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For most people, packing for a trip to Hawaii means throwing together
a couple of T-shirts, a pair of shorts, a bottle of suntan oil and perhaps
a surfboard. For astronomers the packing list is very different: thermal
underwear, gloves designed to prevent frostbite and snow goggles must go
into the suitcase. Not for them the carefree delights of a warm ocean and
a palm-fringed beach. They are heading for a place as cold as the Arctic
and as desolate as the surface of the Moon – the summit of 4200-metre Mauna
Kea.

Mauna Kea is a vent bulging out from Mauna Loa, the gigantic volcano
which forms the entire Big Island of Hawaii. Measured from its base at the
bottom of the Pacific Ocean to its tip, it is the highest mountain in the
world, 600 metres higher than Everest’s 8863 metres. On the summit are nine
telescopes. Three observe the Universe at short radio wavelengths and six
collect visible and infrared light. For the visiting astronomer, none can
rival Keck, the newest, largest and most unusual optical telescope in the
world. Keck’s 10-metre diameter mirror has four times the area of the famous
5-metre telescope on Palomar Mountain in southern California. It can detect
objects a quarter of the brightness of those visible to any other telescope
in the world, making it possible for astronomers to see deeper into the
Universe than ever before.

Keck was built at a cost of $94 million by the California Institute
of Technology, the University of California at Berkeley and the University
of Hawaii. Already, Keck has zoomed in on the most remote object in the
known Universe, and is showing detail not even seen by the Hubble Space
Telescope. Beyond lie undiscovered galaxies and quasars – perhaps astronomical
objects never even dreamt of – which Keck alone can reveal. With plans
already under way to make the telescope even more powerful, Keck’s builders
expect the telescope to dominate world astronomy for the next fifty years.

In 1996, a second, almost identical telescope will be completed close
by. Then early next century, Keck I and Keck II, plus four smaller telescopes
in the 1.5-metre to 2-metre range, will be linked to form an optical interferometer
capable of seeing detail almost 10 times finer than Keck on its own. To
match it, a single telescope would have to have a mirror the size of a football
pitch. With such astonishing power at their disposal, astronomers will be
able to detect Earth-sized planets around nearby stars and determine the
size and age of the Universe.

White mountain

Keck and the other eight telescopes have been placed on Mauna Kea, or
‘white mountain’, for good reasons. First, the volcano is so high and cold
that most of the air’s water vapour has been frozen out. Water vapour absorbs
light, particularly infrared and short-wavelength radio waves. Secondly,
the turbulent trade winds which blow across the Pacific are forced to pass
either side of the mountain, leaving the summit in a pocket of exceptionally
stable air. This provides sharp images of stars free from the ‘jitter’ caused
by atmospheric turbulence. Lastly, the Hawaiian islands are far from any
continents, so the air is free of dust which might obscure the view.

‘The only place on Earth where the ‘seeing’ approaches Mauna Kea is
the central plateau of Antarctica,’ explains Andrew Perala, a former journalist
from Alaska, who fell in love with Keck after writing a story on it for
his newspaper back home. Perala now has a job most amateur astronomers would
kill for, showing scientific visitors how Keck works.

Fortunately for the visitors, the summit of Mauna Kea is easier to reach
than Antarctica, but it is still not without its hazards. To get there you
need a rugged, four-wheel drive vehicle. Leaving behind the palm trees
and proteas of Hilo Bay, you head into Hawaii’s desolate interior on the
Saddle Road from which all rental cars are banned. Stretching away on either
side of the road are endless tracts of black lava which look like solidified
treacle. After an hour, you turn off onto the road to Mauna Kea, straining
and lurching up gradients that make the engine screech like a tortured animal.
At Hale Pohaku, the astronomers’ base camp just below the 3000-metre mark,
where a one-hour’s stop to acclimatise is the law, the road turns to rubble
for the final assault on the summit. After another half an hour of bouncing
and jouncing on a switchback road that deteriorates so rapidly that it has
to be remade by a special machine every two days, the top of the volcano
finally draws close.

At 4000 metres, the landscape seems no longer of this Earth. Among the
conical vents and red volcanic rubble nothing moves or grows. ‘This is
where the Apollo astronauts tried out their Moon buggies,’ says Perala.
‘NASA looked all over the world for a site like the Moon. Mauna Kea was
the closest they found.’

