APOD: Nebula with Laser Beams (2017 Mar 29)

[APOD Robot]

Nebula with Laser Beams

Explanation: Four laser beams cut across this startling image of the Orion Nebula, as seen from ESO's Paranal Observatory in the Atacama desert on planet Earth. Not part of an interstellar conflict, the lasers are being used for an observation of Orion by UT4, one of the observatory's very large telescopes, in a technical test of an image-sharpening adaptive optics system. This view of the nebula with laser beams was captured by a small telescope from outside the UT4 enclosure. The beams are visible from that perspective because in the first few kilometers above the observatory the Earth's dense lower atmosphere scatters the laser light. The four small segments appearing beyond the beams are emission from an atmospheric layer of sodium atoms excited by the laser light at higher altitudes of 80-90 kilometers. Seen from the perspective of the UT4, those segments form bright spots or artificial guide stars. Their fluctuations are used in real-time to correct for atmospheric blurring along the line-of-sight by controlling a deformable mirror in the telescope's optical path.

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[Boomer12k]

Wonderful... too bad my 8" Meade Lightswitch telescope can't do that...

:---[===] *

[heehaw]

Wonderful is the word!

[Coil_Smoke]

Thank You, I always wondered what the excited sodium layer would look like. I never thought it could be seen so vividly and be so easy to detect the returning(emitted) photons.

[Guest]

Not part of an interstellar conflict....until they hit something and they come looking for us

[giles]

Perhaps the caption of this astounding photo says more than what is really here: judging by the direction of the beams the telescope is not pointing towards orion at the time the photograph was taken. Same projection effect as in https://apod.nasa.gov/apod/ap170117.html .

[neufer]

http://asterisk.apod.com/viewtopic.php? ... 85#p268385

[Chris Peterson]

Perhaps the caption of this astounding photo says more than what is really here: judging by the direction of the beams the telescope is not pointing towards orion at the time the photograph was taken. Same projection effect as in https://apod.nasa.gov/apod/ap170117.html .

The beams are stimulating the re-emission of 589 nm light in a thin layer some 90 km high. Unless the photographer was very far from the observatory, the position of the resulting artificial guidestars will closely match the celestial coordinates they target, with only a tiny amount of parallax error. This image is consistent with the telescope pointing at the Trapezium while painting four guidestars around that point. Each guidestar looks to be about 5 arcminutes from the center, which is consistent with the typical size of the region that can be corrected (where the atmosphere is behaving more or less coherently).

[neufer]

[b][color=#0000FF]The beams converge on a vanishing point well to the right of the Trapezium. (And Chris is behaving more or less incoherently.)[/color][/b]

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Perhaps the caption of this astounding photo says more than what is really here: judging by the direction of the beams the telescope is not pointing towards orion at the time the photograph was taken. Same projection effect as in https://apod.nasa.gov/apod/ap170117.html .

The beams are stimulating the re-emission of 589 nm light in a thin layer some 90 km high. Unless the photographer was very far from the observatory, the position of the resulting artificial guidestars will closely match the celestial coordinates they target, with only a tiny amount of parallax error. This image is consistent with the telescope pointing at the Trapezium while painting four guidestars around that point. Each guidestar looks to be about 5 arcminutes from the center, which is consistent with the typical size of the region that can be corrected (where the atmosphere is behaving more or less coherently).

[Whiteelk]

If the Orion Nebula is 1500 light years away how can we observe laser light passing through it? Wouldn't the round trip be ~3000 years?
Thanks,
Bill

[Chris Peterson]

[b][color=#0000FF]The beams converge on a vanishing point well to the right of the Trapezium. (And Chris is behaving more or less incoherently.)[/color][/b]

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Perhaps the caption of this astounding photo says more than what is really here: judging by the direction of the beams the telescope is not pointing towards orion at the time the photograph was taken. Same projection effect as in https://apod.nasa.gov/apod/ap170117.html .

The beams are stimulating the re-emission of 589 nm light in a thin layer some 90 km high. Unless the photographer was very far from the observatory, the position of the resulting artificial guidestars will closely match the celestial coordinates they target, with only a tiny amount of parallax error. This image is consistent with the telescope pointing at the Trapezium while painting four guidestars around that point. Each guidestar looks to be about 5 arcminutes from the center, which is consistent with the typical size of the region that can be corrected (where the atmosphere is behaving more or less coherently).

I think the flaw in your logic is the assumption that the beams are parallel. In fact, the beams are divergent.

[Chris Peterson]

If the Orion Nebula is 1500 light years away how can we observe laser light passing through it? Wouldn't the round trip be ~3000 years?

We only observe the beams for a few kilometers above the observatory, scattering off of particles in the air. And we observe the artificial guidestars at the apparent end of the beams about 90 km up. The system is designed to compensate for the movement of air below that 90 km height. Everything between there and the Orion Nebula doesn't matter.

