Dutch company VDL has been commissioned to build the support structure for the main mirror of the Extremely Large Telescope. The mirror has a diameter of 39 m.
The support structure is essential for the quality of the largest ever optical/infrared telescope, and must keep the 798 segments of the main mirror accurately in place, down to the last nanometre. The quality of the main mirror largely determines the accuracy with which distant galaxies can be examined. Each of the mirror elements has a diameter of 1.4 m and is 5 mm thick, so requires support to prevent distortion.
At the same time, the main mirror must be able to rotate in order to view various positions in the sky. The gravitational force exerted on the structure when the mirror is almost vertical is not the same as when it is lying flat. The effect of the gravitational force on the mirror segments also changes as they rotate, so the distortion varies per mirror element. In order to combat such distortion, the support structure is active: it continuously corrects the shape of the mirror elements. And last but not least, the structure must allow for easy replacement of mirror segments. They need to be replaced every 18 months, which comes down to two elements being replaced each day.
Devised by TNO
The design for the support structure was initially devised by TNO. The support structure resembles a branched tree: three legs with another three legs on each leg, each with three support points, so that each mirror rests on 27 support points. Each of the first three legs has an actuator, and together these keep the mirror in its required position. And so there are close to 2,400 actuators jointly directing the main mirror.
For a long time, it was uncertain whether the TNO/VDL design would be chosen by the European Space Organisation, which commissioned construction of the telescope, as other countries had also made bids for the project. 'This is a fantastic calling card for the Dutch production industry,' is the reaction by spokesperson Miel Timmers on behalf of VDL. A separate production line will be set up to manufacture the 798 support structures, with the company set to supply them within five years.
The mirror segments themselves must be perfectly smooth – the total margin for error for the dish-shaped mirror is 25 nm, less than 1 millionth of the mirror's thickness. The mirrors are to be produced by US company Schott and polished by French company Safran.
The Extremely Large Telescope will be installed on a 3-km-high mountain top in the northern Andes in Chile, close to the Very Large Telescope with its 8.2 m main mirror.
Opening photo: computer animation showing part of the main mirror.
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