Voice-coil technology for the E-ELT M4 Adaptive Unit

Abstract

Abstract. We present our design of the E-ELT M4 Adaptive Unit based on voice-coil driven deformable mirror technology. This technology was developed by INAF-Arcetri, Microgate and ADS team in the past 15 years and it has been adopted by a number of large ground based telescopes as the MMT, LBT, Magellan and lastly the VLT in the frame of the Adaptive Telescope Facility project. Our design is based on contactless force actuators made by permanent magnets glued on the back of the deformable mirror and coils mounted on a stiff reference structure. We use capacitive sensors to close a position loop co-located with each actuator. Dedicated high performance parallel processors are used to implement the local de-centralized control at actuator level and a centralized feed-forward computation of all the actuators forces. This allowed achieving in our previous systems dynamic performances well in line with the requirements of the M4 Adaptive Unit (M4AU) case. The actuator density of our design is in the order of 30-mm spacing for a figure of about 6000 actuators on the M4AU and it allows fulfilling the fitting error and corrections requirements of the E-ELT high order DM. Moreover, our contact-less technology makes the Deformable Mirror tolerant to up 5% actuators failures without spoiling system capability to reach its specified performances, besides allowing large mechanical tolerances between the reference structure and the deformable mirror. Finally, we present the Demonstration Prototype we are building in the frame of the M4AU Phase B study to measure the optical dynamical performances predicted by our design. Such a prototype will be fully representative of the M4AU features, in particular it will address the controllability of two adjacent segments of the 2-mm thick mirror and implement the actuators "brick" modular concept that has been adopted to dramatically improve the maintainability of the final unit.

Self-Tuning Mechanism for the Design of Adaptive Secondary Mirror Position Control

“…The decentralized component offers the definite advantage of scalability. Such a feature makes this solution well suited also for the next generation of telescopes, such as the GMT and the >20-m-class extremely large telescopes [17]- [19] that will require a very large number of actuators. The feedforward contribution is instead adopted so as to guarantee a satisfactory steady-state error to step inputs.…”

Modeling and Control of Deformable Membrane Mirrors

“…However, the computational complexity for high order spatial control of the deformable element typically exceeds available computing power. For this reason, existing deformable membrane mirrors for large telescopes incorporate local feedback instead of global feedback control and neglect some of the global dynamics of the deformable mirror [10,13,14,[46][47][48]. As a side effect, dynamic coupling of separately controlled actuators through the deformable membrane can lead to instability of the individually stable loops and draws the need for carefully designing the control parameters of the local feedback loops.…”

Progress Towards Wide-Field Adaptive Optics for Future Extremely Large Telescopes

“…The LBT adaptive secondary [21] can be considered a prototype for the GMT design, and design and prototyping work is also underway for the E-ELT adaptive mirrors [20,22]. Advances have also been made towards order 64x64 MEMS DMs with adequate dynamic range and predictable open-loop behavior [23].…”