https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/4lgsf/ wrote:
Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

Chris Peterson wrote: ↑Thu Feb 11, 2021 6:21 pm

JohnD wrote: ↑Thu Feb 11, 2021 6:09 pm
Now, please explain why they are visible? So very visible! The beam from a milliwatt hand held pointer is invisible, unless in a smoke filled room.

The shorter the wavelength, the greater the scatter in air. That's why green lasers are so much more visible than red ones. Here we have a wavelength in the middle, so we'd expect more scatter than from a red pointer.

However, the major factor is simply exposure time. In this image we see stuff in the sky that would be far less obvious visually. That tells us that the image is capturing more than the eye... as is typical for most nighttime images made with exposures of a few seconds or more. Go in a dark room, activate a 1 mW red laser pointer, and take an exposure that is several seconds long at high ISO. That beam will stand out dramatically, even if your eye sees only a few sparkles.

Visually, we see AO beams. But they don't look anywhere near as intense as we see them here.

https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/4lgsf/ wrote:
Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

JohnD wrote: ↑Thu Feb 11, 2021 6:09 pm Again, thnaks for ther explanation, Chris! More reassuring.
Now, please explain why they are visible? So very visible! The beam from a milliwatt hand held pointer is invisible, unless in a smoke filled room.

JohnD wrote: ↑Thu Feb 11, 2021 5:08 pm Hmmmmmmmmmmm, really, Chris? A handheld laser pointer should not have more than 1 MILLIwatt, while Class 4 lasers, used for surgery and for cutting wood and metal, are over 500 MILLIWatts and range to 1.5 Watts. These are 22 Watts EACH, at the lower end of theoretical military lasers! Even if a decimal point was misplaced, at 2.2W they would be extremely powerful!

As they are so visible, it's unlikely that birds will fly into the beam. Aircraft should be aware of their location and use, and stay clear, so realistically they should not be a danger. But I feel a Bond villain coming on - "Do you expect me to talk, Goldfinger?" "No, Mr.Bond, I expect you to die!"

Cat Gabrel wrote: ↑Thu Feb 11, 2021 3:00 pm Do these beams pose a danger to wildlife? Airplanes? Spaceships?

NGC3314 wrote: ↑Wed Feb 10, 2021 4:58 pm

johnnydeep wrote: ↑Wed Feb 10, 2021 4:46 pm AO always feels like a case of "extracting information from noise" to me, but obviously it works. Here's a twitter thread from the author of the picture - Juan Carlos Muñoz - explaining in more detail how AO works: https://twitter.com/astro_jcm/status/12 ... 7086560257.

It'a still quite amazing to me that it DOES work!

PS - exactly how do we know how to counteract the laser image flickering? Is it simply because we know what the laser SHOULD look like if there were no atmosphere? And if so, why wouldn't we know the same thing about a distant star that would appear as essentially a dimensionless point source?

Indeed, natural guide stars (AKA stars) can be used as reference sources, but, since the atmospheric effects change over small distances, the quality of the correction gets worse very rapidly moving away from the star (the usable field, known as the isoplanatic patch, is only arcseconds in size - less, say, the the apparent size of Jupiter). A laser can be pointed wherever needed, but a star is still needed to correct for the net wander in the image since that cancels between outbound and return laser trips (that star can be much fainter since the image is already corrected and the system needs much less information from it). Multiple lasers can provide correction for turbulence at multiple layers in the atmosphere, increasing the size of the corrected field of view.

johnnydeep wrote: ↑Wed Feb 10, 2021 4:46 pm AO always feels like a case of "extracting information from noise" to me, but obviously it works. Here's a twitter thread from the author of the picture - Juan Carlos Muñoz - explaining in more detail how AO works: https://twitter.com/astro_jcm/status/12 ... 7086560257.

It'a still quite amazing to me that it DOES work!

PS - exactly how do we know how to counteract the laser image flickering? Is it simply because we know what the laser SHOULD look like if there were no atmosphere? And if so, why wouldn't we know the same thing about a distant star that would appear as essentially a dimensionless point source?