Prototype for 'FIRST' mirror successfully tested
1 June 1999
Imagine you make a large and thin surface out of a special material, and then you polish it to make it so uniform that its 'bumps' are lower than one thousandth of a millimetre. Now guess: would your smooth surface survive if you put it under a load fifteen times that of the usual gravity on Earth? And what if you then made it endure a quick, sharp change in temperature of hundreds of degrees?Well, that is precisely what ESA and Matra Marconi Space engineers did last month with a prototype mirror for future ESA's space telescope FIRST. And they already have the answers to these questions: the prototype mirror passed the tests successfully - not only did it survive without damage, it also changed its shape so little that there were big smiles all around from the engineers.
The main mirror of ESA's space telescope FIRST , due to be launched in 2007, is a technological challenge. With a diameter of 3.5 metres it will be the largest mirror ever sent into space, and it has to be extremely smooth, light and resistant. The technology that will be used to build it is still under study in the United States and Europe; ESA is in fact waiting for the results to come in from the construction of two prototypes, each of them based on different materials and techniques.
The tests last month were performed on the European prototype. This is a 1.35 metre reflector shaped like a shallow bowl built by French-based company Matra Marconi Space from a material called Silicon Carbide (SiC), a ceramic material still rather new in space applications but already in use in industry. SiC seems to have all advantages of metal and glass, such as lightness and high resistance to stress and changes in temperature, and can be polished and smoothed as if it were glass.
To make sure these properties indeed fulfil the requirements of a mirror for ESA's FIRST space telescope, the prototype had to pass a temperature-shape test at the Lihge Space Centre (CSL), in Belgium , The 1.35 metre surface was cooled down from room temperature to about minus 160 °C. Afterwards, laser measurements accurate to half a micron (one micron is a thousandth of a millimetre) were used to check the possible distortions suffered by the surface - there were none!
The test was therefore considered successful.
The second test took place last week at ESTEC (ESA's European Space Research and Technology Centre), in The Netherlands. The purpose of this was to prove the mirror's ability to survive the extreme conditions of a launch. During launch by Ariane-5 the payload has to endure violent shaking, with a force fifteen times that of the normal gravity on Earth, or 15g. In comparison, astronauts withstand at launch a pressure of 'only' three times the Earth's gravity, or 3g.
The shaking causes the payload to vibrate violently, and this is what the engineers did to the FIRST mirror prototype. The surface was vibrated horizontally and vertically, even more violently than expected at launch. Again, there were no changes in the prototype afterwards.
