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November 18, 2017

GTCdigital

All about GTC

How will it work?

Active and adaptive optics

Adaptive optics

If the aberrations produced by the atmosphere are eliminated, the results obtained will be as good as observing from space. This is why the use of adaptive optics is so important.

Adaptive optics is an innovative technique that will greatly assist the scientists’ work. As the project’s Scientific Director, José Miguel Rodríguez Espinosa, has said, it will make the difference between “looking at an object lying on the bottom of a pool of water” and looking at the same object after the water has removed.

With this technology, most of the aberrations suffered by light reaching us from the objects we observe will be identified and corrected. For some observations, when the GTC’s adaptive optics system is up and running, the results will be like those from an 80 m primary mirror rather than a 10 m one.

The principle is simple. Atmospheric conditions will be analysed to find out how light waves are being affected by them, and then the shape of a series of mirrors will be adjusted to compensate. The means of performing this operation are complex since the atmosphere, which is constantly changing, has to be analysed continuously and very rapidly so that the adjustments needed can be transmitted to the flexible mirrors.

The wave-front is the geometric envelope of all the light rays emanating simultaneously from a luminous object. When the light’s origin is a point, the wave front is spherical; but if the origin is sufficiently remote, as is the case with stars, it is virtually flat. As it passes through the atmosphere, the wavefront is deformed and has to be reconstructed.

Light, the information that the telescope receives, is altered as it passes through the atmosphere. It is first analysed by the wave-front sensor to determine what deformations or aberrations it has suffered. The results are then sent to the phase reconstructor, which works out what corrections are needed and how the flexible mirrors need to be adjusted to compensate for the aberrations in the wave-front.

Objects that are to be studied are generally very faint and so to perform all of these actions a reference is needed, a bright nearby star whose light can serve as a comparison. However, it is not always possible to locate stars that are bright enough and close enough to the object of interest to be used to measure the wave-front. What will happen in cases where such a reference cannot be obtained?

We will “make” an artificial star. By using a laser beam to measure agitation in the upper layers of the atmosphere, we will produce artificial stars. It is a technique that requires high potency lasers and is still being developed.