Talk:Laser guide star

Distortion
When you create an artificial guide star in the mesosphere, why doesn't the laser beam get distorted by atmospheric convection just like the light of stars does when it comes down through the atmosphere (Seeing disk)? 193.171.121.30 15:45, 9 Nov 2004 (UTC)


 * It does! IANAAstrophysicist but I'm certain the beam is affected by atmospheric effects on the way up just as the light on the way back down is. Thing is, who cares! All that's being done with the laser is to form a small area of fluorescing sodium atoms (ionized? dunno..) in the upper atmosphere. The area at high altitude emitting light then, is probably no bigger than a beach-ball or so in diameter (it's likely many meters in length though, I'm not intimately familliar with the thickness of the "sodium layer" in the mesosphere) since the divergence of a good quality laser beam is not sufficient to expand it appreciably over a small distance of ~50 miles. So you've got this relatively small glowing area of gas which is above much of the atmosphere, the light it emits then travels back down to the telescope and is "imprinted" with the aberrations intrinsic to the column of atmosphere it went through at that instant. It dosen't matter if the phase or shape or whatever of the laser going UP is distorted because the process of fluorescence destroys that information, and even if it weren't I doubt (I don't really want to do the calculations right now) that the telescope could achieve the resolution to resolve the glowing sodium as anything more than a spot even if it were diffraction limited. Again IANAP, but I'm confident of the accuracy of what I've said here. --Deglr6328 21:25, 27 Nov 2004 (UTC)

I still don't understand it completely - let's say you've got a seeing disc with a diameter of x = 0.5 arcseconds, then, if the divergence of the laser beam is low enough, in height h you should get a spot with the size h*x, which is about 0.2 meters in 90 km height. Now the real spot size is smaller (diffraction limited by laser optics) but it moves around in the 0.2 m region due to atmospheric convection.

Now I'm a bit confused. The timescale of fluorescence (the time the excited atoms need to fall back to ground state) could be much larger than the timescale of atmospheric fluctuations, so the laser excites the whole seeing disc up there (I know it's not really a disc but more like a cylinder) and the telescope mirror is adjusted to see this well-defined disc undisturbed. But according to this webpage, the lifetime is about 10-8 seconds, while the twinkling of the stars can be seen with the naked eye. So this is not the case. Now the timescale of the fluctuations seems to be longer than the time the light needs to go up there, get re-emitted and come back down (0.6 milliseconds). So the light should come back to the laser along the same path - interesting, but I just realize this is not relevant for the telescope, which is not at exactly the same position as the laser, and the spatial variability scale is about the size of a human pupil or a bit larger. Now the spot stays small but moves around there in the 0.2 m seeing disc up there? So we can adjust the mirror in a way that a dot appears but this dot moves around a bit? 193.171.121.30 01:20, 28 Nov 2004 (UTC)


 * ???? I don't know what you're not understanding. Na fluorescence time delay is quite irrelevant and it only takes .3 ms for light to travel down from the guide star. Plenty short enough for you to take an "instant" snapshot of the aberration in the whole air column. Most AO systems have a closed loop refresh time of less than a few hundred Hz anyway.....


 * So it's basically like i said in my last sentence - through the telescope the guide star looks like a dot (if the adjustments are done) but moves around a bit? 193.171.121.30 14:41, 28 Nov 2004 (UTC)


 * I guess it would move around a bit but we're not talking Km here. I doubt its more than a few m, and when youre looking at it from 90Km away it would really be pretty minor, no?--Deglr6328 02:55, 29 Nov 2004 (UTC)


 * Yes - the region it moves around should be about equal to the maximum resolution of the telescope without adaptive optics. 193.171.121.30 03:36, 29 Nov 2004 (UTC)

Explanation of all this stuff about seeing
Indeed, the laser is affected by astronomical seeing. It breaks up into speckle patterns (like the one shown here: Image:Ast_seeing_20r0.png), and these speckles move around. The adaptive optics system tracks this pattern of speckles (mostly the brightest speckle) and does its correction of the shape of the incoming wavefronts. But as you point out, the speckle pattern changes and moves around. Because of this, even a laser guide star AO system needs a real star to do the tip-tilt (pointing) correction and remove the effects of the laser speckles moving around. Because only tip and tilt need to be corrected, it can be a much fainter star than is required for normal adaptive optics (where the shape of the wavefront is also measured using the real star).

Rnt20 13:33, 8 May 2005 (UTC)

Liquids and commercial ad
The sentence "the use of fluid gain media including toxic chemicals is a disadvantage" is only true if bad and sloppy engineering leads to dye spills. Also this type of assertion should be referenced. The same applies to the consumption of electrical power.

And the advertisement of particular commercial products is not part of the Wikipedia culture. — Preceding unsigned comment added by 172.101.163.28 (talk) 19:20, 4 December 2015 (UTC)

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