Starship Asterisk*

giles wrote: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 .

Chris Peterson wrote:

giles wrote:
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 wrote:

[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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Chris Peterson wrote:

giles wrote:
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 wrote: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?

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

neufer wrote:

Chris Peterson wrote:
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.

vendetta wrote:
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.

MarkBour wrote:
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.

https://en.wikipedia.org/wiki/Sodium-vapor_lamp#Light_pollution_considerations wrote:
<<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.>>

MarkBour wrote: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 wrote: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.)

Chris Peterson wrote:

neufer wrote:
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.

Chris Peterson wrote:
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 wrote:

Chris Peterson wrote: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).

Chris Peterson wrote: 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.