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September 22, 2023



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When any part of the mechanical structure of the Gran Telescopio CANARIAS (GTC) is installed, precise measurements are taken so that any changes caused by the additional weight can be countered. It is an immense structure over 20 metres high, a meccano® set with parts weighing several tonnes a piece, and it has to be adjusted on the tiniest of scales. Precision is demanded by every telescope in the world, but the bigger the telescope the harder this is. Especially when we are talking in microns.

A precise skeleton

The telescope's mechanical structure is the skeleton that supports the rest of the body - with its muscles, veins, organs and very special eyes which send information to the brain for processing and forwarding for analysis by us.

We have given our skeleton four legs, which float on a film of pressurised oil so that it can move by rotating in the horizontal and azimuth planes. The mount, which plays the role of the feet, is made up of the fork and the Nasmyth platforms to which the heaviest instruments will be attached.

The axis of movement has to pass through a set point, located on a line running from the centre of the Cassegrain focus to the centre of the secondary mirror 18.13 metres away in the upper part of the structure.

The second axis of movement, through which the "trunk" will be hauled through the vertical into the elevation plane, will also be made possible by the hydraulic system at the Nasmyth foci.The axis of movement has to be fixed at the centre of the imaginary line that passes trough the centre of the two Nasmyth foci, 14.8 metres away.

The degree of accuracy that needs to be obtained in measuring the azimuth and elevation axes is 0.1 mm or 100 µm (microns). To put this in context, the human eye, which has a resolution of around 100 µm, can see nothing smaller than a human hair.

To continue with the analogy, the muscles are the motors; the veins the system of tubes and pipes; the eyes the jigsaw of mirrors formed by the primary, secondary and tertiary, all of which can move to adjust and fix their position; finally, the instruments make up the brain.

Some of these components of our 'telescope body', which weighs 300 tonnes, can 'feel' changes in temperature and understand how they are affected. They have systems which compensate for these changes, like that fitted to the primary mirror segments or that used when the telescope is moving and the axis of movement has to be maintained.

During the day the GTC prepares for night-time work by anticipating the temperature that will be needed and maintaining it using the ventilation system.

This constant process of adjustment does not just happen while the telescope is being erected. It is a process that will continue throughout the lifetime of this unique body.

Natalia R. Zelman

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