The Gran Telescopio CANARIAS (GTC): 300 tonnes of steel pieces that will fit together perfectly. Before they can start to move, though, they will have to be carefully balanced.
To achieve this, the GTC will have a system of counterweights installed in different sections of its mechanical structure. Tests were performed at the factory after the structure was built and these gave us an initial counterweighting. The structure was then taken apart and reassembled at the Observatorio del Roque, where further tests have to be carried out.
With the weight of the structure and information about the mass of the parts that will be fitted later, we will be able to provide for the addition of counterweights to the lower part of the primary mirror cell, the elevation ring and the secondary mirror spider.
To help calculate the weight that will be added later, the GTC has models to substitute for three future components: the instrument that will be located at the Cassegrain focus, the secondary mirror and one for each segment of the primary mirror (a total of 36).
The GTC will have a total of around 11 tonnes of counterweights in 50, 25, 10 and 5 kg units, although not all of them are likely to be used. 2 tonnes of counterweights will be used in the first series of tests, during which the telescope will be fixed in 5 different positions: 0º, 20º, 45º, 70º and 90º.
This will give the static counterweighting but, once the telescope is in operation, there will be a dynamic or "fine" counterweighting system. As the tertiary mirror moves in its tower, for example, a counterweight will be activated to compensate for changes in the centre of gravity caused by the shift in mass.
It is a process devoted to the search for perfect balance.
Natalia R. Zelman