page_title: Deformable mirror
section_title: Deformable mirror concepts
hierarchical_section_title: Deformable mirror / Deformable mirror concepts
caption_attribution_description: English: Picture of a ferrofluid deformable mirror
context_page_description: Deformable mirrors are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics. In 2006 they found a new use in femtosecond pulse shaping.
  The shape of a DM can be controlled with a speed that is appropriate for compensation of dynamic aberrations present in the optical system. In practice the DM shape should be changed much faster than the process to be corrected, as the correction process, even for a static aberration, may take several iterations.
  A DM usually has many degrees of freedom. Typically, these degrees of freedom are associated with the mechanical actuators and it can be roughly taken that one actuator corresponds to one degree of freedom.
context_section_description: Segmented concept mirrors are formed by independent flat mirror segments. Each segment can move a small distance back and forth to approximate the average value of the wavefront over the patch area. Advantageously, these mirrors have little or zero cross-talk between actuators. Stepwise approximation works poorly for smooth continuous wavefronts. Sharp edges of the segments and gaps between the segments contribute to light scattering, limiting the applications to those not sensitive to scattered light. Considerable improvement of the performance of the segmented mirror can be achieved by introduction of three degrees of freedom per segment: piston, tip and tilt. These mirrors require three times as many actuators compared to piston segmented mirrors. This concept was used for fabrication of large segmented primary mirrors for the Keck telescopes, JWST, and the future E-ELT. Numerous methods exist to accurately co-phase the segments and reduce the diffraction patterns introduced by the segment shapes and gaps. Future large space-based telescopes, such as the NASA Large UV Optical Infrared Surveyor will also possess a segmented primary mirror. The development of robust methods to increase the contrast is key for the direct imaging and characterization of exoplanets.
  Continuous faceplate concept mirrors with discrete actuators are formed by the front surface of a thin deformable membrane. The shape of the plate is controlled by a number of discrete actuators that are fixed to its back side. The shape of the mirror depends on the combination of forces applied to the faceplate, boundary conditions (the way the plate is fixed to the mirror) and the geometry and the material of the plate. These mirrors allow smooth wavefront control with very large - up to several thousands - degrees of freedom.
  Magnetics concept mirrors are based on continuous reflective surface motioned by magnetics actuators. They feature large strokes, linearity and fast settling time.
  MEMS concept mirrors are fabricated using bulk and surface micromachining technologies. They consist of a thin reflective membrane controlled by a multitude of actuators. MEMS mirrors could break the high price threshold of conventional adaptive optics. They enable a higher actuator count at a more cost effective price allowing for accurate wave-front correction. MEMS mirrors offer fast response times from the actuators with limited hysteresis. An additional benefit is that micromachining technologies allow for the benefit of economies of scale to create cheaper and lighter deformable mirrors with a greater number of actuators.
  Membrane concept mirrors are formed by a thin conductive and reflective membrane stretched over a solid flat frame. The membrane can be deformed electrostatically by applying control voltages to electrostatic electrode actuators that can be positioned under or over the membrane. If there are any electrodes positioned over the membrane, they are transparent. It is possible to operate the mirror with only one group of electrodes positioned under the mirror. In this case a bias voltage is applied to all electrodes, to make the membrane initially spherical. The membrane can move back and forth with respect to the reference sphere.
  Bimorph concept mirrors are formed by two or more layers of different materials. One or more of (active) layers are fabricated from a piezoelectric or electrostrictive material. Electrode structure is patterned on the active layer to facilitate local response. The mirror is deformed when a voltage is applied to one or more of its electrodes, causing them to extend laterally, which results in local mirror curvature. Bimorph mirrors are rarely made with more than 100 electrodes.
  Ferrofluid concept mirrors are liquid deformable mirrors made with a suspension of small (about 10 nm in diameter) ferromagnetic nanoparticles dispersed in a liquid carrier. In the presence of an external magnetic field, the ferromagnetic particles align with the field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibri
caption: A ferrofluid deformable mirror
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