I’m writing this on a plane from Honolulu to Los Angeles having enjoyed the first few days of my book tour in the paradise on Earth that is the Hawaiian archipelago. Most memorable for me was the expertly guided tour of the Keck Observatory on the “big” island of Hawaii – on a 14,000 foot mountain – high enough for them to offer me oxygen. I tried it, hoping for some sort of metaphorical as well as literal high, but I couldn’t detect any difference from ordinary air and soon discarded it.
There are lots of big telescopes on the mountain. The Keck Observatory has two of them: identical reflecting telescopes side by side on the bare mountain, connected by a corridor laden with complicated infrastructure. My first surprise was the discovery that astronomers themselves seldom need to go up the mountain. You might expect that of theoretical astrophysicists, but I’m talking about observational astronomers making real measurements of real stars and galaxies. When you think about it, it’s not so surprising. A giant reflecting telescope with its 34 foot parabolic mirror has to be controlled by accurate computer, not clumsy human hand. Almost as obviously, the measurements it takes are destined to be fed straight into a computer: no human eye actually “looks” into an eyepiece of a big telescope. Indeed, I doubt that the word “eyepiece” has any applicability to leviathans such as the Keck reflectors. So why would the astronomers need to stir themselves up the mountain? Instead, they are cosily ensconced at “ base camp”, staring at banks of computer screens under a commonplace ceiling. On the mountain itself the personnel are mainly engineers, ceaselessly engaged in the round-the-clock business of maintaining the giant instruments in precision working order.
When I heard the learn'd astronomer,
When the proofs, the figures, were ranged in columns before me,
When I was shown the charts, the diagrams, to add, divide and measure them,
When I sitting heard the astronomer where he lectured with much applause in the lecture room,
How soon unaccountable I became sick and tired,
Till rising and gliding out I wander'd off by myself,
In the mystical moist night air, and from time to time,
Look'd up in perfect silence at the stars.
No scientist could really go along fully with Walt Whitman, but we might have a little more sympathy given the fact I have just told you, that the learn’d astronomers, far from gazing rapt at the night sky, don’t even go near a telescope. The sympathy should not last long, however. And even Walt Whitman might have been inspired by the poetry of the laser beam that shoots into the sky, straight as a . . . well, similes are superfluous, for what in the world is straighter than a laser beam?
What is the laser for? This is where the story becomes truly wonderful. It is a story of which I had known nothing, and I was grateful to be enlightened by one of the learn’d astronomers, Dr Roy Gal of the University of Hawaii Institute of Astronomy. Before I tell the story, a brief preamble . . .
The resolving power of a reflecting telescope is directly proportional to the diameter of the mirror at its back. Moreover, the bigger the mirror, the greater the number of photons, speeding along parallel paths, that can be examined to give us information about the distant star or galaxy which is their source. Mirrors larger than a certain size inevitably sag, so giant mirrors have to be assembled by jigsawing a number of smaller hexagonal mirrors, each one precisely curved to its place in the giant parabola. The two Keck mirrors each have 36 hexagonal components. The 36 are swapped in and out, two at a time at bimonthly intervals, following a careful regimen of cleaning and polishing. Indeed the sheer perfectionism of the cleaning and polishing rituals, to say nothing of the minuscule tolerances of the milling procedures by which the hexagons are made in the first place, took my breath away.
But however large the mirror, and however exquisitely machined, cleaned, polished, fitted and adjusted, there is an additional limit on the quality of the image it can serve up. Unlike the Hubble telescope (of which more in a moment) and unlike any projected telescope built on the moon (maybe one day) Earth-bound telescopes are smothered beneath the thick blanket which is the atmosphere. Air refracts – bends the light rays. This wouldn’t matter if the air were perfectly uniform and still, but it isn’t. The refractive index of air varies with temperature; and winds and turbulence constantly mess about with the refraction of the light rays coming from any star. If only we could park our telescope outside the atmosphere!
Well we can, and this is why the Hubble telescope is so good. This is why the Hubble can compete with the largest Earthbound telescopes, even though its mirror is a fraction of their size. But there is a cunning trick by which an Earthbound telescope such as the Keck pair can achieve the equivalent of getting outside the atmosphere. The trick is known as Adaptive Optics.
Think about it. Atmospheric turbulence is a known phenomenon. If we could only measure it, from moment to moment and from place to place, we could compute the effect of turbulence on the image of a star or galaxy, compensate for it, and reconstruct what the starlight would tell us if only there was no atmosphere. How to do it? This is where the laser enters the story.
