Utrecht
HE Martians have landed on the island of La Palma in the Canaries. At least,
that鈥檚 your first impression when you drive up the steep, winding road to the
top of the Roque de los Muchachos, one of the islands鈥� highest peaks. This is
among the world鈥檚 best sites for observing the heavens and the mountain is
littered with the eerie domes of astronomical telescopes.
But amid the white domes another unworldly structure rises on seemingly
fragile metal legs, like a Martian fighting machine from H. G. Wells鈥檚 The
War of the Worlds. But this is no invader from outer space. It is the Dutch
Open Telescope (DOT), a relatively small instrument designed to observe not the
stars or the Moon or the planets, but the searing disc of the Sun. This has
always been a difficult task to achieve with any precision. During the day, the
Sun鈥檚 raging heat creates so much turbulence in the Earth鈥檚 atmosphere that good
observations from the ground are all but impossible.
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DOT鈥檚 unearthly shape is designed to combat this problem. Its designers
believe the telescope will produce images that rival the quality of much larger
instruments, and at a fraction of their cost. And DOT, which came into service
late last year, is paving the way for yet more ambitious projects to come.
Through the maelstrom
In the past, solar astronomers have fought a losing battle against the
Earth鈥檚 atmosphere. The air above our heads is an angry maelstrom of eddies and
currents that bend incoming rays at random, like the heat haze above an asphalt
road on a summer鈥檚 day. This is why all astronomers prefer to work on
mountaintops where they can sit above the thickest, most turbulent part of the
atmosphere. But even on the highest peaks, turbulence in the thin air above is a
major problem. As the currents swirl, the image through a telescope dances back
and forth. Astronomers call this phenomenon 鈥渟eeing鈥�. Poor seeing has ruined
many a night鈥檚 work.
While seeing may be bad at night, it is impossible during the day when solar
astronomers must do their work. The Sun heats the Earth irregularly, causing
giant cells of air to rise through the atmosphere like huge bubbles. This drives
the wind and creates turbulence, which is worst near the ground because trees,
buildings and the topography of the land turn the smoothest flow of air into a
choppy sea.
But strong winds can also help astronomers. La Palma is one of a lonely group
of islands off the coast of Morocco. The northern flank of the Roque de los
Muchachos is buffeted by constant strong winds that have travelled thousands of
kilometres across the Atlantic. These winds are unhindered by land and so,
although strong, they are extraordinarily smooth. For this reason, the seeing
here is unusually good, which is why the site has long been favoured by
astronomers.
Solar astronomers also have another problem to contend with. Anyone who has
played with a small magnifying glass will know it can focus enough energy to
burn through paper, grass and even human skin. The amount of energy concentrated
by a mirror or lens many times bigger rapidly heats the air inside a telescope,
creating 鈥渕icroturbulence鈥濃�攕wirling currents that severely degrade any
image.
The largest solar telescope in the world is the McMath-Pierce telescope run
by the US National Solar Observatory at Kitt Peak in Arizona, which has a mirror
1.6 metres across. 鈥淚n terms of image sharpness, the telescope isn鈥檛 that good,鈥�
says Rob Rutten, project scientist for DOT, who is based at the Astronomical
Institute of Utrecht University in the Netherlands. Rather than take pictures of
the Sun, astronomers use it for spectroscopy, a technique that splits sunlight
into its component frequencies. 鈥淔or this you need as much light as you can
get,鈥� explains Rutten.
To get good images, solar astronomers need to reduce the amount of turbulence
inside their telescope. The conventional way to do this is to seal the tube with
a clear window and pump out the air. The US National Solar Observatory has a
telescope of this type on Sacramento Peak in New Mexico. Its mirror is half the
size of the one used by the McMath-Pierce instrument, and yet it produces
sharper images. Only a few metres from DOT on the Roque de los Muchachos, the
47-centimetre Swedish Vacuum Solar Telescope produces still sharper images by
using a lens to seal the tube and doing away with an internal imaging mirror
altogether.
