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Deformable mirrors (DM) 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. They are also finding a new use in femtosecond pulse shaping.〔http://www.adaptiveoptics.org/News_0106_2.html〕 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. == Deformable mirror parameters == Number of actuators determines the number of degrees of freedom (wavefront inflections) the mirror can correct. It is very common to compare an arbitrary DM to an ideal device that can perfectly reproduce wavefront modes in the form of Zernike polynomials. For predefined statistics of aberrations a deformable mirror with M actuators can be equivalent to an ideal Zernike corrector with N (usually N < M) degrees of freedom. For correction of the atmospheric turbulence, elimination of low-order Zernike terms usually results in significant improvement of the image quality, while further correction of the higher-order terms introduces less significant improvements. For strong and rapid wavefront error fluctuations such as shocks and wake turbulence typically encountered in high-speed aerodynamic flowfields, the number of actuators, actuator pitch and stroke determine the maximum wavefront gradients that can be compensated for. Actuator pitch is the distance between actuator centers. Deformable mirrors with large actuator pitch and large number of actuators are bulky and expensive. Actuator stroke is the maximum possible actuator displacement, typically in positive or negative excursions from some central null position. Stroke typically ranges from ±1 to ±30 micrometres. Free actuator stroke limits the maximum amplitude of the corrected wavefront, while the inter-actuator stroke limits the maximum amplitude and gradients of correctable higher-order aberrations. Influence function is the characteristic shape corresponding to the mirror response to the action of a single actuator. Different types of deformable mirrors have different influence functions, moreover the influence functions can be different for different actuators of the same mirror. Influence function that covers the whole mirror surface is called a "modal" function, while localized response is called "zonal". Actuator coupling shows how much the movement of one actuator will displace its neighbors. All "modal" mirrors have large cross-coupling, which in fact is good as it secures the high quality of correction of smooth low-order optical aberrations that usually have the highest statistical weight. Response time shows how quickly the mirror will react to the control signal. Can vary from microseconds (MEMS and magnetics mirrors) to tens of seconds for thermally controlled DM's. Hysteresis and creep are nonlinear actuation effects that decrease the precision of the response of the deformable mirror. For different concepts, the hysteresis can vary from zero (electrostatically-actuated mirrors) to tens of percent for mirrors with piezoelectric actuators. Hysteresis is a residual positional error from previous actuator position commands, and limits the mirror ability to work in a feedforward mode, outside of a feedback loop. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Deformable mirror」の詳細全文を読む スポンサード リンク
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