Figure 1: Video entitled "why do stars twinkle" by SciShow.
Transcript: "Lots of kids grow up singing that tune about little stars twinkling, but even as adults, many folks don’t know why stars seem to flicker and wiggle. Astronomers describe the twinkling of the stars as stellar scintillation. Technically, it refers to variations in apparent brightness or position of a distant, luminous object as its light travels through a medium. In other words, that twinkling isn’t caused by the stars which are super constant in their brightness -- it’s caused by the Earth’s atmosphere. As the light from a star travels toward you through the atmosphere, it passes through many layers of air of varying temperatures and densities. Each of the boundaries between those layers of air refract the light, bending it in a slightly different direction from moment to moment. So, the starlight zigs and zags as it passes through the air to your eye or to a telescope on the ground. From our perspective, this makes the star appear to shift ever so slightly. Of course, we can get around this astronomical nuisance by taking advantage of the fact that stars do not twinkle in outer space. This is why our best images of stuff in outer space have so far have come from the Hubble Space Telescope, which makes its observations outside our atmosphere. With no medium to refract the light hitting its lens, Hubble is able to take brilliantly beautiful, completely crisp, twinkle-less photos of the stars. To be fair though, twinkling may frustrate astronomers here on the ground, but it can actually be useful as well.
Thanks to scintillation, when you’re gazing up at the night sky, you can tell the difference between a star and a planet. That’s because planets don’t twinkle - at least not as much. Stars are so far away that they’re what we call point sources of light. Like, if I were to carry a flashlight far, far away from you, it would appear as one point of light, shining out of the darkness. But because planets are so much closer, the light we see bouncing off of them consists of many points of light. So, kind of like me shining a flashlight in your face, planets essentially appear as disks. Even if one individual point is disrupted by the atmosphere, several others reach you at the same time. This averages out the twinkling effect, making the planet appear relatively stable.
So next time you wish upon a star, make sure it’s twinkling! Otherwise, you may very well be wishing on Saturn. Which, which honestly is fine; you can wish on whatever you want. It’s up to you.
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Excerpt from http://bit.ly/2wJjd2s
"(...) certain elements are segregated from others at specific altitudes. One of the elements that’s very rare is sodium, which happens to be concentrated in a thin layer about 100 km (60 miles) up. If you fire a sodium laser into the air, it will excite those sodium atoms found at that particular altitude, which then spontaneously de-excite, creating an artificial light source to be used as a guide star.
The light from this artificial star then travels back to the telescope through that 100 km of atmosphere, and gets distorted by that same turbulent air column that all the other light coming to your telescope must pass through. Only this time, we know for absolute certain that this should be a single, point source of a particular wavelength at a particular location. So no matter what the light that we actually get back from that artificial star looks like, we know what it should look like: that single point source.
So what do we do? We adapt."
By ESO/M. Kornmesser - http://www.eso.org/public/images/potw1136a/, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16361914
"We can compute exactly what the shape of a mirror would need to be — at any instant — to undo the turbulent effects of the atmosphere, and return our artificial guide star to simply being a single point of light at the correct location. What we then do is we delay the light from all the other sources coming into the telescope, and actually mechanically adapt a mirror along the light path to be the exact shape it needs to be to undo the effect of the atmosphere, which we then pass the delayed light through.
We update the shape of this mirror on a continuous basis, and this allows us to obtain — to the best of our ability — an image that undoes all the negative effects of the atmosphere. This entire setup is the most advanced technique in the field known as adaptive optics, and it’s perhaps the most spectacular, revolutionary advance in ground-based astronomy since the invention of photography. Here’s a lovely video from Gemini Observatory (Figure 3), detailing how the entire process works."
Figure 3: "The Gemini Observatory's adaptive optics system cancels out the scintillation of starlight caused by the atmosphere."
The Hubble telescope takes perfect images because it is in space where there is no atmosphere.
The atmosphere has different temperature and density areas, it has winds and as a result the light has a rough trip and gets distorted. How can we correct this?
Please refer to the previous section.
If you have sent a light point and you got a reflection in the form of a smear or a complicated light pattern, you can do image processing computations to understand how your original signal was transformed. We could maybe say, what kind of “filter” was applied.
You can then use your filter to process the twinkling light of a star to get its real non-distorted signal.
The concept is similar to that of fun house mirror! You get a distorted image but you know what your real image is. If it shows that a person is taller and their height can be calculated with a specific ratio (cf. double height), you use that information to process distorted images of stars.
Figure 4: Fun house mirrors. From http://bit.ly/2IHGpTN mentioning "Image credit: retrieved from Isa Garcia's blog".
Figure 5: Facebook post by ESO - http://bit.ly/2k2b4Np