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A steel telescope

23/07/2004

To the man in the street the idea of a telescope conjures up romantic images of someone peering through a thick tube at all the wonders of the Universe: the moon’s craters, our nearest planets in the Solar System, planetary nebulas...

For anyone who has seen a film about space exploration in recent years, the word ‘telescope’ might evoke the image of a radiotelescope, given the role that their huge size and amazing abilities have led them to play in a number of films. The Arecibo telescope in Puerto Rico and the VLA (Very Large Array, a group of 27 radio antennae in New Mexico) both feature in the film “Contact” starring Jodie Foster, for example.

A number of documentaries have also presented the image of huge spherical domes rotating in a dramatic twilight, searching the horizon for the starting point of their quest.

As technology has advanced, telescopes have become metal giants that have to house mirrors to collect light from the night sky and instruments to analyse it. As mirrors have increased in size, to collect greater amounts of light and look further back in time, so telescopes have become larger and larger.

The Gran Telescopio CANARIAS (GTC), which is under construction at the Observatorio del Roque de los Muchachos at Garafía (on the island of La Palma), weighs 300 tonnes even before the mirrors and instruments are added. When all the parts are included, it will weigh some 350 tonnes – and that is still without counting the 500 tonnes of the dome, the enormous cover that will protect the telescope during the day.

The telescope proper is made up of three main parts, each with its own role to play: the azimuth ring, the mount and the tube. Once the azimuth ring and the mount had been installed, work began on the tube.

The azimuth ring will support horizontal movement; it is a kind of circular girder which will allow the hydrostatic bearings, on which the telescope will rest, to rotate.

The mount (the “feet” of the telescope), which will sit on top of these bearings, is made up of the fork and the Nasmyth platforms. The heaviest instruments will be mounted at the foci of the Nasmyth platforms. The elevation axes are at the top of these platforms – they will provide vertical movement for the telescope tube.

The third part, the tube, comprises a lower structure, the elevation axis, and an upper structure. In the lower part will be the secondary mirror assembly, made up of the ring, the spider and the mirror support structure.

The elevation axis will contain the folded Cassegrain and Nasmyth foci. It will also house the motors for the tube’s vertical movement, which will raise the tube to the necessary height for observation.

The lower structure will contain the Cassegrain focus, the tertiary mirror turret and the complex primary mirror cell. The cell, which is currently being installed, is an intricate assembly of welded metal supports. It will hold the primary mirror’s 36 segments in place, keeping them aligned and adjusting their position to counter the effects of expansion and contraction. It will use 168 onboard sensors and 108 ‘actuators,’ or primary mirror adjustment systems, to do this.

Once the primary mirror cell had been assembled outside the telescope, it was manoeuvred into place through the observation slit and is now being adjusted, levelled and centred.

Everything is in place, balanced and calibrated, for this enormous pile of carbon-steel to start to move.

Natalia R. Zelman

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