Adaptive optics

If the largest terrestrial telescopes are located at altitude, it is because our atmosphere is a real embarrassment for the observation of the sky.
The Earth's atmosphere is dynamic, its air layers are heterogeneous in temperature, move, mingle, exchange energy, and disruptions due to pressure, heat, humidity, movement, greatly hinder astronomical observations made from Earth. Indeed this constant and unpredictable atmospheric turbulence shook the images received at the pace variations in air molecules of atmospheric layers. As the air molecules are agitated very quickly, form vortices move in a few milliseconds, the observed objects are also agitated and mingle making blurred images. Is said that the wavefront is disturbed. The plane wavefront, who traveled for billions of years is broken in the last millisecond of his journey on the Earth's atmosphere.
From 2000s adaptive optics systems (OA) are experienced on existing telescopes and since 2010 the OA system is part of the standard equipment of the great observatories.
What is the principle of an adaptive optics system?
Adaptive optics can observe the sky in reducing the effects of dynamic optical distortion, the disruptive effects of turbulences, which blur images are thus "eliminated".
The system real-time adaptive optics, interference analysis of light by the atmosphere, the adaptive optics computers calculate corrections and operate every millisecond mini deformable mirrors (of the order of 16 to 50 mm), machined at the microscopic level to compensate advances and delays for light wavefronts.

These wavefronts as they are almost planes in the interstellar void, are increasingly distorted as they pass through the layers of the atmosphere (see image). The images focused by telescopes are even more blurred as turbulences are strong. This is extremely annoying when scientists observe objects at low light the early universe.
An adaptive optics system needs a reference, a "guide star" to calibrate the wavefront sensor of the telescope. This star must be sufficiently bright and located in the vicinity of the observed star. However, despite the high number of stars, the operation is not easy, so the scientists had the idea to create a virtual star. This star is a laser star. The adaptive optics system sends a laser beam in the sodium layer of the mesosphere (between 50 km and 100 km altitude), it "bounces back" and an artificial star appears. Although this laser star poses a lot of technical problems, it's thanks to it that the system determines "correctly" the instability of the air in the vicinity of the observed object.

NB: Deformable mirrors are adaptive optics with dynamic faces capable of modifying the wavefront of the reflected light for a specific application. With a time control, a deformable mirror can focus a beam at different points at different times in an optical system. They can improve the optical images in the telescopes and other imaging systems. Deformable mirrors are available in a variety of sizes. The standard sizes range from 16 mm diameter with 37 actuators (DM 25-37) to 50 mm in diameter with 61 actuators (DM 50-61).

In adaptive optics, laser star allows no longer be dependent on a real star, close to the observed object. The disruptive effects of Earth's atmosphere are reduced and scientists obtain sharper images.
This laser guide star, artificially created in the field of observation, improves the quality of the correction of terrestrial telescopes using adaptive optics.
The laser beam Yepun, one of the four VLT telescopes, crosses the southern sky and creates an artificial star at an altitude of 90 km in the mesosphere of the Earth. The Laser Guide Star (LGS) is used as a reference to correct the blurring effect of the atmosphere on images. The color of the laser is precisely tuned to energize a layer of sodium atoms present in one of the upper layers of the atmosphere.
The color of the laser is the same color as the sodium street lights of our cities. When the sodium atoms are excited by the laser light, the atoms become incandescent, forming a small bright spot that can be used as an artificial reference star for the adaptive optics. Using this technique, astronomers can obtain much sharper observations. For example, when looking towards the center of our Milky Way, researchers can better observe the ballet of stars, gas and dust orbiting the central supermassive black hole.