But vacuum telescopes have their limitations. The windows needed to seal them
are hugely expensive and limit the size of the telescope. Large windows cannot
be supported evenly, unlike mirrors, and so tend to bend under their own weight.
This ultimately limits the quality of the images that can be made.
DOT does not have this problem. The telescope was conceived in the 1970s by
Rob Hammerschlag, an astronomer at Utrecht University and DOT鈥檚 chief designer,
who realised that the strong winds that produced such good seeing at La Palma
could also be used to cool an instrument. His idea was to build a telescope with
a structure that allowed the northeastern trade winds at La Palma to blow
straight through. His telescope鈥檚 45-centimetre mirror would sit in the open, it
would not be encased in a closed tube and it would not even shelter under a
dome. With this design, any microturbulence generated by focused sunlight would
simply be blown away.
But designing such a telescope is not simple. For a start, the 16-tonne
instrument must be as far away from the ground and the turbulence it creates as
possible. So DOT perches on legs 15 metres above the ground, giving it its
Martian appearance. This structure must provide a rock-steady platform for
observations, even against winds of up to 70 metres per second that occasionally
hit La Palma.
This has turned out to be one of Hammerschlag鈥檚 biggest challenges. He says
the trick is to allow the platform to make small horizontal movements that don鈥檛
affect the direction in which the instrument is pointing, while preventing any
twisting, angular movements that change the aim. The design he came up with has
eight legs that maintain the height of each corner of the platform to within
0.001 millimetres.
DOT will only be shielded from the elements in really bad weather. The
telescope has a covering made from two hinged halves like a clamshell that can
be erected in gales blowing at 30 metres per second. During the winter months,
bad weather also brings ice and snow to the mountaintop. The extra weight this
adds has destroyed observatories in the past, so DOT is designed to cope with a
huge extra load. Hammerschlag believes the weight of snow and ice could add an
extra 30 tonnes to the structure.
The problems don鈥檛 end there. The huge amount of energy concentrated at the
focal point is too much for the sensitive apparatus that records the images.
Most of this energy must be reflected away using a mirrored disc with a tiny
aperture at its centre that only allows a small portion of the focused light to
pass through. This disc or aperture has to be water-cooled so that its
temperature is within a fraction of a degree of the surrounding air. Even this
might cause unwanted microturbulence, so a pump ensures that air flows smoothly
across its surface.
Even with these challenges solved, Hammerschlag has had to wait more than
twenty years to see his project come of age. Lack of funds kept the idea on the
drawing board until the Dutch Technology Foundation provided roughly a million
pounds sterling to meet construction costs.
Late last year, DOT took its first images. Hammerschlag and his colleagues
are using the early observations to look for heat sources and sinks within the
telescope that could cause microturbulence, and to fine-tune the water cooling
and air suction accordingly. Hammerschlag also wants to monitor the telescope鈥檚
pointing accuracy so that, in future, he will be able to take into account tiny
effects such as unequal heating of the tower鈥檚 legs, which may cause them to
expand unevenly. With these housekeeping tasks finished, DOT will begin its
scientific programme. Early goals include photographing sunspots to build up a
better understanding of the magnetic patches on the Sun鈥檚 surface that create
them.
If this work is successful, Hammerschlag and Rutten believe that the
open-telescope concept can easily be applied to larger instruments. Hammerschlag
designed DOT with this in mind. At the moment, the telescope has a 45-centimetre
mirror, but it could easily be fitted with one almost twice this size, greatly
improving its resolving power. In the US, Jacques Beckers, director of the
National Solar Observatory, is already planning a 4-metre solar telescope based
on DOT鈥檚 design.
For DOT, attracting further funding remains a challenge though this task may
be easier now, given that its first sunspot pictures have been so good. Rutten
says DOT has already secured its place as one of the sharpest telescopes in the
world. For the time being at least, the future for DOT looks bright.
Further information: the DOT Web site is at http://www.fys.ruu.nl/~rutten/dot