APOD: Firing Lasers to Tame the Sky (2021 Feb 10)

[APOD Robot]

Firing Lasers to Tame the Sky

Explanation: Why do stars twinkle? Our atmosphere is to blame as pockets of slightly off-temperature air, in constant motion, distort the light paths from distant astronomical objects. Atmospheric turbulence is a problem for astronomers because it blurs the images of the sources they want to study. The telescope featured in this image, located at ESO's Paranal Observatory, is equipped with four lasers to combat this turbulence. The lasers are tuned to a color that excites atoms floating high in Earth's atmosphere -- sodium left by passing meteors. These glowing sodium spots act as artificial stars whose twinkling is immediately recorded and passed to a flexible mirror that deforms hundreds of times per second, counteracting atmospheric turbulence and resulting in crisper images. The de-twinkling of stars is a developing field of technology and allows, in some cases, Hubble-class images to be taken from the ground. This technique has also led to spin-off applications in human vision science, where it is used to obtain very sharp images of the retina.

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[JohnD]

Is that what you would see on the ground?
The lasers on the Paranal telescope " are tuned to a color that excites atoms floating high in Earth's atmosphere -- sodium left by passing meteors" and are very powerful, 22 Watts each, when a Class 4 laser is more than half a Watt, but is there enough sodium in the atmosphere at ground level to see those bars of light? Paranal was built because at nearly 3000 meters high, the air is cold, dry and dustfree.
JOhn

[heehaw]

Like JohnD I am suspicious - those solid yellow beams look too good to be true. Is that what you would SEE, standing outside the observatory? No fake news please! We are sick of it with Trump.

[Holger Nielsen]

Inspection of the image on pixel-level shows
1) light from bright stars is visible through the yellow beams
2) the lower end of the beams display a white core not visible at the top
3) the beams have not quite the same diameter
If the beams were faked I would not expect to see these effects. Perhaps a long exposure time made the beams look so bright.
It would be interesting to hear what the impression would be of a bystander.

[Chris Peterson]

Is that what you would see on the ground?
The lasers on the Paranal telescope " are tuned to a color that excites atoms floating high in Earth's atmosphere -- sodium left by passing meteors" and are very powerful, 22 Watts each, when a Class 4 laser is more than half a Watt, but is there enough sodium in the atmosphere at ground level to see those bars of light? Paranal was built because at nearly 3000 meters high, the air is cold, dry and dustfree.
JOhn

We aren't seeing sodium glow here. We're seeing the scattered light of the lasers directly. The sodium emission is a different color.

The long exposure makes everything brighter and more saturated (as we see in the sky), but yes, this is what we'd see. The lasers shooting up from observatories that have such AO systems are readily visible to the naked eye.

[JohnD]

Thank you, Chris!
I think I understand the principle, but it still seems extraordinary that astronomers, ever desperate for good observing and a dark sky, should shine such powerful lasers directly up their line of sight!
John

[Chris Peterson]

Thank you, Chris!
I think I understand the principle, but it still seems extraordinary that astronomers, ever desperate for good observing and a dark sky, should shine such powerful lasers directly up their line of sight!
John

The laser outputs its light at a single, extremely narrow wavelength. That is filtered out from the imaging optical path.

[wallyware]

I thought this technique had already been in use for years. Didn't we talk about it once, Jerry?

[Chris Peterson]

I thought this technique had already been in use for years. Didn't we talk about it once, Jerry?

Nothing new. But always improving.

[johnnydeep]

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?

[johnnydeep]

Inspection of the image on pixel-level shows
1) light from bright stars is visible through the yellow beams
2) the lower end of the beams display a white core not visible at the top
3) the beams have not quite the same diameter
If the beams were faked I would not expect to see these effects. Perhaps a long exposure time made the beams look so bright.
It would be interesting to hear what the impression would be of a bystander.

Excellent observations! I really have to remember to always click on the image and examine it at maximum size/resolution!

[orin stepanek]

Detwinkle; detwinkle; little star;
Now I see what you really are!

Wow! Now that's a smart telescope!

[NGC3314]

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.

[Chris Peterson]

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?

Yes. And the first AO systems did look at actual stars. And amateur AO systems still do. But most of the time, there are no stars in the field that are bright enough to use. It takes a fair bit of signal at the telescope to track the motion at high speed (many samples per second). Using a laser to generate an artificial star (or better, several of them) means you always have a reference in your field.

[johnnydeep]

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.

Thank you NGC3314, and Chris! I had forgotten about those real guide stars’ use for AO.

[Cat Gabrel]

Do these beams pose a danger to wildlife? Airplanes? Spaceships?

[Chris Peterson]

Do these beams pose a danger to wildlife? Airplanes? Spaceships?

Realistically, no. They're not even as bright as a lot of flashlights, with a mere 22 watts spread out in a 30 cm diameter beam.

[JohnD]

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!"

[Chris Peterson]

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!"

It is not the absolute power of the beam that matters, it is the power density. In this case, you have 22 watts spread out in a beam that is 30 cm in diameter. At most your eye could intercept about a 5 mm diameter sample out of that. Or about 1/3600 of the total. Meaning that visually, in this beam, you'd receive about the same energy on your retina as you would from a little laser pointer. Harmless. A bird flying through the beam would absorb considerably less energy than it would flying through a typical searchlight of the sort used to advertise store openings (several thousand watts over a few square meters), and we don't hear about birds dying or bursting into flames.

A power density of 5 mW/mm² simply doesn't present any significant hazards to anything overhead.

[JohnD]

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.

[Chris Peterson]

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.

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.

[neufer]

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.

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]

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.

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?

[neufer]

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.)

Tribute in Light set up atop Battery Parking Garage in 2018

---

<<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.>>

[Chris Peterson]

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.)

"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?

And the sodium content of air at ground level is very low. We're not seeing any stimulated emission.