Adaptive optics (AO) systems are used to enhance the performance of optical systems. A classical AO system consists of the wavefront corrector with the wavefront sensor (WFS). Wavefront correctors are able to compensate for aberrations in real-time with the measured aberrations. Compared with traditional wavefront correctors, the major advantage of the magnetic fluid deformable mirror (MFDM) features large deformation strokes that can be easily up to more than 100μm both for the single actuator or inter-actuators. However, the measuring range of WFS is normally small, which could limit its usage in the applications with large aberrations. Considering the idea of taking full advantages of the MFDM’s stroke strengths and the limitations of the AO system with the WFS, this paper proposes a model-based wavefront sensorless control algorithm for the adaptive optics systems with magnetic fluid deformable mirror. Compared with the model-free wavefront sensorless AO systems, the model-based control algorithm for the wavefront sensorless AO systems features faster convergence without dropping into the local optima. The model-based control approach is developed based on a relationship between the second moments of the wavefront gradients and the far-field intensity distribution by taking Zernike polynomials as the predetermined bias functions, therefore, the unknown aberrations can be corrected without the wavefront measurement in the closed-loop AO control system. The control algorithm is evaluated in a wavefront sensorless AO system setup with a prototype MFDM, where a parallel laser beam with unknown aberrations is supposed to produce a focused spot on the CCD. Experimental results show that the model-based control method can effectively make the MFDM to compensate for unknown aberrations in an imaging system
Liquid mirror telescope (LMT) is a viable kind of telescope with a thin rotating mercury layer, which can generate excellent parabolic surface under the constant pull of gravity and a centrifugal acceleration. The cost of LMT is normally smaller than 1% of the cost of a traditional glass mirror telescope. However, the LMT cannot be tilted to observe larger field of sky due to the liquid property so that the field of view of LMT is very limit. In order to observe a larger field of the sky with LMT, the large off-axis aberrations must be compensated. Since the aberrations of a parabolic mirror increase rapidly with the increase of field angle, the classical correctors used in adaptive optics (AO) systems cannot correct the large off-axis aberrations of LMT when the field angles are significantly greater than 1 degree. In this paper, the magnetic fluid deformable mirror (MFDM) has been proposed as a new perspective to wavefront correction technology, which could produce a large stroke to compensate the large off-axis aberrations of LMT. The designed MFDM has a radius of 95 mm with 1141 actuators, which is able to correct the large off-axis aberrations of a 3-m f/4 LMT and permits the LMT to be operated at 10 degree off-axis observation from the zenith. The type and the order of off-axis aberrations generated by the liquid mirror telescope are first studied analytically and then the 3-m f/4 LMT operated at 10 degree off-axis observation is simulated in ZEMAX, where the off-axis aberrations and the Zernike coefficients of those aberrations are obtained from the simulation result. The required surface shape of MFDM can be calculated from the obtained Zernike coefficients of off-axis aberrations. Since the shape of the magnetic fluid surface is controlled by the combined magnetic field generated by a Maxwell coil and an array of micro-electromagnetic coils, the Maxwell coil and micro-electromagnetic coils are hence optimally designed to generate the required magnetic field. The correction performance of MFDM for the large off-axis aberrations of LMT is finally co-simulated by MATLAB, COMSOL and ZEMAX software. The simulation results show that the off-axis aberrations can be compensated with the designed MFDM and the Zernike coefficients of wavefront are substantially reduced.
Magnetic fluid deformable mirror (MFDM) is a new type of wavefront corrector that features large deformation strokes, smooth continuous mirror surface, low manufacturing cost, and easy scalability. Considering the idea of taking full advantages of the MFDM’s stroke strengths and the limitations of the adaptive optics (AO) system with the wavefront sensor, this paper proposes a MFDM based wavefront sensorless adaptive optics system. In order to make the MFDM produce desired deformation to eliminate unknown aberrations, this paper introduces the stochastic parallel gradient descent (SPGD) algorithm in the control system. Experimental results show that this SPGD algorithm can effectively control the MFDM to compensate for the unknown aberration in a parallel laser beam so that a perfect focused spot is produced on the CCD image.
In this paper, a rectangular magnetic fluid deformable mirror (MFDM) with dual-layer actuators is proposed, which is designed to improve the correction performance for full-order aberrations. The shape of the magnetic fluid surface is controlled by the combined magnetic field generated by a Maxwell coil and a two-layer array of miniature copper coils. Compared to conventional adaptive systems that use two mirrors, the proposed MFDM combines the two mirrors into one device. In order to evaluate the performance of MFDM on the correction of full-order aberrations, a decoupling control algorithm based on Zernike mode decomposition is adopted for the MFDM with dual-layer actuators. The experimental results based on a fabricated prototype MFDM show the effective correction performance of the mirror for the full-order aberrations.