The resolution of large ground based
telescopes is limited by atmospheric turbulences. Placing the telescope
on a satellite would avoid this, but space telescopes are limited in
size and expensive.
The atmospheric turbulences can be corrected in real time by an
adaptive optics system. Therefor the wavefront of a bright star is
analyzed and corrected by a deformable mirror (inspired
by Claire Max, CfAO)
So the resolution of a telescope is no longer limited by the
atmosphere. This example shows the galactic center.
The adaptive optics system needs a bright light source to work. If
is no suitable object near the scientific object, an well defined,
artificial star is generated by a laser. The picture shows PARSEC
, the natrium
Layer Adaptive Optics
-profile shows the amount of air
turbulences over height from ground.
It is obvious, that the strongest turbulences are close to the ground.
The aim of a Ground Layer Adaptive Optics system is to compensate these
turbulences over a wide field of view. (figure by E.
As the used laser is pulsed with 10kHz, it looks like a continuous
beam. But in fact the facility deals with single pulses.
How it works:
- A laser pulse is launched above the secondary mirror
and focused on a point in a height of about 12km. With the pulse propagating through atmosphere, a small
percentage of its energy is
scattered by air-molecules on its path (rayleigh scattering).
- As the speed of light is known, the time can be
calculated when the pulse reaches a height of about 12km. Twice this
time is when the scattered light will be back at the telescope. Here it
is detected by the wavefront sensor, which is focusing on the same
point as the laser beam. Due to the use of a single laser pulse, we see
only the scattered light from the desired height.
As the scattered light passes the ground layers on its way back, the
distortion of these layers can be optained from the wavefront.
But as seen on the figure, one star covers only a small field above the
By generating more stars, a wider field of the air above the telescope
can be analyzed.
Green lasers are often used because:
- rayleigh scattering works best for short
- the telescope optics are optimized for visible and infrared light
- visible light simplifies the alignment of the instrument
- they are robust and cheap. They are usually used for industrial