Ken Tapping, 16 October, 2013
A telescope does two things. Firstly it provides a magnified image, making it easy to see details, and secondly, it captures more light, making it possible to see faint objects. In astronomy, that second capability is the really important one. If you grab a pair of binoculars and scan along the Milky Way, this becomes immediately apparent. Typical binoculars collect up to a hundred times as much light as your unaided eyes. That faint fuzz arching across the sky becomes clouds of dust, gas and stars. Of course all those stars lie in our neighbourhood in our galaxy. When we want to probe further out into space, the objects we want to see become even fainter, making it very desirable to collect a lot more light.
Until recently the largest astronomical light collector available was the telescope on Mount Palomar. This has a 200-inch (5-m) mirror. The problem with large mirrors is that their surfaces have to be shaped with high precision in order to focus the light, and to maintain this precision as the telescope tilts from horizon to zenith. In the case of the 5-m mirror this was done by making it thick and stiff, which inevitably made it very heavy. If we try to make a mirror larger than 5 m, using the same engineering approach, as the Soviet Union did when it made a 5.9 metre mirror, we find that adding more material increases the bendiness faster than the stiffness. Trying to make bigger mirrors using that approach does not work.
Over the last half-century we have made great advances in materials science and in precision computer controls. This makes it possible to manufacture mirrors of sizes limited mainly by the available funding. Modern telescopes use mirrors that are quite thin. However, instead of sitting directly on a rigid support, the mirrors now sit on a large number of computer-controlled actuators.
Instead of passively resisting the tendency for the mirror to change shape as the telescope moves, its shape is continually monitored and corrected. Using this technique we have made telescopes with eight and ten metre mirrors, and are designing telescopes with mirrors 30 metres or more in diameter! These telescopes are revolutionizing astronomy. However, for telescopes on the Earth’s surface there is another problem we have to deal with to get the full return on our investment: the stability of the Earth’s atmosphere.
A dark, clear night, with a sky filled with twinkling stars, is a beautiful thing to behold. However, if you now get out a telescope and try to observe, you will find that the images are swimming and dancing around. Moments when you can see fine detail are few and far between.
The problem is that we are looking from the bottom of a turbulent sea of air. We can reduce the problem a bit by putting our telescopes on top of mountains. However, we can take the idea of computer controlled, deformable mirrors and use it to almost eliminate those atmospheric distortions. The technique is called adaptive optics.
We know that stars images are dots in the sky, which do not dance around. Any blurring or dancing around is due to the atmosphere. A computer looks at the image many times per second, and controls a small mirror with lots of actuators behind. It then distorts the mirror to undo the atmospheric distortions, leaving us with almost as good an image as we would get if our telescope were in space, above the atmosphere.
When you see adaptive optics in action it almost seems like magic. With it switched off you see the telescope image blurring and dancing around. Then you switch the adaptive optics system on. Almost immediately the image becomes sharp and steady. It is an amazing transition.
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