SCANDINAVIAN astronomers have opened an ‘intelligent’ telescope in the
Canary Islands that automatically adjusts itself to produce the sharpest
possible images of the sky. And they have done it at little more than half
the cost of a conventional telescope of the same size.
The Nordic Optical Telescope is the result of a collaboration between
Denmark, Finland, Norway and Sweden. Until now, astronomers in these countries
have been able to use some telescopes in the southern hemisphere, but their
only instruments to look at the northern sky were in Scandinavia. There,
they experience problems with the weather and with turbulence in the atmosphere,
which blurs the images of stars and other astronomical objects.
In 1981, the Scandinavian astronomers decided to take advantage of the
much better conditions on the Spanish island of La Palma, at an observatory
site already opened up by British and Spanish astronomers. With a budget
of only Pounds sterling 4.5 million, the astronomers could not design a
large telescope. Instead, they chose a medium-sized instrument, designed
to produce the sharpest possible images.
The Nordic Optical Telescope has a mirror 2.56 metres in diameter, making
it the twentieth largest in the world. It can gather only 25 per cent as
much light as the largest instruments, such as the neighbouring William
Herschel Telescope on La Palma.
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A small team designed the telescope to weigh as little as possible,
but sufficiently rigid to ensure it does not shake and blur the images of
the sky. This makes it economical to build and operate. The telescope’s
mirror also has an extremely short focal length, only twice its diameter.
As a result, the telescope is only half the length of a more conventional
instrument.
The designers came up with a protective dome that was very small, fitting
snugly around the short telescope. They tested the system in wind tunnels
to ensure that the wind blowing around and into the dome would not stir
up the air and so distort the incoming starlight. They also analysed the
way that heat from the ground disturbed the air, and decided to put the
telescope 8 metres above the ground.
A computer constantly checks the positions and alignments of the main
and a second mirror, to ensure the sharpest images.
Paul Murdin, who was in charge of the British telescopes on La Palma
for several years, acknowledges the team’s achievement but also points out
they have had to pay some penalties. In particular, light can come to only
one focus, behind the main mirror. With most large telescopes, astronomers
can also examine the light at the focal point of the main mirror, at the
top of the tube, or with the use of extra mirrors at other positions.
The director of the new telescope, Arne Ardeberg, of the Lund Observatory
in Sweden, says that the sharp images will provide clearer views of distant
galaxies, so telling us how galaxies change as they grow older. The sharpness
of the images will also help astronomers to see planets going around other
stars.
The telescope is designed to make accurate measurements of the polarisation
of light from stars, nebulae and quasars, which will reveal details of their
magnetic fields.
