Adaptive Deformable Mirrors
The adaptive secondary concept has been introduced by Piero Salinari (Osservatorio Astrofisico di Arcetri - Florence) in 1993. The adaptive deformable mirror is currently one of the most attractive adaptive correction technologies for present and future telescopes.
From the beginning, Microgate has been one of the major contributors to the development of the "contactless" adaptive mirror concept, having conceived and realized a fully dedicated control electronics that is currently at its third generation.
In its current implementation, the "contactless" adaptive mirror is based on a continuous, thin mirror (~1.6mm) actively controlled in position and shape by a large number of force actuators (voice coil motors). During operation, the thin mirror "levitates" controlled by the electromagnetic field generated by the actuators. There is no mechanical contactbetween actuators and thin mirror. Each actuator is made by a fixed coil wound on a cold finger and a moving magnet, glued to the rear surface of the thin adaptive mirror. The actuators are fixed to a cold plate that provides cooling and mechanical support. A stiff (~50 mm thick), thermally stable structure (backplate) provides the position reference for the thin mirror.
The gap between backplate and thin mirror is measured by capacitive sensors, co-located with the actuators. Typical operating gap between backplate and deformable mirror is between 40 and 120µm. The adaptive mirror control system is based on custom-designed high speed digital and analog electronics. About 70,000 times every second, the gap of the mirror at each actuator position is read, compared with the desired gap, and a proper actuator force is computed and commanded.
The mirror position and shape is updated at a typical rate of 1 millisecond.
The adaptive mirror concept has several advantages over other conventional adaptive optics layouts for telescopes:
- For most applications, the adaptive secondary can be the only adaptive mirror in the telescope. This allows the reduction of the number of "warm" reflecting surfaces, with great benefit for IR observation, exposure time and transmission efficiency.
- High actuators stroke allows the correction of large tip-tilt aberrations and also the implementation of IR chopping
- Force actuators have no intrinsic stiffness, in case of failure of some actuators the system can be operated with minimal performance degradation
- Force actuators are hysteresis –free
- The high accuracy position sensor allow high speed monitoring of the actual wavefront correction
- The mirror can be actively "frozen" in a rigid shape and operated as an ordinary secondary