Active optics is a complicated process that will manipulate the primary and secondary mirrors. On the primary mirror, it will move and bend the 36 mirror segments to ensure that they are correctly aligned at all times. The weather (wind, temperature fluctuations), mechanical tension and unavoidable manufacturing defects will all affect the way the segments fit together. To make sure they are properly aligned there will be an ‘assembly line’ made up of a number of different devices:
The onboard sensors are high precision devices - accurate to within nanometres (millionths of a millimetre) - that will be used to determine the relative position of each of the 36 segments of the primary mirror and to identify any unwanted shifts and movements.
The primary mirror’s actuators, or adjustment systems, will physically move the mirrors to correct their position.
This is how it will work: the information collected by the onboard sensors will be sent to the control system, which will process it and send it to the actuators. These have been designed to apply very precise movements to the mirrors, correcting deformities and ensuring that, together, the 36 segments form the correct shape of the telescope’s primary mirror.
The onboard sensors are so accurate that it will be possible to measure not only any deformity in the mirrors but also the force that gravity is exerting on each of them, so that these can then be compensated for by the control system.
The acquisition, guiding and calibration devices will provide the control system with the information it needs to control the telescope’s mechanisms and optical systems. They will measure the movement of stars 200 times per second in order to calculate the movements that the telescope and the secondary mirror need to make, monitor image quality so that the shape and position of the primary mirror segments can be adjusted, monitor the telescope focus so that it can be adjusted by the secondary mirror and measure the quality of the atmosphere to provide the control system with information about observing conditions.