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The ESO’s ELT M4 adaptive mirror is based on the large, contactless voice-coil adaptive mirror technology, with 5352 actuators controlling the shape of the six mirror segments. The system will dissipate up to 8 kW, which are removed by a direct gas expansion cooling system, specifically designed for the application. The key advantage of such solution is related to the fact that any possible leakage from the cooling system would not cause damages to the optics and electronics of the M4 only, but also potentially to the ones of M1, M3 and M5; with AdOptica’s system, the gas spillage would cause a local evaporation with consequent temperature drop, without any further consequences.
The whole cooling system, including the evaporator embedded in the M4 and the dedicated compressor unit, have been completely assembled. To facilitate the testing and calibration, we designed and built a dedicated large evaporator test unit, called Heat Load Emulator: such device allowed testing and tuning the whole cooling system over the full functional power and environmental ranges, well before the integrated M4 became available, without putting at risk the precious M4 embedded control system.
In this work we present the overall cooling system, the dedicated test setup and performance tests results.
Aim of this paper is to present the latest developments on the main issues related to the fabrication of a breadboard, covering two project critical areas identified during the preliminary studies: the design and performances of the long-stroke actuators used to implement the mirror active control and the mirror survivability to launch via Electrostatic Locking (EL) between mirror and backplane. The described work is developed under the ESA/ESTEC contract No. 22321/09/NL/RA.
The lightweight mirror is structured as a central sector surrounded by petals, all of them actively controlled to reach the specified shape after initial deployment and then maintained within specs for the entire mission duration. The presented study concerns: a) testing the Carbon Fiber Reinforced Plastic (CFRP) backplane manufacturing and EL techniques, with production of suitable specimens; b) actuator design optimisation; c) design of the deployment mechanism including a high precision latch; d) the fabrication of thin mirrors mock-ups to validate the fabrication procedure for the large shells.
The current activity aims to the construction of an optical breadboard capable of demonstrating the achievement of all these coupled critical aspects: optical quality of the thin shell mirror surface, actuators performances and back-plane - EL subsystem functionality.
1) control accuracy in the mirror surface shaping. 2) mirror survivability to launch.
The aim is to evaluate the effective performances of the long stroke smart-actuators used for the mirror control and to demonstrate the effectiveness and the reliability of the electrostatic locking (EL) system to restraint the thin shell on the mirror backup structure during launch. The paper presents a comprehensive vision of the breadboard focusing on how the requirements have driven the design of the whole system and of the various subsystems. The manufacturing process of the thin shell is also presented.
The unit has entered the final design and construction phase in July 2015, after an advanced preliminary design. The final design review is planned for fall 2017; thereafter, the unit will enter the construction and test phase. Acceptance in Europe after full optical calibration is planned for 2022, while the delivery to Cerro Armazones will occur in 2023.
Even if the fundamental concept has remained unchanged with respect to the other contactless large deformable mirrors, the specific requirements of the E-ELT unit posed new design challenges that required very peculiar solutions. Therefore, a significant part of the design phase has been focused on the validation of the new aspects, based on analysis, numerical simulations and experimental tests. Several experimental tests have been executed on the Demonstration Prototype, which is the 222 actuators prototype developed in the frame of the advanced preliminary design. We present the main project phases, the current design status and the most relevant results achieved by the validation tests.
The deformable secondary mirror of VLT: final electro-mechanical and optical acceptance test results
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