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Giving the atmosphere the slip
17/02/2005
Getting rid of the perturbations caused by the atmosphere will allow us to see as clearly as we would from space. This is why adaptive optic systems are so important.
Adaptive optics is an innovative technique that will make astronomers’ work much easier. As José Miguel Rodríguez Espinosa, the Gran Telescopio Canarias’ Science Director, put it, the difference is like “looking at an object on the bottom of a swimming pool through the water” or looking at it “with the water taken out.”
This technology will help us detect and correct most of the aberrations present in the light that reaches us from the objects we observe. When the adaptive optics system is in operation at the GTC it will, on some observations, be like having an 80 metre primary mirror instead of one of 10.
What is adaptive optics?
The principle is simple. Atmospheric conditions are analysed to determine what effects they will have on light waves. Then, straight away, the shape of a series of mirrors is altered to compensate for these effects. Applying this principal in practice is complicated, because the changeable nature of the atmosphere means that it has to be analysed rapidly and continuously, some 700 times per second, so that instructions can be transmitted to the deformable mirrors.
We know that the wave front (the geometric envelope of every ray of light emanating from a luminous object at a given time) is spherical if its origin is clear, but that if it is far away, like stars, it is virtually flat. When it passes through the atmosphere the wave front is deformed.
When this deformed light reaches the telescope it is examined by the wave front sensor to identify the changes caused by its passage through the atmosphere. This information is sent to the phase reconstructor, which calculates the necessary corrections and the changes in shape required for the deformable mirrors to compensate. This results in a much clearer image.
Artificial stars
The objects being observed are generally very weak, so we have to use a nearby bright star to guide the Adaptive Optic system. However, it is not always possible to locate sufficiently bright stars close to the object we are interested in. What can be done when no such reference is available?
We have to ‘make’ an artificial star by agitating the upper layers of the atmosphere with a laser. This is the last remaining element of our adaptive optics system.
Natalia R. Zelman