Strangely, for the final few minutes of the ascent, the track changes
again to tarmac. ‘It cost $12 million, for a four-mile stretch,’ explains
Perala. It proved essential because vehicles were kicking up dust which
drifted over the nearby telescopes, taking the edge off their vision.

Standing on the summit of Mauna Kea is like being on top of the world.
The view is breathtaking. The summmit is so high that it is possible to
see the other islands of the Hawaiian chain stretching away like stepping
stones across the blue Pacific. Even the cloud layer is several thousand
feet below. The sunlight is very intense, even through dark glasses, and
you can get badly burnt in just 15 minutes.

The hazards do not deter 1200 astronomers from coming to Mauna Kea each
year. The telescopes they are desperate to use are housed in shiny white
domes which dot the summit, looking like eggs in the world’s biggest and
highest nest. ‘There’s room up here for 20 telescopes in total,’ says Perala.

Inside the largest dome is the Keck Telescope. Not far away is the half-built
dome intended for Keck II. Reaching Keck from the car is an effort: up here,
there is only 60 per cent of the oxygen available at sea level. Every step
has to be taken in exaggerated slow motion, the lungs straining, the heart
racing to pump more blood around the body. Originally, it was planned that
the Keck control room would be pressurised. Ducts for piping in oxygen were
put in at a cost of $300 000. But it was later decided that airlocks would
be a hassle for people going in and out of the dome. Instead, plans are
being made to run Keck remotely from its headquarters in Waimea, at 750
metres.

Berkeley brainchild

The dome is eight storeys high, the same size as Mount Palomar’s, even
though Keck’s mammoth 10-metre-diameter mirror is twice as big. ‘Keck weighs
297 tonnes and floats on silicone oil,’ Perala explains. ‘If you release
the brake, you can push it with your hand.’ Its mirror is breathtakingly
beautiful. Unlike other telescope mirrors, it is made up of 36 hexagonal
segments fitted together in a quicksilver mosaic.

The revolutionary design is the brainchild of Jerry Nelson of Lawrence
Berkeley Laboratory in California. By choosing to employ a composite of
small mirrors, he overcame the major problem facing builders of large telescope
mirrors. When a mirror tracks a star as it moves across the sky, it must
be stiff enough to keep its shape as it moves. For a conventional mirror,
that means it must be thick, which also makes it heavy. For a mirror more
than about 5 metres in diameter, the framework needed to support it would
be so massive that the telescope would be impossibly expensive. ‘A single
‘monolithic’ mirror would have cost half a billion to a billion dollars,’
says Perala.

Each of the mirror segments is 1.8 metres across and only 7.5 centimetres
thick, and is supported by three actuators that adjust its position and
tilt relative to its neighbours every 2 seconds. This arrangement compensates
for distortions in the mirror caused by wind pressure and the changing effects
of gravity as the telescope moves. The position and tilt of each half-tonne
segment are measured by sensors along its edge and read out by a computer
100 times a second. ‘It takes 90 000 lines of computer code to actively
control the shape of the mirror,’ says Perala. The segments are never allowed
to slip out of alignment by more than a thousandth the thickness of a human
hair.

The mirror segments are sections of an off-axis paraboloid, asymmetric
shapes well-nigh impossible to grind. Nelson achieved the unachievable with
an ingenious technique known as stressed-mirror polishing. First he distorted
each mirror disc in a carefully calculated way. He then ground a symmetric
surface on the warped disc. Finally, he released the warping forces, allowing
the mirror to spring back into the required asymmetric shape.

The technique has worked perfectly and Nelson admits to being ‘enormously
pleased’ at the reaction of astronomers who have used Keck. ‘It has worked
better than they had hoped, giving extremely sharp images and allowing the
study of very faint objects,’ he says. ‘This is what the astronomers had
dreamt of.’

Indeed, astronomers’ dreams are already coming true. Since going into
full operation in March last year, Keck has succeeded in obtaining near-infrared
images of the most distant known object in the Universe – a quasar – and
the most distant known galaxy, 4C 41.17. In both it picked out more detail
than even the Hubble Space Telescope has seen. Its latest observations of
a gravitational lens system provide a new estimate for the Hubble constant,
and thus of the size and age of the Universe (see Science section in this
issue).

