2014

Enclosure Design for the European Extremely Large Telescope

A talk by Jac Cross, Trinity 1995-9, on Jenkin Day 2013

Jac is a structural engineer. He had worked with Atkins and then Frazer-Nash before joining Arup seven to eight years ago. He has worked on a wide variety of projects with Arup apart from this one, the most recent being foundations for offshore wind generators.

The European Extremely Large Telescope will be built in the Atacama Desert of Chile, in the foothills of the Andes, where the climate gives clear dry air favourable to astronomy. The location will be about 10-15 km inland, at an altitude of 4000 m, so there is much less air to see through than at sea level. It will be an optical telescope with a reflector 39 m in diameter, and a successor to the "European Very Large Telescope", an array of four 8 m reflectors, already constructed. The client for whom it is being built over the next ten years is the European Southern Observatory, whose members include several European countries, and also Chile and Brazil. Their headquarters are in Munich.

To scan the whole sky, the telescope has to be rotatable 360° in azimuth, and from horizontal to vertical in elevation. The movement in elevation causes the reflector to deflect under its own weight, so it is made up of about 800 individual panels around 1.5 m across, each of which is adjusted automatically to preserve the correct overall parabolic shape. The image is reflected by a sequence of five mirrors altogether, including the main one, and one of the others too is continually adjusted automatically to reduce image distortions due to atmospheric variations. A narrow-band sodium light beam is shone skyward, and the scattering of it used for calibration. The whole rotating structure will weigh about 9000 tonnes. It is mounted on seismic isolators to protect it from earthquakes. The chosen site has onshore winds which rise up the hillside and over the top of the telescope, leading to a narrow boundary layer which reduces atmospheric distortion. But the site is remote and somewhat barren, so quite a lot of basic civil engineering works will be necessary before construction can start.

The functions of the enclosure, which rotates in azimuth with the telescope, are:

1. To protect the telescope against the weather and dust when it is not being used, i.e. normally in daylight. For this purpose the enclosure is sealed as far as possible, and kept above atmospheric pressure, so that any leakage is from inside to out.
2. At night, to open the reflector to the sky. The opening is in the shape of a short quarter-cylinder, of length 40 m axially and 80 m circumferentially, to allow for the 15-90° range of elevation. One cause of image distortion is air turbulence and temperature variation near the telescope itself, so the enclosure is designed to be well ventilated to minimise this. There will also be air-conditioning to hold the daytime temperature close to what is expected at night, thus avoiding temperature changes in this massive structure. And any vibration has of course to be minimised too. Cranes will be provided to remove mirror segments for cleaning.

The shutter panels which open the telescope to the sky are made of six overlapping segments, three on each side, parallel to the long curved sides of the opening, which move across the aperture from either side to close it. Arups had previous experience of designing an opening roof for a stadium in the USA, using a similar technique, and this turned out to be very helpful. There is also a windshield, exterior to the shutters.

The lecturer answered several questions, some about the organisation and financing of the Observatory Group, others about the structure itself.

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