快猫短视频

THE YEAR is 2016. High on a remote mountain top, a titanic telescope scans
the star-spangled sky. Its light-collecting mirror is as big as a football
pitch, and the whole structure is half the height of the Eiffel Tower. On
previous nights, this 40 000-tonne leviathan has produced detailed images of the
surfaces of the nearest stars and determined the atmospheric compositions of all
the known extrasolar planets. Tonight, provided no clouds move in to obscure the
view, it will zoom in on individual stars at the edge of the Universe.

A telescope with such mind-boggling capabilities might seem like an
astronomical fantasy. But a team from the European Southern Observatory (ESO)
based near Munich, Germany, is convinced it could become a reality. Roberto
Gilmozzi and his colleagues at the ESO say the telescope could be built for less
than $1 billion and finished by 2015.

The OverWhelmingly Large telescope (OWL), as Gilmozzi鈥檚 team call it, would
have a mirror 100 metres in diameter. That would give it 100 times the area of
either of the Keck telescopes in Hawaii, whose 10-metre mirrors make them the
biggest optical instruments in the world. 鈥淚n fact, OWL would have 10 times more
light-gathering area than all the telescopes that have ever been built put
together鈥� says Gilmozzi. 鈥淭he more light a telescope gathers, the fainter and
more distant the objects it can see.鈥�

Although Keck can see fainter objects than the Hubble Space Telescope, which
has only a 2.4-metre mirror, the distorting effect of the atmosphere means it
can鈥檛 match Hubble鈥檚 resolution. OWL, however, will far outstrip both of them.
It will even surpass the Next Generation Space Telescope, an orbiting instrument
that NASA hopes to launch in 2007 as Hubble鈥檚 successor.

Other astronomers are stunned by the audacity of the OWL vision. 鈥淚 was
extremely impressed,鈥� says David Tytler of the University of California in San
Diego. 鈥淚t was a huge surprise to all of us in the astronomical community that
something of this size was feasible and at a reasonable cost.鈥�

By daring to propose a 100-metre optical telescope, Gilmozzi and his
colleagues have broken out of a mental straitjacket that has bound astronomers
since the beginning of the century. No one has ever dreamt of building a
telescope more than twice as big as the biggest that already exists. For
instance, the 2.5-metre telescope built on Mount Wilson in 1908 was superseded
by the 5-metre telescope on Palomar Mountain in 1948, which in turn was
superseded by the first 10-metre Keck Telescope on Mauna Kea in Hawaii in 1994.
鈥淏y considering a telescope 10 times bigger than anything around today,
Gilmozzi鈥檚 team has shattered the usual `factor of two鈥� way of thinking,鈥� says
Tytler. 鈥淥WL is a quantum jump in telescopes,鈥� says Gilmozzi.

A 100-metre telescope certainly presents formidable technical challenges. But
the main reason nobody has proposed building one before is probably money. The
cost of a telescope goes up very rapidly as the diameter of its main mirror
increases. The Keck telescopes cost about $100 million each, and,
according to the historical trend, that would make a 100-metre telescope cost
more than $30 billion. 鈥淣obody could afford this,鈥� says Gilmozzi.

But Gilmozzi and his colleagues believe that they can cut the cost to less
than $1 billion with a few clever tricks. One is to make OWL鈥檚 primary
and secondary mirrors spherical rather than parabolic. All parts of a spherical
mirror have the same curvature, making them easier and cheaper to make.

The drawback of a spherical mirror is that light falling at different places
is brought to a focus at different points. However, Gilmozzi鈥檚 team says this
spherical aberration can be eliminated by adding two corrective mirrors. These
extra mirrors鈥攁n 8.2-metre tertiary and a 5.6-metre quaternary鈥攎ust
have complicated shapes to compensate for the spherical aberration and produce a
sharp image. Grinding them will not be an easy task, admits Philippe Dierickx, a
member of the OWL team.

A second way to bring down the cost is to make the primary and secondary
mirrors out of segments. In fact, there is no choice here, because it is
impossible to make a single mirror 100 metres across. The segments will be
hexagonal pieces of glass 2.3 metres across, each supported by three pistons
which will keep the segment perfectly aligned with its neighbours.

