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Giant mirror will boldly go to explore the galaxies

Polishing the 3.5-metre Herschel space mirror took more than six months of hard work


Giant mirror will boldly go to explore the galaxies
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By Nils-Erik Backman in Piikkiö
     
      At the Tuorla Observatory in Piikkiö, near Turku, most of the last eight months have been spent in polishing the world's largest space mirror, a behemoth of 3.5 metres in diameter. In February 2007, the mirror and the telescope to which it belongs will be launched on the back of an Ariane-5 rocket as part of the Herschel Space Observatory, with a mission to study the formation of galaxies and their evolution.
      The work of polishing the primary Herschel mirror has been the responsibility of Opteon Labs, a specialist company set up by scientific staff working with the Observatory.
      "Over the last few weeks we have been working around the clock on the mirror", says research associate Aimo Sillanpää.
     
Last Thursday, representatives from the European Space Agency (ESA), the customer ordering the mirror, came to inspect the work. They went away well satisfied that the product meets the tolerance demands set for it.
      This is no ordinary telescope, and no ordinary mirror. Sillanpää says that we are talking about the most difficult task of polishing that has ever been attempted anywhere.
      "The parabolic form has caused problems. The shape of the mirror is designed precisely so as to reflect parallel rays of light to a single point and form a sharp image."
      The purpose of the mirror is to collect collect long-wavelength infrared radiation from some of the coolest and most distant objects in the Universe. On the basis of the results sent back, the aim is to study the birth of stars, the evolution of galaxies, and even possibly to discover new planets.
     
If the shape delivered up a challenge, the materials used in the mirror presented an even greater one. It is made of silicon carbide (SiC), and it is the largest ceramic structure ever created by the isostatic pressing and sintering of SiC.
      Silicon carbide is both light and extremely hard, making it a very painstaking task to polish to the tolerance requirements of such a sensitive device.
      When complete, the mirror will be the largest single-component telescope reflector ever made for use in space. The shaped mirror blank on which the staff in Piikkiö have been working has a price-tag of EUR 20 million.
      Aimo Sillanpää does not reveal what the value of the completed article might be. NASA was reportedly also interested in the polishing contract, but lost out to the Finns.
     
The mirror blank weights 290 kg, and is considerably lighter than the special glass or metal used in conventional reflecting telescopes. The ultra-thin shell surface is only a few millimetres thick, where a glass mirror would be 10-20 centimetres. So the smoothing and polishing process has removed only a few tenths of a millimetre from the surface all over.
      The very hardness of the material creates the difficulties: without the development of a novel and patented method, it would not be possible to obtain a perfect reflecting surface with a "roughness" that is almost negligible - less than 30 nanometres, or three thousand millionths of a metre - while preserving the precise parabolic shape required.
     
The Tuorla Observatory of Turku University has long traditions in optics and in building telescopes, dating back to the observatory's visionary founder Yrjö Väisälä (1891-1971), who was Professor of Physics and of Astronomy at the university. Tuorla was set up in the 1950s, and celebrated its 50th anniversary a few years ago.
      Opteon's managing director Dr. Tapio Korhonen is the Laboratory Manager at Tuorla, and a former pupil of Väisälä's. He also took part in the polishing work, and was the brains behind the new technique adopted.
      Not only has it been patented, but parts of the equipment are discreetly covered over with sheeting in order to keep out prying eyes and prevent industrial espionage.
     
Opteon has made a number of mirrors for reflecting telescopes in service around the world, and work is currently going ahead on a smaller 1.5-metre diameter mirror of the same silicon carbide material for another ESA satellite.
      The Herschel primary mirror blank and its smaller cousin were brought from Toulouse in France to Turku last summer, first by road and then in the belly of the world's largest cargo-carrying aircraft, an Airbus A300-600ST Beluga.
      The same plane took back the large mirror from Turku Airport on Monday (April 18th).
     
Early in 2007, as part of a multi-billion euro venture, the larger mirror, fitted into a Cassegrain telescope manufactured in France by EADS Astrium, will be blasted into space.
      The mission's ultimate destination is in orbit around the so-called second Lagrange point of the Earth-Sun system (L2), approximately 1.5 million kilometres from Earth.
      The choice of location is not an accident: L2 is an orbital "sweet-spot" in space, where all the gravitational forces acting between two objects cancel each other out and it therefore can be used by spacecraft to 'hover' almost motionless relative to Earth for a practically unlimited time.
     
As for the smaller mirror, that, too, will be going into space in 2007, but as part of a telescope on board the ADM-Aeolus satellite. ADM stands for Atmospheric Dynamics Mission, and the project - part of the ESA Living Planet Programme - will further our knowledge of the Earth's atmosphere and weather systems.
      Aeolus will not be travelling far: the satellite will go into low Sun-synchronous orbit around 400 kilometres above the surface of the Earth, and it will beam down laser pulses into the atmosphere. The telescope then measures the return signal, generating a three-dimensional observation of wind patterns and their movements.
     
Helsingin Sanomat / First published in print 16.4.2005


Links:
  Opteon Labs
  Tuorla Observatory
  A short history of Tuorla
  World´s largest space mirror nearing completion
  European Space Agency: Herschel Space Observatory
  ESA: Herschel Primary Mirror Fabrication
  Herschel Mirror on the Move (ESA, July 2004)

NILS-ERIC BACKMAN / Helsingin Sanomat
nils-eric.backman@hs.fi


  19.4.2005 - THIS WEEK
 Giant mirror will boldly go to explore the galaxies

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