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January 20, 2021



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"Actuators" to perform actions


In engineering, just as the functions of sensing or detecting are carried out by ‘sensors,’ so actions are performed by ‘actuators’. This is the name given to the components that will be used to move the 36 segments of the Gran Telescopio CANARIAS’ primary mirror. They will ensure that the segments always make up a perfect concave hyperboloid surface. This technique for moving the mirrors is called active optics.


The 36 segments of the GTC’s primary mirror must always be joined together to create one single surface. Between one segment and the next will be a space of just three millimetres. To achieve this, sensors will be installed at the edge of each segment. Every two seconds these will send information about the segment’s position, registering any unwanted movement and indicating the amount of correction needed to keep the segments in exactly the right position.

They will not just measure the distance between the mirrors, though. They will also control the “relief” of each segment, as this can be affected by gravity and other factors. To do this, the 168 edge sensors (made by IDS-UTE) will measure deformations in the surface as well as the distance between the segments so that the control system can compensate for any changes as they occur.

The sensors will contain around 90 grams of gold, in the form of capacitor plates. These will be used to show when the segments are in the right position for optimum ‘vision’.


The information captured by the sensors will be sent to the control system, where the ‘actuators’ will come into play.

Each segment will have 3 actuators, which will able to move it with absolute precision. They will keep the segments correctly aligned so that they make one single entity - the telescope’s primary mirror. The motion actuators, deformation actuators and edge sensors will comprise the primary mirror’s active optics system.

Irregularities in the primary mirror caused by the relative position of the segments or by changes in their surface must be kept to within a margin of error of around 90 nanometres. In a surface the size of the Iberian peninsula this would be the same as having ‘mountains’ no more than 10 millimetres high.

Natalia R. Zelman

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