Growing a telescope

Mass production is the third way to cut costs. Gilmozzi鈥檚 team say that the
2000 polished segments needed for the primary mirror and the 100 or so needed
for the secondary mirror could be made in five to six years at the rate of one a
day. 鈥淲e already have experience in making identical elements,鈥� says Gilmozzi,
鈥淓SO鈥檚 Very Large Telescope in Chile is equipped with four 8.2-metre mirrors.鈥�
Tytler adds: 鈥淎fter you have had experience building something, it gets
肠丑别补辫别谤.鈥�

One advantage of the segmented design is that OWL could begin observing long
before the main mirror is finished. 鈥淚n effect, we鈥檇 be growing a telescope. By
putting down the segments either randomly or in a special spiral pattern, we
could make an interferometer,鈥� says Gilmozzi. This is a configuration that would
have the resolution of a filled 100-metre telescope鈥攊ts ability to see
fine detail鈥攚ithout such a large light-collecting area.

The resolution of a telescope depends on the size of its mirror because of
the wave nature of light. All waves bend around obstacles in their path, as
anyone who has watched water waves bending round a moored boat will know. In the
same way, light bouncing off a mirror is bent because the mirror doesn鈥檛 go on
for ever鈥攖he edges of the mirror define the obstacle. This causes
the image of a point-like star to be smeared into a smudge. But the bending
becomes less severe as the size of the mirror, or aperture, is increased, so a
bigger mirror means better resolution.

According to this relation, the theoretical resolution of OWL is equivalent
to being able to distinguish between two adjacent coins about 1000 kilometres
away. But in practice, the twinkling caused by turbulence in the atmosphere
means that the resolution of any ground-based telescope is hundreds of times
worse than that鈥攁nd not much better than Galileo achieved in 1609 with his
version of the newly invented telescope.

Gilmozzi and his colleagues plan to overcome this limit by using adaptive
optics, in which atmospheric distortion is measured by monitoring a reference
star. This information is used to very rapidly flex one of the mirrors of a
telescope in such a way as to exactly compensate for the atmospheric turbulence.
The technique is difficult and still in its infancy, but, remarkably, it becomes
easier with a very large telescope. 鈥淲ith a big telescope there is a lot more
light to tell you what is happening to the atmosphere,鈥� says Tytler.

But flexing a mirror 100 metres across is out of the question, especially as
the glass segments will be about 10 centimetres thick. Instead, Gilmozzi鈥檚 team
plan to use a fifth mirror鈥攁 quinary. This mirror, a mere 65 centimetres
across and made of very thin material, would have its shape altered 100 times a
second by 500 000 piezoelectric actuators. This is no mean feat. Simply
calculating how to flex the mirror in real time will require a supercomputer 300
times faster than any that exist today. He is optimistic, however. 鈥淏y the time
we need them in 10 to 15 years time, such computers will almost certainly be
around,鈥� he says.

Great leap

The result of all this innovation will be uniquely powerful instrument. All
pioneering telescopes in the past have either embodied a jump in
light-collecting area, like Keck, or a jump in resolution, like the Hubble Space
Telescope. OWL will provide both. 鈥淲ith 100 times the area of Keck and 40 times
the resolution of Hubble, it will be a gargantuan leap forward,鈥� says
Tytler.

As OWL will collect 100 times as much light as Keck, it might seem that it
will be able to see objects up to 100 times fainter or, equivalently, gather
enough light to see the same objects 100 times faster. But for some objects,
those that are both point-like (such as stars) and fainter than the background
glow of the sky, the telescope is much better than this. Because its resolution
is 10 times finer than Keck, it will focus the light from a star into a spot 100
times smaller鈥攚hich therefore contains 100 times less confounding
background light. This means OWL will be able to see objects 10 000 times
fainter than Keck. So will astronomers who are used to applying for a night鈥檚
observing time on Keck apply for just 1 second of time on OWL? Tytler thinks
not. 鈥淭hey won鈥檛 want to do anything they can do on other telescopes,鈥� he says.
鈥淭hey鈥檒l want to do things that no other telescope can do.鈥�

Like what? Well, it鈥檚 hard to think of an area of astronomy that won鈥檛
benefit. With its phenomenal resolution and light gathering power, OWL will
enable astronomers to see and count individual stars at red shift of about 3, so
distant that we are seeing the Universe at about a quarter of its present age
and size. Hubble can barely see single stars in the Virgo cluster, with a red
shift of about 0.003. OWL will also be able to see supernova explosions or star
clusters at a red shift of 10, when the Universe was less than a tenth of its
present age. 鈥淭his will enable us to determine the star formation history of the
Universe,鈥� says Gilmozzi.

