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Richard Walker

The Compound Eye

Visible to anyone in the world
A dragonfly's

Steady gaze.
Mount Fuji.


Our eyes let us see further than a dragonfly can, but from the time of Galileo we have explored how we can extend our reach, not just to gaze at the mountains, but to penetrate the heavens. And for that we use telescopes.

The first optical telescopes were very small, at least by modern standards. For example, one of Newton's seems to have a diameter of about 50 mm. 

But subsequently the diameter of telescopes has doubled and redoubled many times, as various construction problems have been overcome.

The first telescopes used lenses, but starting with Newton's, reflecting telescopes have dominated, because it's much less difficult to manufacture big mirrors than it is big lenses.

Why does size matter? Because a bigger diameter means, in simple terms, a bigger magnification.

So there are three really big telescopes planned to come into operation 2020s (hopefully). The biggest telescope at present is the Keck, which is 10 m across, so collects 40,000 times as much light as Newton's, if I have my sums right. The new ones proposed will be 25 m, 30 m, and the largest of all, the European Extremely Large Telescope, at 39 m.

This giant dragonfly eye won't be just one huge mirror, it will be made of 798 hexagons, each 1.44 m across a compound mirror, just as dragonfly sees through compound lenses.

And in case you wondered the EELT won't physically be in Europe, for practical reasons, but in Chile.

The EELT and the other big telescopes will let us take pictures of exoplanets. These are planets orbiting other stars from ours. Many of these have been discovered but usually their existence is only inferred, from regular periodic changes in the light we observe from the star, as the planet comes between it and us. Only in a score of cases can we even observe the planet as a separate object from its star.

When a promising new exoplanet is discovered the media often feature illustrations of what it looks like, particularly if there is any possibility that it could harbour life. But these are merely the imaginative work of artists, not real pictures captured by telescopes.

The new generation telescopes will change this. For the first time we shall be able to actually see exoplanets orbiting their stars. We shall have come a long way from Galileo's observation of Jupiter being circled by its moons. 

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