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Life's twists and turns...

07/04/2003

Have you ever paused to consider that, from the day we are born, we never keep still - and not just because some of us cannot do as we are told! Think about it ? how much does ?movement? affect our lives? Not only do we move from one place to another on foot or by car, bus, metro, train, aeroplane or boat....but we carry on moving even while we are sleeping.

We are not talking about the involuntary twitches that happen before we fall asleep, nor even about changing position or turning over in bed. Every night the world keeps rotating, so that when we are told to “keep still!” we can never, in the wider sense, truly obey.

The earth moves, and we move with it!

This constant movement affects everything on the earth - including, of course, the Gran Telescopio Canarias (GTC). The earth’s motion (it spins at the same time as it orbits the sun) creates field rotation, a phenomenon we have to deal with in order to carry out our work. Field rotation causes problems for observation, but it can be successfully countered using the techniques explained below.

In the article “How will the giant move?” we use a 3D animation to show how instrument rotators work: they are devices that compensate for field rotation, and they will be installed on the GTC so that its instruments can function with no interference from all this movement. We are going to take a closer look at these vital components of the GTC, which are currently being built by the Basque company Tekniker.

Imagine we have an instrument that weighs 2,400 kg. We need to literally ‘hang’ it on one of the rotators mounted at the GTC’s Nasmyth foci, and then rotate it at a speed of just 0.005 revolutions per minute! In other words, each full rotation will take three and a half hours. Think about how strong, slow and precise the rotator needs to be for everything to run perfectly so that we can track a star faultlessly through the whole of the night. As you will appreciate, our instrument rotators - each 4 metres in diameter with a weight of 6 tonnes - are components that must be designed to cope with very demanding conditions: they will rotate without a break for a period of six hours (turning through a total of 617º).

As if this was not complicated enough, the instruments will also have to be connected up to many different services, like the helium, dry compressed air and nitrogen gas supplies needed for refrigeration. Without these services, the instruments would be like a car with no petrol: they would not work.

All of these services will be connected to a console on the Nasmyth platform by a large number of cables, which will run from there into the telescope enclosure. At the other end, the instruments will be turning on their rotators and so the cables will need to turn with them to avoid becoming jumbled up or twisted. Each group of cables will need to be “plaited,” - they will be grouped together and placed in a cable drag chain. In all, 27 metres of cable will be used to connect the moving consoles to the fixed one, and they will weigh somewhere in the region of 600 kg.

All this cable and so much weight, perfectly sorted and organised. Do you see how complicated it all is? We always have to take account of life’s twists and turns...

Natalia R. Zelman

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