Imaging stars and other heavenly bodies from earth based telescope is difficult because of the atmosphere that envelops the earth. The atmosphere acts as veil between the distant sky and the telescope, and that veil is dense, disturbing and unpredictably variable. This causes fluctuations in the brightness of the star as it refracts through different layers of the atmosphere. The effect is that we get a smeared image of the star instead of a bright point of light.
To measure how the Earth's atmosphere is changing astronomers monitor fluctuations in brightness of a known bright star, but many times no bright star exists in the direction where atmospheric information is needed. To solve this problem, an artificial guide star is created. A beam of laser is projected up through the atmosphere. At about 100 km, the laser beam hits a layer of sodium atoms created by micrometeorites, which vaporize as they enter the upper atmosphere, and excites the sodium atoms. The excited atoms emit a yellow light in all directions, creating a glowing guide star in the upper atmosphere which the astronomer uses to carry measurements.
The blurring effect of the atmosphere is then compensated by employing a special kind of rapidly flexing mirror, a technique known as adaptive optics.
Below are some magnificent images of laser beam shooting out of observatory domes.
Astronomers observing the centre of the Milky Way using the laser guide star facility at ESO’s Paranal Observatory.