[neufer]

I think the flaw in your logic is the assumption that the beams are parallel.
In fact, the beams are divergent.

The distance between the beams is greatest at the left.

[Chris Peterson]

I think the flaw in your logic is the assumption that the beams are parallel.
In fact, the beams are divergent.

The distance between the beams is greatest at the left.

I'm not sure what your point is, however. The beams don't have a vanishing point because they aren't parallel. And because the photographer wasn't centered on the telescope, there is some degree of parallax effect (which we might estimate by the length of the sodium emission sections, given knowledge of the thickness of the sodium layer). But given that we can reasonably assume the imager was quite close to the telescope, and that we are seeing the artificial guide stars at 90 km high, we do, in fact, know that the field that was being corrected is very nearly centered on the Trapezium.

[vendetta]

The picture was taken with a small telescope several dozen meters away from the UT4 telescope where the lasers are mounted. So there's indeed some small parallax.

As for whether the beams are divergent or not, in this picture they are. At the telescope the laser launchers are separated by about 8 meters, so when the beams are parallel they subtend an angle of about 20 arcseconds at an altitude of 80 km. That's much closer to each other than what you see in this picture.

In practice the lasers can be actually pointed independently, so that the artificial stars are closer or further away from each other. This depends on the instrument you want to use, and whether you want an average correction over a large field of view or a very good correction over a smaller field.

JC

[BDanielMayfield]

I was wondering why there is a layer of sodium about 90 Km up in Earth's atmosphere. Searching "sodium layer" revealed that this sodium comes from meteor ablation.

I guess that, where it not for continuing meteor activity, all the Na would settle out and this method of making artificial guidestars wouldn't work.

Bruce

[kgbudge]

"Now witness the firepower of this fully armed and operational emission nebula!"

[neufer]

The picture was taken with a small telescope several dozen meters away from the UT4 telescope where the lasers are mounted. So there's indeed some small parallax.

As for whether the beams are divergent or not, in this picture they are. At the telescope the laser launchers are separated by about 8 meters, so when the beams are parallel they subtend an angle of about 20 arcseconds at an altitude of 80 km. That's much closer to each other than what you see in this picture.

In practice the lasers can be actually pointed independently, so that the artificial stars are closer or further away from each other. This depends on the instrument you want to use, and whether you want an average correction over a large field of view or a very good correction over a smaller field.

• OK, I get it now.

Assuming the sodium layer is one sixth its altitude in thickness each individual laser beam will converge to a different spot ~6 times it's (sodium layer) length further along to the right. It is that square (at the edge of the Orion Nebula) that defines the region (De Mairan's Nebula M43 ) the telescope is focused on. (Unless, of course, that laser defined square region was intentionally place up[jet]stream from the region of interest.)

[MarkBour]

Interesting to read the viewtopic above that pointed to a discussion where someone was trying to figure out why they saw some lights in the sky one night. Whether that incident was from artificial guide stars or not, it's a bit ironic to think that those who are the most enthusiastic proponents of dark skies seem to have found a reason to add some rather significant light pollution to the skyscape.

[neufer]

Interesting to read the viewtopic above that pointed to a discussion where someone was trying to figure out why they saw some lights in the sky one night. Whether that incident was from artificial guide stars or not, it's a bit ironic to think that those who are the most enthusiastic proponents of dark skies seem to have found a reason to add some rather significant light pollution to the skyscape.

• At least it's not broad band light pollution.

<<For locations where light pollution is a consideration, such as near astronomical observatories or sea turtle nesting beaches, low-pressure sodium is preferred (as in San Jose and Flagstaff, Arizona). Such lamps emit light on just two dominant spectral lines (with other much weaker lines), and therefore have the least spectral interference with astronomical observation. The yellow color of low-pressure sodium lamps also leads to the least visual sky glow, due primarily to the Purkinje shift of dark-adapted human vision, causing the eye to be relatively insensitive to the yellow light scattered at low luminance levels in the clear atmosphere. One consequence of widespread public lighting is that on cloudy nights, cities with enough lighting are illuminated by light reflected off the clouds. Where sodium vapor lights are the source of urban illumination, the night sky is tinged with orange.>>

[starsurfer]

Looks like a still from the music video of a disco song.

[Chris Peterson]

Interesting to read the viewtopic above that pointed to a discussion where someone was trying to figure out why they saw some lights in the sky one night. Whether that incident was from artificial guide stars or not, it's a bit ironic to think that those who are the most enthusiastic proponents of dark skies seem to have found a reason to add some rather significant light pollution to the skyscape.

The light doesn't scatter enough to have any impact on the surrounding sky. If you're near enough to the observatory, you'll see the faint yellow lines pointing up into the sky, but that won't interfere at all with either your dark vision or the appearance of the rest of the sky. So I don't think this could ever qualify as "significant" light pollution.

