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Excerpt of Stargazer by Fred Watson
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Of course, optical telescopes also predate all the other types by more than 300 years. So perhaps it was not surprising that those attending our forum paid little more than lip-service to the newer, 'invisible' astronomies. There was a clear underlying message: at this meeting, optical astronomy ruled, OK. Wavelength chauvinism was alive and well in Munich.

Then there was the obsession with size. Just why do optical telescopes have to be so big?

Unlike Gary Larson's cartoon telescope, today's real telescopes have at their heart a shallow, concave mirror to collect and focus the incoming light. Bigger mirrors collect and concentrate more light, and an insatiable appetite for light—even in very small quantities—is the most common dietary complaint among astronomers. The more light that can be collected, the fainter the objects that can be studied.

But there is another craving that draws astronomers towards ever bigger telescope mirrors, one known as resolution—the fineness of the detail that can be seen in a magnified image of the sky. The quest for resolution is as old as the telescope itself, for it was the instrument's ability to reveal invisible detail that made it such an astonishing invention in the first place. Today, the physics of the situation is well understood: given the necessary degree of optical perfection, the bigger the telescope mirror the finer the detail it is capable of recording.

Like all dimensions in the sky, resolution is measured as an angle. It is expressed in arcseconds—microscopic units that are to angles what nanometres are to length. Geometry tells us that an arcsecond is 1/3600th of a degree. So much for geometry; it's much more instructive to imagine a person 5 kilometres away holding up a coin. An Australian dollar, a British pound and a US quarter are all about the right size. To your eye, the coin's diameter at that distance is one arcsecond—and you would need a sizeable telescope to be able to see it.

Putting some figures on resolution, a one-metre diameter telescope mirror is theoretically capable of showing detail on a scale of a little more than 0.1 arcseconds—the coin at 50 kilometres. But a 4 metre mirror could resolve detail of one-quarter the size—0.03 arcseconds. That is fine enough to detect surface markings on the planet Pluto, or the disc of the giant star Betelgeuse. Bigger is definitely better.

Unfortunately, there is a wholly unwelcome natural phenomenon that plays havoc with resolution, and that is atmospheric turbulence. We're all familiar with what happens when a jet aircraft ploughs into turbulent air 10 kilometres or so above the ground. The nerve-racking shaking and juddering happens even in cloudless skies. That same turbulence has an equally alarming effect on rays of light coming down through the atmosphere. It gives the stars their appealing twinkle when seen with the naked eye—but in the telescope, what should be infinitesimally small points of light are blown up into fuzzy, trembling balls.


Seeing the unseeable

What can be done about the problem of seeing? The direct approach is to take your telescope above the atmosphere, but that is very expensive. The Hubble Space Telescope, launched in 1990, was designed primarily with this in mind (though its high-level vantage point also provided unprecedented access to the ultraviolet waveband). The story of the Hubble, its flawed 2.4 metre mirror and the 1993 rescue mission that enabled engineers to recover most of its intrinsic resolution is well known, but less widely appreciated is its cost. The eventual bill to build, launch and fix it was well over US$2 billion (1990 dollars), and by the time the project is completed sometime beyond 2010, it will have notched up more than US$6 billion.

From Stargazer by Fred Watson, pages viii - x of the Prologue, and pages 1-17 of Chapter 1. Copyright Fred Watson. All rights reserved. Excerpt reproduced by permission of Da Capo Press.