Choosing the main eye of FIRST
4 January 1999The European Space Agency investigates two technologies to choose the main eye of its next infrared space telescope, FIRST.
ESA's Far-Infrared and Submillimetre Telescope (FIRST), due to be launched in 2007, will be the first of a new millenium's generation of space observatories operating at 1.5 million kilometres from Earth, the farthest distance ever achieved by a space telescope. Its main mirror will be 3.5 metres in diameter, the biggest ever built for an orbiting telescope.
These features together pose a great technological problem: how to build such a large mirror light enough to allow the telescope to be placed so far away and still not deform at extremely low temperatures?
The problem is not an easy one to tackle, and ESA's engineers are considering two technologies to solve it. There are, in short, two possible approaches: one uses a fairly well known technology being developed in the US; the other, provided by European companies, is based on a new material and an original and ingenious technique designed up in the North, in a Finnish cave.
A demonstration model of the FIRST mirror made with this European approach was presented on 11 December in Turku, Finland. Fabrizio Felici, FIRST Project Manager, says that "the new technique is highly promising".
The 'US option' would be delivered free of charge by NASA to ESA in return for the participation of American astronomers in the FIRST observations. It uses the material called Carbon Fibre Reinforced Plastic (CFRP), well known in space technology but never used before in a space telescope so big and so stable and accurate. It is a very stiff and light material.
The 'European solution', lead by French-based Matra Marconi Space and Finnish Opteon, proposes the ceramic material Silicon Carbibe (SiC). SiC is generally used in the industry but its qualities have only very recently attracted attention from space scientists. According to their manufacturers (MMS), it will revolutionise space astronomy.
"Silicon Carbide has all advantages of metal and glass. It is light, it resists stress and fatigue and it's able to withstand low and high temperatures without any notable loss of properties", says Jean Dauphin, from MMS.
And there's something more: SiC can be polished as if it was glass, its surface becoming so uniform that its 'bumps' are less than one micron high - one micron is the thousandth fraction of a milimeter. This makes SiC a suitable material to build the mirror of a telescope, in which the slightest roughness causes the image of the astronomical objects to deform and is thus a disaster.
Diamonds in a cave
A FIRST Silicon Carbide reflector would be polished with a new technique patented in Finland, very accurate. To polish a surface, engineers have to bruise it softly with a very hard material, a diamond dust. But while doing it they also apply a strong pressure in order to create a very uniform surface. Or that was, at least, the traditional approach. Tapio Korhonen, a Finnish expert from Opteon, has developed a totally new concept in which the surface is not pressed but sucked, by means of several actuators in which vacuum is applied. This reduces any risk of deformation.
The demonstration model presented last week is being polished right now by Korhonen. The whole process takes place in an underground cave especially designed to preserve the machinery from unstabilities. When the 1.3 m silicon carbide surface is ready, it will be tested to ensure that the polishing has achieved the required roughness.
The final decision between the American and the European option will be taken by ESA in 2000.