Actuators for the E-ELT

Nanometer accuracy with centimeter stroke

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Dutch industry wants to be involved in the technical challenging part of the E-ELT. The actuators for the segments of the main mirror are such an area. In order to be accepted by ESO, it is necessary to demonstrate the capabilities of the Dutch industry by making and testing a demonstrator. That is what will be done in this project.


TNO has designed various prototypes of this actuator, thereby paying specific attention to possibilities for cost saving and manufacturability once it is mass produced (ESO will need about 2800 of them).

The prototypes have been manufactured by VDL-ETG, and extensively tested at TNO. These tests have revealed excellent dynamic behavior of the actuator and closed-loop control experiments have demonstrated a 1.4 nm RMS tracking accuracy over its 15 mm stroke, even in the presence of realistic disturbance sources.

Involved Partners
Space Science and Astronomy Instrumentation from TNO, together with VDL-ETG

Progress, Finished


The Dutch industry would like to be involved in the design and construction of (parts of) the E-ELT. TNO is especially interested to support industry in the area of opto-mechanical design work. The hope is that the Netherlands can be involved in the high tech systems and not only in flattening a mountain top, like we did for the VLT. TNO and VDL-ETG have been involved in the design and production of three prototypes for the support of the mirror segments of M1 (the primary mirror of the E-ELT). These support structures have been delivered to ESO and brings Dutch industry in an excellent position to later deliver the required 931 support structures (798 for installation on M1 and 133 spares). In the same spirit TNO is involved in the development of the actuators which will be used to control the position, tip and tilt of each of the individual segments. It is our ambition to convince ESO of the quality of the Dutch design to bring Dutch industry in an excellent position to win the eventual production contract for these actuators; this encompasses about 2800 pieces (of which 400 spares), i.e. three for each mirror segment.


TNO has designed various prototypes of the actuators. Each prototype utilizes a two-stage concept: a fine stage to achieve high accuracy, combined with a coarse stage to achieve a large stroke and reduce power consumption. Moreover, each actuator is equipped with an on-axis nanometer-resolution encoder. The different prototypes vary in the choice of coarse stage (stepper-motor or DC-motor) and concept for support of lateral loads. In the design special attention has been paid to cost reduction. As such the actuator design uses as many commercial off-the-shelf components and easy-to-manufacture parts as possible. ESO has been informed about these design developments and has shown interest in the performance of such a design.

Status & Results

The prototypes have been manufactured by VDL-ETG and delivered to TNO for extensive testing. A picture of one of the prototypes in its testing environment is shown in the picture below.


Testing of the actuator at TNO

During the tests the dynamic response of the actuator has been thoroughly assessed and dedicated controllers for both the fine and coarse stage have been designed. Final performance experiments have demonstrated that the actuator achieves 1.4 nm RMS accuracy, in the presence of (simulated) wind force disturbances and structural vibrations, and while tracking a 1.2 µm/s reference velocity over its full 15 mm stroke. This is well within the requirement of 1.7 nm RMS, and thus successfully qualifies the Dutch design.

In future steps TNO will broadcast their results and try to demonstrate the performance at the ESO test facility, where the actuators can be integrated with the M1 support structure.

Involved Partners

The work has been done by Space Science and Astronomy Instrumentation of TNO together with VDL-ETG.


Extractor function
An old requirement for the actuators that they should be able to lift a whole mirror segment above the other segments, so that they can easily be extracted. This requirement has been lifted in the new list of requirements ofr the actuators. 

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