The industrial contract for the design and supply of a prototype of a deformable secondary mirror with coil actuators for EST is well underway.
Coil-actuator ASM baseline design. Credit: AdOptica.
One of the distinguishing features of EST is that it includes a deformable secondary mirror (Adaptive Secondary Mirror or ASM) to correct the distortions caused by the atmosphere in an optimised way. In addition, this ASM allows to reduce the number of optical surfaces of the telescope, decreasing the loss of light in the optical path. The result is that more sensitive measurements can be taken, which makes EST one of the most advanced tools to observe the Sun from the ground.
Although there are nighttime telescopes with an ASM, currently no solar telescope has one. There are many differences in the technical requirements for an ASM for night or daytime observing and state-of-the-art development is needed to build the EST ASM. To ensure that the results of this development are aligned with the scientific needs of EST, the two technologies available for constructing ASMs are being evaluated: internal reluctance actuators or non-contact moving coil actuators in combination with reference capacitive sensors. A thorough analysis of the pros and cons of each technology, together with a small-scale test bench built in the IAC laboratories, allows us to demonstrate the concepts that will then be scaled appropriately to the EST dimensions. By approaching this issue in a thorough and methodical manner, we aim to mitigate the risks of such a novel system.
The contract for the design and construction of a prototype based on coil actuators was awarded to AdOptica. AdOptica is a consortium of two companies, A.D.S. International and Microgate, and is the only company that manufactures ASMs based on this technology. It currently has ASMs operating on night telescopes with excellent results (e.g. the MMT telescope in Arizona, the Large Binocular Telescope in Arizona and the Very Large Telescope in Paranal).
The project held its kick-off meeting in May 2023, starting the Phase 1 where a number of trade-offs were conducted. This resulted in a baseline design presented at the End-of-Phase 1 Review Meeting in November 2023 (see figure).
The coil actuator technology allows no contact between the reflecting surface of the mirror and the actuator itself. Each actuator is accompanied by a capacitive sensor that allows the position of the mirror's reflective surface to be known with respect to the main body at the nanometre scale. Theoretically, this allows the mirror deformation to be controlled very precisely and at all times to correct any type of disturbance, induced not only by the atmosphere but also by gravity and wind. In addition, non-contact technology makes the mirror intrinsically fail-safe with respect to the failure of individual actuators.
The ASM prototype will have around 100 actuators, which represents 5% of the actuators needed by the EST ASM. These are separated by a distance of 16.2 mm, the same distance they will have in the EST ASM. In Phase 1, trade-offs have been made as to the best strategy for cooling this system and the manner in which the prototype will be tested to verify its performance has been established.
We are now in Phase 2 and about to hold the Phase 2 review meeting. The fundamental task of this phase is to design the prototype and define the test plan. In particular, the prototype shall allow verifying the requirements related to distance between actuators, thermal control for electronics, tip-tilt dynamics, and mirror deformation dynamics. Phase 2 will end in June 2024 with a design of a prototype to be built and tested during Phase 3. The contract will conclude in January 2025.