[Chris Peterson]

Assuming the sodium layer is one sixth its altitude in thickness each individual laser beam will converge to a different spot ~6 times it's (sodium layer) length further along to the right. It is that square (at the edge of the Orion Nebula) that defines the region (De Mairan's Nebula M43 :?: ) the telescope is focused on. (Unless, of course, that laser defined square region was intentionally place up[jet]stream from the region of interest.)

No. It doesn't matter where the laser beams "converge". What matters is the position on the sky where they stimulate the sodium layer. That is, where the beam intersects a 5 km thick layer of atmosphere 90 km high. If you want to place an artificial guidestar at some particular celestial coordinate, that's exactly where you aim your laser. Moving your viewpoint a few meters to the side doesn't significantly change the position of that point with respect to the background. Assuming that the target lies at the center of the square (which is a reasonable assumption), the target in this case is, in fact, very close to the Trapezium.

BTW, it is common these days to use a green laser pointer instead of a finder scope in aiming small telescopes. The laser is mounted to the telescope tube, coaxial with its optical axis. Where you see the laser beam end in the sky is where the scope is pointed, and that doesn't change if you move away from the scope.

[neufer]

Assuming the sodium layer is one sixth its altitude in thickness each individual laser beam will converge to a different spot ~6 times it's (sodium layer) length further along to the right. It is that square (at the edge of the Orion Nebula) that defines the region (De Mairan's Nebula M43 ) the telescope is focused on. (Unless, of course, that laser defined square region was intentionally place up[jet]stream from the region of interest.)

No. It doesn't matter where the laser beams "converge". What matters is the position on the sky where they stimulate the sodium layer. That is, where the beam intersects a 5 km thick layer of atmosphere 90 km high. If you want to place an artificial guidestar at some particular celestial coordinate, that's exactly where you aim your laser. Moving your viewpoint a few meters to the side doesn't significantly change the position of that point with respect to the background. Assuming that the target lies at the center of the square (which is a reasonable assumption), the target in this case is, in fact, very close to the Trapezium.

I am quite certain from my above analysis (assuming a 15 km thick layer of atmosphere 90 km high) that the target in this case was, in fact, De Mairan's Nebula M43. (In the unlikely case that the atmosphere layer was only 5 km thick layer, however, then the target would be shifted three times the amount I have indicated.)

In any event, so far as the telescope is concerned, the target is dead center for both the four separate vanishing points as well as the wider square of the four sodium "artificial guidestars." However, I estimate that the APOD picture was taken a good 400m to the side of the telescope causing the sodium "artificial guidestars" to shift by about a quarter of a degree (from the vanishing point square).

BTW, it is common these days to use a green laser pointer instead of a finder scope in aiming small telescopes. The laser is mounted to the telescope tube, coaxial with its optical axis. Where you see the laser beam end in the sky is where the scope is pointed, and that doesn't change if you move away from the scope.

Try moving 40m away.

[Chris Peterson]

No. It doesn't matter where the laser beams "converge". What matters is the position on the sky where they stimulate the sodium layer. That is, where the beam intersects a 5 km thick layer of atmosphere 90 km high. If you want to place an artificial guidestar at some particular celestial coordinate, that's exactly where you aim your laser. Moving your viewpoint a few meters to the side doesn't significantly change the position of that point with respect to the background. Assuming that the target lies at the center of the square (which is a reasonable assumption), the target in this case is, in fact, very close to the Trapezium.

I am quite certain from my above analysis (assuming a 15 km thick layer of atmosphere 90 km high) that the target in this case was, in fact, De Mairan's Nebula M43. (In the unlikely case that the atmosphere layer was only 5 km thick layer, however, then the target would be shifted three times the amount I have indicated.)

A 15 km thick sodium layer would be quite extraordinary. The layer is rarely much thicker than 5 km. Without knowing more details about how the image was acquired, and what they were doing with the telescope at the time, I'd be cautious using the apparent length of the fluorescing region in any calculations. We don't even know that the lasers were being held in one place.

In any event, so far as the telescope is concerned, the target is dead center for both the four separate vanishing points as well as the wider square of the four sodium "artificial guidestars." However, I estimate that the APOD picture was taken a good 400m to the side of the telescope causing the sodium "artificial guidestars" to shift by about a quarter of a degree (from the vanishing point square).

Assuming that the earlier post describing the image was accurate, the distance of the imager was more like 50 meters from the telescope, which would create a 2 arcmin parallax shift, which can be compared with the 10 arcmin diagonal length of the square.

BTW, it is common these days to use a green laser pointer instead of a finder scope in aiming small telescopes. The laser is mounted to the telescope tube, coaxial with its optical axis. Where you see the laser beam end in the sky is where the scope is pointed, and that doesn't change if you move away from the scope.

Try moving 40m away.

People 100 meters away can easily tell where such a telescope is pointing.