Mirror Shaping Actuators

It is impossible to observe outer space through a telescope with a deformed mirror. How can you ensure high-precision positioning and compensate for any deformation? This is the task that was entrusted to Micromega Dynamics for a future telescope in Turkey.

At Micromega Dynamics, we feel at ease in applications where there is vibration present. Specialising in structural control, we also develop high-precision mechanisms for scientific applications. These mechatronic devices are used in particular for measuring and actively controlling the vibrations and the form of large ground-based telescopes. We have equipped the primary mirror of the future DAG telescope, which will be inaugurated in 2020 in the Turkish mountains.

The stakes are high. The mirror measures 4 m in diameter, it is made of glass and the slightest deformation would without doubt distort the data collected by astronomers. Consequently, its shape must remain constant within a few nanometres, after every movement and in all environmental conditions, such as gravity, wind and temperature variations – there will be a combination of all of these where the telescope will be installed at an altitude of 3,000 m.

A rigid primary mirror would have been 50 cm thick and 40 tons. The selected mirror is 16cm thick and weighs 13 tons, which is much less, but still enormous. Preventing its deformation under its own weight is a real challenge.

In order to achieve this, we installed 44 actuators on the rear side of the mirror, each of them generating a force of 100 kg with a precision of 30g. They dissipate less than 1.6 W, as heat produced by electricity also distorts data. Each actuator is comprised of a pneumatic power generator controlled by high-speed piezoelectric valves, a high-precision force sensor (0.03 %) and control electronics capable of manoeuvring the whole unit and communicating with the telescope’s control programme. Since each actuator is factory calibrated, it can be replaced in its entirety. This implies full integration of the control electronics in the actuator.