The formation of stars is inextricably bound up with the formation of
galaxies. It was partly to search for the origins of galaxies that the Hubble
telescope was kept pointing at the same small region of the sky for 10 days
during Christmas 1995. In this marathon exposure, the embryonic precursors of
galaxies showed up鈥攂ut only as smudges. 鈥淭his has tantalised the
astronomical community,鈥� says Tytler. 鈥淭he image contains lots of objects at
high red shift, which are galaxies in the process of forming.鈥� OWL will let us
see these objects clearly and even take their spectra, which will reveal how
many heavy elements have already been created and spewed out by short lived
stars, for example.

Nearer to home, OWL will enable astronomers to analyse the atmospheres of
planets around other stars鈥攁nd especially to search for oxygen, the
signature of life. It should also be able to map the surface of a star.
According to Tytler, such mapping would be possible for eight nearby Sun-like
stars, for red giant stars as far away as 300 light years and for supergiant
stars 3000 light years away. OWL would be able to make a 40 by 40-pixel picture
of R Doradus, the star with the largest apparent size. 鈥淓ven with such crude
images, we would see hotspots, concentrations of magnetic field, mass being lost
and vibrations of the surface which reflect internal structures,鈥� says
Tytler.

Amazing potential

In one way, OWL is overdue. In radio astronomy, the technique known as very
long baseline interferometry already produces high-resolution images of the
cores of 鈥渁ctive galaxies鈥� such as quasars, among the brightest objects in the
Universe. Radio astronomers have been waiting decades for optical astronomy to
catch them up. 鈥淥WL will allow us to overlay radio and optical images, giving us
crucial information on the supermassive black holes thought to power such
objects,鈥� says David Hough of Trinity University in San Antonio. Smaller
versions of such black holes are thought to reside in nearby galaxies. 鈥淲ith
OWL鈥檚 resolution we will be able to see the motions of stars deep in the hearts
of nearby galaxies, thus determining how common and how massive such black holes
are,鈥� says Tytler.

But the most exciting prospect is that OWL might discover objects of a kind
never even suspected. 鈥淭he possibility of finding new astronomical objects is
tremendous,鈥� says Gilmozzi.

It all sounds wonderful. But will OWL really be built? Gilmozzi admits that
there are problems. 鈥淭he optics are viable and could be built today,鈥� he says,
鈥渂ut we haven鈥檛 yet solved all the mechanical problems.鈥� Somehow, about 20 000
tonnes of mirrors and scaffolding will have to be supported and pointed. That
will require a metal cradle weighing another 20 000 tonnes. Gilmozzi鈥檚 team
favour mounting the telescope within a cup that floats on oil on a turntable. A
similar system is used by the 470-tonne Very Large Telescope in Chile, which is
so well balanced it can be moved by hand. 鈥淏ut OWL may be a little much for even
Arnold Schwarzenegger,鈥� says Gilmozzi.

Another problem is how to stop unwanted vibrations. The 鈥渢ube鈥� of the
telescope will an open lattice of girders 135 metres tall, with a natural
frequency of about 1.8 hertz. Such vibrations could easily be produced by the
buffeting of the wind. Gilmozzi says that this may require the development of
鈥渁daptive mechanics鈥�, in which vibrations are generated with just the right
amplitudes and frequencies to cancel out the natural oscillation.

Gilmozzi is confident that all the outstanding problems can be solved. 鈥淲e鈥檙e
pretty sure there are no unforeseen show stoppers,鈥� he says. But, mechanical
problems aside, will the ESO fund OWL? Gilmozzi is reasonably optimistic. At
$1 billion, the cost of OWL is less than the observatory鈥檚 $80
million annual budget over the 15-year time span of the project. 鈥淲e may even be
able to get the US interested in an international collaboration,鈥� says Gilmozzi.
Two American consortia are already considering the possibility of building
25-metre telescopes.

If a 100-metre telescope does get built, what will be next? 鈥淚鈥檓 sure there
will be someone crazy enough to propose a 1-kilometre optical telescope,鈥� says
Gilmozzi. Tytler agrees. 鈥淲e鈥檙e going to build bigger telescopes, even though we
don鈥檛 know how to do it today,鈥� he says.

Tytler points out that Dan Goldin, the head of NASA, has redefined the
concept of a large telescope by asking the astronomical community to design one
that could make high-resolution pictures of terrestrial planets around other
stars. This is going to require telescopes far bigger than 100 metres across. To
resolve a continent the size of Asia on a planet 10 light years away would need
a mirror, or an interferometer, about 10 kilometres across. 鈥淭hey will almost
certainly have to be in space,鈥� says Tytler. 鈥淭hey鈥檙e going to be expensive but,
let鈥檚 face it, it鈥檚 going to be cheaper than actually going to the stars.鈥