Amazing Laser Application 12: Laser Guide Stars!

What's the application? Producing artificial "stars" to serve as a reference for telescopes using adaptive optics to correct for atmospheric turbulence. This allows ground-based telescopes to produce images that are as good as those from the Hubble Space Telescope.

What problem(s) is it the solution to? "How can I make this giant telescope produce even more impressive pictures?"

How does it work?The basic problem with ground-based telescopes, as anyone who has ever looked at the stars or listened to nursery rhymes can tell you, is that stars "twinkle." They appear to fluctuate in brightness from one moment to the next, and if you look at them closely enough, they appear to shift position very slightly. This is caused by turbulence in the atmosphere-- as the light from a distant star passes through the air above us, it encounters shifting patches of slightly different density and temperature, which bend the light onto slightly different paths. The overall effect is to "smear" the image of the star over a wider area. This isn't too awful when you're just looking at a star (a point just becomes a slightly fuzzier point), but it's fatal if you'd like to see the details of a galaxy or some such object.

i-2dbf8055ea5e5080e45b479154af305f-adaptive.gif"Adaptive Optics" is a technique for fixing this problem by adjusting the mirror of the telescope to compensate for the blurring in the atmosphere. The idea, shown in the figure (and explained at the CTIO site) is to use the telescope to image two objects, one of them something like a single star that we know should be a single point source, the other something like a galaxy with lots of fine detail. You split up the light from those two objects, and use a deformable mirror to correct the image of the star so that it appears as a single sharp point, rather than a blurred mess. If the star is close to the target object you'd like to see in detail, then the correction used to get the star back to a single image will almost certainly fix most of the blurring in the object you care about, as well, giving you a much sharper image.

This system can produce dramatic results-- see the images below with and without adaptive optics corrections, taken from the Center for Adaptive Optics. It requires constant small adjustments to the mirror to keep correcting the focus, as the atmospheric disturbances shift around and change, but that's something that can be automated, and run by a computer during the lengthy image runs that astronomers need to pick up details.

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Why are lasers essential? You might notice that the above description doesn't mention lasers at all. What do lasers have to do with this?

The maddening thing about stars is that they're scattered fairly randomly across the sky, and there's never one right where you want it. Depending on what object you'd like to view, there may or may not be a star in the same region of the sky that can serve as a point source for adaptive optics correction. That limits the applicability of the technique.

With lasers, though, you can make your own "stars," wherever you need them. You just use a telescope to project a laser beam up into the sky, illuminating a tiny spot in the upper atmosphere, above most of the problematic distortion. That spot then serves as the reference "star" for the image correction, and you proceed as described above. This can have really impressive results, as in these really spiffy pictures of the Orion Nebula (this also includes a shot of the laser beam shooting up into the sky, like they're blasting alien invaders...).

For this to work, you need to project a very small spot onto the sky fifty-odd miles above the surface of the Earth. While you could do this with the Bat-Signal and a great big lens, this is really a job for a laser.

Why is it cool? Dude, look at those pictures! Or this one, comparing ground-based telescopes with adaptive optics to the Hubble:

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You can't say that isn't cool...

Why isn't it cool enough? The basic technique doesn't require lasers per se; lasers just make it more convenient. Also, the laser correction isn't always enough, because there's still a bunch of atmosphere above the layer where they project the laser spot.

Also, you could argue that the technology that enables adaptive optics isn't the laser, but the computer-- what makes this a practical tool is the ability to do real-time corrections of the mirror to adjust the focus, which is a formidable computing problem.

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How do project a laser spot 50 miles up? What's there to reflect the light? Don't you get reflections from all the dust and whatnot on the way?

This stuff came along after I left the trade, so I missed out on all the fun and I don't understand how it works, but I have never come across a description that gives anymore detail than yours.

By Keith Harwood (not verified) on 22 Apr 2010 #permalink

This is a great summary of the benefits of laser guide stars and the principles of adaptive optics. I love the fact that you open with AO and mention how laser guide stars are not the most important component of the system: It's just that it's hard to find good natural guide stars (I've heard that natural guide stars can only be used for about 10% of the sky).However, I do need to correct/clarify a few points:
1) While a laser guide star is projected from a telescope enclosure, it is not projected from the telescope itself. A small detail, but an important one considering that novices might wonder,"do telescopes have lasers shooting out of the lens?" The laser is a separate piece of equipment.
2) In your final section, you mention that "the technology that enables adaptive optics isn't the laser, but the computer." While this isn't false, you're leaving out the other enabling technologies: The sensor and the corrector. Without the right sensor, you can't measure the aberrations well and without the corrector (in most cases a deformable mirror or series of deformable mirrors for large telescopes), you can't correct for the aberrations. Both of these need to be equally as fast and accurate as the computer. If you want to see gory details on a project that is currently in the construction phase, check out the PALM 3000 at Palomar (http://ow.ly/1ChJu). If you'd like to see something even cooler, check out the Gemini Planet Imager (http://planetimager.org/index.html). It will be used to image planets outside our solar system. Both are supposed to see first light early next year.

@Keith Harwood: I don't know a lot more than you about the reflecting mechanics, but what I do know is that the type of laser that is usually used is a sodium laser. This type of laser bounces off a layer in the upper atmosphere that reflects a large portion of this type of laser light, thus giving you a reference close to that of a natural guide star.

The laser guide stars are made using a yellow-orange laser tuned near the sodium D lines at 589 nm. There's a layer of the upper atmosphere at around 90km that contains a bunch of sodium atoms. These absorb light from the laser, and re-emit it, creating a bright spot at high elevation. Lower down, there's very little sodium, so the beam passes through without scattering much.

CCD detectors and deformable mirrors are a big part of the process as well, to be sure. Thanks for the reminder.

Thank you for that explanation. The next bit is to wonder at how covenient it is that such a layer exists and how it came to be, but that's a long way off topic.

By Keith Harwood (not verified) on 26 Apr 2010 #permalink