Design achievements for actuators and sensors of the main mirror of ESO's ELT

M1, the main mirror of ESO’s Extremely Large Telescope (ELT), is moving closer to completion with the approval of the final design for its edge sensors, with its positions actuators also nearing this key milestone. The sensors and actuators are crucial to the optical quality and functionality of the giant 39-metre mirror. The ELT is planned to start operating later this decade in the Chilean Atacama Desert.

The primary mirror on the ELT, M1, is composed of 798 hexagonal segments, made of a thin glass-ceramic material. To ensure the optical quality of the mirror, its segments have to work together to maintain the mirror’s shape, despite changes in the telescope’s position and in external conditions, such as temperature, pressure and wind. Movable supports and sensors for the mirror, namely the M1 position actuators and edge sensors, allow each mirror segment to move independently and with nanometric (1 millionth of a millimetre) precision and are crucial for the telescope to achieve its potential.

M1 position actuators

Lying underneath each of the 798 mirror segments, the M1 segment supports connect each mirror to the primary mirror support cell. The first set of M1 segment supports, which are being produced and tested to high precision by VDL ETG Projects BV in the Netherlands, was recently delivered to Safran Reosc in Poitiers, France, where the mirror segments are being polished and assembled.

Once in Chile, each individual mirror segment, some 1.4 metres across and weighing about 250 kg with its support structure, will be installed on three position actuators. These actuators constantly adjust the position of the mirror segments through tiny, precise movements.

The position actuators are built by the German company Physik Instrumente (PI). Specialising in precision motoring and positioning, the company will produce the required 2394 position actuators to adjust the segments of the ELT’s main mirror with pinpoint accuracy. The final design for these high-precision actuators is now nearing approval, with series manufacturing starting once the final design review is completed.

M1 edge sensors

Another important component of the main mirror system, the edge sensors, has already passed its final design review. The M1 edge sensors, which are being designed and manufactured by the FAMES consortium (comprising Fogale Nanotech in France and Micro-Epsilon in Germany), are the most precise edge sensors ever designed for a telescope. Two on each side of each segment, the sensors can detect when the segments move out of position, even if only by a millionth of a millimetre. A total of 4608 edge sensors will measure the relative positions of all M1 segments, allowing the overall shape of the mirror to be corrected when the segments move out of place.

Given the high number of edge sensors and position actuators, a consequence of the size of the ELT, the final design phase required the companies to produce dozens of units to carefully test and validate not only the products themselves, but also their manufacturing and production processes. With the completion of the final design review for the edge sensors, FAMES will begin in-series production of these crucial components of the telescope.

In mid 2021, the first batch of edge sensors and position actuators is anticipated to be delivered to the ELT Technical Facility in the Chilean Atacama Desert for future assembly into the M1 segments.

Courtesy of European Southern Observatory