APOD: Attack of the Laser Guide Stars (2018 May 18)

[geckzilla]

appears to be CG superimposed on photo background. Actual photos are in gallery and are even more impressive.

Or, I mean, you could just not say this if you're not sure. These lasers are relatively rarely photographed, and because of their unfamiliar appearance and similarity to actual cg animation, it is easy for your brain to pattern match it to what is more familiar to you. There are enough real photos of them from various photographers that you'd find they all pretty much look like this if you looked elsewhere.

[Bellerophon]

My first thought was that the lasers could damage the drone. I found out they are powerful enough to do so (each beam delivers 22 watts), but the beam diameter is 30 cm, so they'd do no more harm than a reading light (not to say that the laser might not damage the drone's camera, though). https://www.eso.org/public/teles-instr/ ... str/4lgsf/

• https://youtu.be/N6vOy2YPTg0?t=265

It sure looks like the drone's camera passes right through the beam at 4:26, so apparently it's not enough to damage the drone's camera.

[Chris Peterson]

My first thought was that the lasers could damage the drone. I found out they are powerful enough to do so (each beam delivers 22 watts), but the beam diameter is 30 cm, so they'd do no more harm than a reading light (not to say that the laser might not damage the drone's camera, though). https://www.eso.org/public/teles-instr/ ... str/4lgsf/

• https://youtu.be/N6vOy2YPTg0?t=265

It sure looks like the drone's camera passes right through the beam at 4:26, so apparently it's not enough to damage the drone's camera.

Yeah, not enough energy for that. The camera aperture is way too small. Those cameras can image the Sun without damage; the lower power laser isn't going to be a problem.

[alter-ego]

...
The smaller spot will actually cause a less damage. But in either case, blood flow will carry away the heat. (In reality, the optics of the eye will probably not allow such a small focus for a point source light.)

A good eye will focus to ≤20 microns on the retina

Anyway, again, the point is that these lasers pose no safety risk to anybody passing through them from above.

That is not a true statement, at least the ANSI safety criteria and ESO's operation protocol will not agree with you. For a night-time fly-over the irradiance at a pilot's eye is 10x higher than the 0.25sec blink response MPE (Maximum Permissible Exposure).

All the operations at the 4LGSF will follow a protocol to avoid any risk to aircraft. The laser system is equipped with an automatic aircraft avoidance system that shuts down the lasers if an aircraft ventures too close to the beams.

For obvious reasons, these safety limits have a certain amount of margin built in, and the exposure details are complicated by several variables. There would likely be cases of temporary vision impairment, but there is not data to support the certainty of your statement. Having worked in the laser field for decades, I think your comment is unsafe speculation.

[neufer]

...
The smaller spot will actually cause a less damage. But in either case, blood flow will carry away the heat. (In reality, the optics of the eye will probably not allow such a small focus for a point source light.)

A good eye will focus to ≤20 microns on the retina

Anyway, again, the point is that these lasers pose no safety risk to anybody passing through them from above.

That is not a true statement, at least the ANSI safety criteria and ESO's operation protocol will not agree with you. For a night-time fly-over the irradiance at a pilot's eye is 10x higher than the 0.25sec blink response MPE (Maximum Permissible Exposure).

All the operations at the 4LGSF will follow a protocol to avoid any risk to aircraft. The laser system is equipped with an automatic aircraft avoidance system that shuts down the lasers if an aircraft ventures too close to the beams.

For obvious reasons, these safety limits have a certain amount of margin built in, and the exposure details are complicated by several variables. There would likely be cases of temporary vision impairment, but there is not data to support the certainty of your statement. Having worked in the laser field for decades, I think your comment is unsafe speculation.

Interesting information, AE.

Of course it is dangerous for a pilot to be even temporarily blinded...especially during landing.

