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Adaptive Optics

         The primary issue that large, research telescopes face is the turbulence of the Earth's atmosphere through which the rays of light from celestial objects must pass before they are enhanced by the telescope. Looking at objects in space from the Earth's surface is much like looking at the overhanging tree branches from the bottom of a stream.  To the extent that the water is clear and still, our view is not too bad; but as the stream's flow picks up speed or turbulence, our view becomes distorted.  The larger the telescope, the more this issue is exacerbated. This is why all large telescopes are put on the tops of high mountains, especially selected for the lack of turbulence in the atmosphere above them and far from city lights and pollution. It is also why the Hubble Telescope (and its planned successors) is in outer space.


Photograph of Neptune without adaptive optics

Photograph of Neptune with adaptive optics

Neptune without AO, 10-meter Keck Neptune with AO, 10-meter Keck



         There is, however, another way to address this problem, referred to by some as the most revolutionary advance in astronomy since 1609 when Galileo Galilei introduced the telescope. Known as adaptive optics, it is a procedure of measuring the atmospheric distortion in a celestial object's light beam, and then sending signals to deform a multitude of mirrors in the light path to exactly compensate for the atmospheric turbulence. This can be accomplished by dividing the light reflected from the telescope mirror into hundreds of smaller beams, each of which is examined for deviation due to turbulence from a standard. The system's electronic circuitry computes the shape of many small mirrors, so that when each is altered their individual distorted beams realign in the same direction. Then these deformable mirrors are sent a signal that in fact does change their shapes, producing a composite, undistorted beam.

         One of the hurdles for an adaptive optics system is having an optical system mechanically capable of correcting the incoming light beam. This can be done by two mirrors acting in concert: one providing coarse corrections to the incoming light, the other a deformable mirror. The deformable mirror's purpose is to make distorted light entering the telescope appear as it did before entering the Earth's atmosphere.

         In order to know how to deform the deformable mirror, the shape of both the distorted and undistorted image need to be known. The undistorted shape for stars is usually just a point without detail. In fact, since stars are so far away anything more than just a geometrical point must be an artifact of the optical system. To determine the final distorted shape of an ideal point source once it reaches the Earthís surface, adaptive optics systems initially used a bright star near the telescopeís target as a comparison source for the distorted wavefront. This, however, required that the object being observed was close to a relatively bright star, which is usually not the case.

         Subsequent adaptive optical techniques use a laser beam instead of a guide star. There's a naturally occurring layer of sodium atoms 60 miles above the Earth's surface (in the mesospheric layer) whose atoms fluoresce when excited by a laser beam. A laser directed into the sky very close to the direction of a celestial object will cause the mesosphere's sodium atoms to fluoresce, and that directional location, if you will, will become a "guide star" for the celestial object of interest. This allows any object to be observed even when no bright stars are nearby.


go to . . .

telescopes in general
reflecting telescopes
refracting telescopes
compound-catadioptric telescopes
charge-coupled devices (CCDs)





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© Carl Woebcke, Adaptive optics, 1991-2017. All rights reserved.