The laser points in exactly the same direction as the telescope and slightly to the side. It is pure sodium light, with the known spectral lines characteristic of sodium. High above the atmosphere is a layer of “sky” where the beam excites sodium atoms. The reflected light shoots straight back to the observatory, where it is picked up by sensitive and fast-responding instruments. Do you see the elegant cunning of this? The reflected sodium light has been subjected to exactly the same atmospheric turbulence as the light from the distant star or galaxy we are studying. Since we know the precise quality and intensity of the sodium light, the fluctuations in the reflected light tell us exactly what must also be happening to the incoming starlight. And our Earthbound instruments therefore have all the information needed to reconstruct what the starlight would look like if only there was no atmospheric turbulence to distort it.
You might think that the compensation could be done by adjusting the big hexagonal mirrors that constitute the giant reflector. Indeed the hexagons can be adjusted, and they are precisely lined up before the evening’s observations begin, by means of delicately refined measurements of beams reflected by them. But the hexagons are massive. Any attempt to move them would inevitably be too slow. The correcting adjustments couldn’t be fast enough to keep up with the fluctuations in the turbulent atmosphere. Instead, the adjustments are made to smaller mirrors downstream from the main mirror in the chain of light transmission. The result of all this ingenious precision engineering and computational wizardry is that a large Earthbound telescope can closely approach the optimal performance theoretically allowed by the diameter of its mirror.
One strange coda to this lovely story. The only person on the strength who actually does what Walt Whitman would wish – at least in a professional capacity – is a man in a thick fleece sleeping bag, lying flat on his back on an electrically operated turntable outside the telescope. Armed only with a pair of binoculars, he passes the hours gazing up at stars and Milky Way. His job is to watch for aircraft which might stray into the airspace above the observatory. Why? Laser beams of the power needed are extremely dangerous: more than 2,000 times the power of an ordinary classroom pointer laser. They would instantly blind anybody who happened to look down from a plane, with especially drastic consequences in the case of the pilots. The task of the lonely sentinel in the sleeping bag is to keep a vigil for planes, and cut the laser off if one should stray dangerously close to the beam.
The master plane-spotter to whom we spoke had been doing the job for eight years. He had only once thrown the emergency switch, and even then it turned out that there was no danger. The risk has been calculated as 1-in10,000. Scientists calculate that automated instruments could do the job with greater reliability than even the most conscientious human observer. Yet the Federal Aviation Authority has decreed that only the human eye should be trusted with such a responsible job. That’s what happens when non-scientists terrified of lawyers are allowed to take decisions that are in the proper domain of scientists. It’s reminiscent of the exaggerated respect given to eye-witness testimony in courts of law, despite frightening – in some cases comical – demonstrations of the unreliability of the human eye and memory.
A great observatory like the Keck is one of those human achievements which, like the Large Hadron Collider, the Human Genome Project, William Shakespeare and Franz Schubert – render me tearful with pride at belonging to the species Homo sapiens. After nearly four billion years of evolution, our species awoke from a sub-intelligent stupor, saw the stars and wondered. Within a few millennia, we had worked out that they wheel around the sky in a way that could sensibly be explained only on the hypothesis that we are sitting on a spinning ball of tiny dimensions compared to the distances between the stars. Within centuries we knew the awe-inspiring truth that the stars visible to the naked eye are all near neighbours, clustered in one galaxy, the Milky Way, which is but one among 100 billion galaxies. Within decades we knew that space itself is expanding, and the galaxies receding from us and each other, at ever increasing velocities until they outpace their own light and disappear, beyond the faintest hope of detection, over the unbridgeable Event Horizon.
We know all this because of precise, quantitative data gathered by instruments such as the twin Keck telescopes. These giant, staring eyes are honed and adjusted to a perfection which allows us to leap the parsecs – to escape the home planet and spring across literally billions of light years toward the far reaches of forever and wherever. And to me perhaps most moving of all is the fact that no one man or woman could even begin to achieve such feats. This is a cooperative enterprise, bringing together the finest that our remarkable species has to offer: not just astronomers but mathematicians, engineers, physicists, computer scientists, spectral chemists – one day, who knows, maybe even biologists. We climbed the high mountain on the island of Hawaii and broke through the cloud layer to the summit: a metaphor, perhaps, for one of the loftiest pinnacles of human achievement.
I am indeed proud to celebrate humanity. And surely Walt Whitman – who celebrated himself as a man – would do the same. On reflection . . .