Observations like this are just the beginning for Keck. The light from
the most distant object seen so far in the Universe began its journey to
Earth when the Universe was only about a fifth its present age. Keck will
be able to detect fainter light and thus probe the Universe at an even earlier
epoch. With luck it will see far enough back in time to catch a glimpse
of the first galaxies congealing out of the virgin stuff of the big bang.
No one understands how galaxies are born, but with Keck astronomers could
nail the problem once and for all.

Telltale fingerprints

Among its targets in this quest will be quasars, the prodigiously bright
cores of newborn galaxies billions of light years away. As light from a
quasar travels towards Earth, it passes through countless galaxies that
are themselves far too faint to be seen directly by any telescope, even
Keck. But each galaxy leaves a characteristic dip or ‘absorption line’ in
the quasar’s spectrum, a fingerprint that tells astronomers about the galaxies’
properties. With Keck capable of picking up quasar light at record distances,
astronomers will be able to learn about the ordinary objects that populated
the Universe at the dawn of time.

Closer to home, Keck will explore faint stars and galaxies more thoroughly
than has ever been possible before. Because Keck collects more light than
other telescopes, astronomers will be able to pick out unprecedented detail
in the spectra of stars. One strong possibility is that this could launch
a new science of stellar seismology. Just as the vibrations caused by earthquakes
have allowed seismologists to probe the interior of our planet, vibrations
on the surface of the Sun have opened a window into its interior. With Keck,
astronomers are hoping to see similar vibrations on other stars. The problems
are formidable because even the closest stars are nearly a million times
as far away as the Sun. Nevertheless, if a star’s spectrum is sufficently
detailed, it may record small changes in frequency caused by the Doppler
effect as the surface of the star vibrates.

But what most thrills astronomers about Keck is that is leading them
into an unknown world, certain to be full of surprises. After the 5-metre
telescope on Palomar Mountain was completed in 1948, astronomers made a
host of exciting new discoveries, including pulsars, interstellar masers,
quasars, gamma-ray bursters and the microwave afterglow of the big bang
itself. No one doubts that a new world lies over the horizon. ‘If anything,
Keck will be more influential than Palomar,’ says Nelson.

Wal Sargent, Professor of Astronomy at the California Institute of Technology,
who has pioneered the use of quasar absorption lines to probe the Universe’s
past, agrees on the prodigious power of Keck. ‘It’ll be 1998 before any
other telescope catches this one up,’ he says. And by that time, when several
other telescopes with mirrors almost as large as Keck’s will be complete,
Keck’s twin brother will be finished and the team will be ready for the
next big leap: coupling the two Kecks and several smaller telescopes together
to make a telescope with nearly 10 times the resolving power.

Linking Keck I and Keck II to form an interferometer will bring huge
technical problems. For the interferometer to work, the beams of light from
the telescopes must travel precisely the same distance before they are combined.
This means that, as the two telescopes track a star, the two light paths
will have to be continually adjusted to within a fraction of a wavelength.
Given that the wavelength of visible light is less than a thousandth of
a millimetre, this will be no mean feat. It has been achieved over short
distances, using sliding mirrors, but never over anything approaching the
85 metres that will separate Keck and its twin.

A way will also have to be found to compensate for turbulence in the
atmosphere, which will smear images of stars at this resolution. One possibility
is to flex the telescope mirrors very rapidly to cancel out the atmospheric
jitter. But the technical problems of such ‘adaptive optics’ systems are
formidable. ‘Keck will move towards adaptive optics in small steps,’ Sargent
says. Nelson’s hope is that Keck I and Keck II, working in concert, will
be able to detect planets around other stars.

Such possibilities are enough to make any astronomer’s head spin. Which
is how a trip to Keck – limited to half an hour for a first visit, because
of the risk of altitude sickness – usually draws to an end. Coming out of
the dome, the stars seem curiously dim and you wonder what is happening
to you. ‘At this altitude, your retina gets starved of oxygen,’ Perala explains.
‘You get tunnel vision and colours don’t seem so bright.’ A few lungfuls
of pure oxygen from a cylinder he carries is enough to rip away the veil.
The sky suddenly fills with more and brighter stars than you have ever seen
before.