While Chris & I were having a more academic debate about the danger of permanent eye damage it is always important to keep in mind that there are folks out there who will play around with staring at either the Sun or a laser pointer (and certainly far more who will play around with aiming their laser pointer at others). Regardless of who is correct in our little academic debate I am glad to have landed on the more responsible side of our debate (for a change).

[orin stepanek]

That's the same sharpness you would get if the telescope were in space.

That's pretty darn good; I'd say

Yes, though the observation is a little misleading. This performance is only at longer wavelengths, and only in regions right around the artificial guide stars. If you want a sharp, wide field image, the little bitty Hubble mirror still outperforms any ground-based telescopes, regardless of their size or adaptive optics capabilities.

Hi Chris! Oh; I'll never take Hubble for granted as it really opened up Space imagery!

[Chris Peterson]

...
The smaller spot will actually cause a less damage. But in either case, blood flow will carry away the heat. (In reality, the optics of the eye will probably not allow such a small focus for a point source light.)

A good eye will focus to ≤20 microns on the retina

That would be a good eye indeed! For a bright point source, it will usually be quite a bit larger. And for somebody making the effort to stare, they will almost instantly start producing tears and that will further degrade the spot.

Anyway, again, the point is that these lasers pose no safety risk to anybody passing through them from above.

That is not a true statement, at least the ANSI safety criteria and ESO's operation protocol will not agree with you. For a night-time fly-over the irradiance at a pilot's eye is 10x higher than the 0.25sec blink response MPE (Maximum Permissible Exposure).

Well, I was discussing eye safety here. Yes, there are specs for pilots. However, realistically, there is still no danger. A pilot will experience more dazzle from a close lightning strike than she will for the microseconds that one of these lasers will illuminate the cockpit. It's not like a plane flying over this observatory is about to land! Pilots are trained to deal with light dazzle. They will experience more energy in their eye from a conventional advertising searchlight over a city than from one of these lasers.

Having worked in the laser field for decades, I think your comment is unsafe speculation.

Not at all. I was a laser safety officer at Beckman Instruments for some years, and then spent many more years developing surgical eye equipment, including tools designed to focus lasers onto the retina. This is an area I'm very familiar with.

[alter-ego]

...
Well, I was discussing eye safety here. Yes, there are specs for pilots. However, realistically, there is still no danger. A pilot will experience more dazzle from a close lightning strike than she will for the microseconds that one of these lasers will illuminate the cockpit. It's not like a plane flying over this observatory is about to land! Pilots are trained to deal with light dazzle. They will experience more energy in their eye from a conventional advertising searchlight over a city than from one of these lasers.
...

I did more research and calcs to reconcile a apparently large discrepancy between the MPE calculation and your confident claim that retinal damage would not occur with an LGS. There are factors that both reduced and increased exposure times for damage thresholds:

• LGS profile is Gaussian, therefore the peak irradiance at the beam center = 2x higher than average = 62mJ/cm²

• I found a reference claiming that in order to assure a safe exposure limit, tabulated MPE values are an "order of magnitude" below damage thresholds

I calculated exposure time limits for the LGS incorporating the above corrections, and the exposure time for damage from solar radiation (400nm to 1400nm:

• ~0.25 seconds is coincidentally the time limit for on-axis viewing of the LGS beam

• 10 to 20 seconds is the limit one can look at the sun
  Note: This is in reasonable agreement with the retinal burn a woman suffered while looking at the 2017 solar eclipse from New York. She estimated her viewing time was ~20 seconds, and she was wearing some bogus eclipse glasses.

Allowing for significant uncertainty in iris aperture size, I've concluded that:

• One must exercise more caution than I believe you said (or implied) for intrabeam viewing of a 22W LGS laser, and

• Exposure time is longer for retinal damage from the sun than required for damage from the LGS source

However:

• The calculation(s) do support the very low probability that damage from a fly-over due to a short exposure time (unless you're in a helicopter)
  I completely agree with you on this point.

BTW, these new calculations have also reconciled my own benign experience with above-MPE exposure to 5mW to 10mW HeNe lasers without damage.