The Shack-Hartmann wavefront sensor is composed of a lenslet array generating the spot images from which local slope
is calculated and overall wavefront is measured. Generally the principle of wavefront reconstruction is that the spot
centroid of each lenslet array is calculated from pixel intensity values in its subaperture and then overall wavefront is
reconstructed by local slope of wavefront obtained by deviations from reference positions. Hence the spot image of each
lenslet array has to remain in its subaperture for exact measurement of wavefront. However the spot of each lenslet array
deviates from its subaperture area when wavefront with large local slopes enters the Shack-Hartmann sensor.
In this research, we propose the spot image searching method that finds area of each measured spot image flexibly and
determine the centroid of each spot in its area. Also the algorithms that match these centroids to their reference points
unequivocally even if some of them are situated off the allocated subaperture are proposed. Finally we verify the
proposed algorithm with the test of a defocus measurement through experimental setup for the Shack-Hartmann
wavefront sensor. It has been shown that the proposed algorithm can expand the dynamic range without additional
Adaptive optics is widely used in astronomy. Recently, most of newly built telescopes have adaptive optical systems. In
order to apply adaptive optics into astronomical telescope, high-speed wavefront sensing is required. In high-speed
operation, exposure time per frame of a sensor is quite short. In this case, optical intensity of one-frame image is pretty
low. Thus signal-to-noise ratio of the wavefront sensor is also low. Therefore, in this case, noises such as readout noise
and photon noise greatly influence the accuracy of wavefront sensing. The center of mass method is widely used due to
its low computational cost. However it is very weak to noise. The correlation method is robust to noise. But the
computational cost is expensive. In this paper, Multi-Resolution Correlation method is proposed. This method, by
employing multi-resolution images, considerably reduces the computation time when compared to the FFT correlation
method. Also the accuracy of Shack-Hartmann wavefront sensor using the proposed algorithm is proved to be almost
same as that of the conventional correlation method. The verification is done through the simulation.
Lightweight mirrors experience optical image degradation due to mechanical loadings such as self-weight, polishing pressure, and vibration. Optical surface deformation of a lightweight primary mirror is an important factor that affects optical performance. We use topology optimization to design a lightweight primary mirror under self-weight and polishing pressure. For the optimization, we used a 3-D model of the mirror and based our calculations on the rms surface error of the mirror as an objective function constrained by the maximum weight of the mirror. In the first example of topology optimization, we consider the mirror's self-weight loading. In the second example, we include the polishing pressure. We present the results of the optimized design topology for the mirror with various mass constraints. To examine the optimal design results, we manufacture a prototype of the mirror.
The performance of ground telescopes is limited by atmospheric distortion. Nowadays all large ground telescopes adopt adaptive optics for overcoming this limitation. Sending space telescopes outside the troublesome atmosphere might be a natural solution to overcome the atmospheric distortion. However, the cost of development and launch, and the size of launch fairing severally limit this option. Inflatable optics is major candidate for overcoming these technical and budget limits. In this study, we performed thickness optimizations of a membrane mirror for the mirror to be parabolic. Our optimization showed that this optimized mirror is still not good enough for visible observation. However, with limiting the effective optical surface area, the mirror was demonstrated to be used as a primary mirror in infrared bands. In addition, the wavefront errors are also shown to main contributors: piston, defocus and spherical aberrations. Adapting an adaptive secondary mirror with 19 actuators, which has been developed for ground telescopes, could remove the major wavefront errors. Therefore, combining an inflatable primary mirror and an adaptive secondary mirror can be a candidate for future large space telescopes.
The continuous thin-plate type with discrete actuators is widely used for active or adaptive mirrors in the medium size range from about 10cm up to 2m in diameters. The performance of a thin-plate deformable mirror could be characterized by the influence function of an actuator and the layout of the actuators. This paper first derives an equation which estimates influence functions of thin-plate deformable mirrors based on the analytic calculation and finite element analysis. Then the performance analysis for the case of equi-spaced actuators is presented.
In this work, multi-physics simulation software (CA/MEMS) and design-optimization software (DS/MEMS) tailored for MEMS devices are introduced. The CA/MEMS, which is a simulation engine for DS/MEMS, is a 3-D multi-physics analysis code utilizing various numerical methods such as FEM, BEM and FVM to efficiently model MEMS application problems. The current CA/MEMS includes analysis- modules for structural, thermal, electric, electromagnetic and fluidic fields and is capable of the analyses of various coupled- field problems for MEMS applications. DS/MEMS is design optimization engine for MEMS devices. With integrating CA/MEMS and pre/post processor into CAD environment, DS/MEMS is organized to work in parametric CAD platform. DS/MEMS consists of optimal design module and robust design module. The optimal design module provides users three methods nonlinear programming, Taguchi parameter design and the response surface method. The robust design module, which is specially developed for MEMS application, can be used to minimize the perturbation of performances of MEMS devices under uncertainties of MEMS devices, such as process tolerance and the change of operating environments. To verify the efficiency and accuracy of CA/MEMS and the practical usefulness of DS/MEMS, we have been comparing the simulated results of CA/MEMS with those of other commercial codes and experimental data of manufactured MEMS devices, and investigating the performances of the optimized designs through DS/MEMS.
An eigenvalue analysis of a tunable micro-mechanical actuator is presented. The actuator is modeled as a continuum structure. The eigenvalue modified by the tuning voltage is computed through the linearization of the relation between the electrostatic force and the displacement at the equilibrium. A staggered algorithm is employed to perform the coupled analysis of the electrostatic and elastic fields. The stiffness matrix of the actuator is modified at this equilibrium state. The displacement field is perturbed using an eigenmode profile of the actuator. The configuration change of the actuator due to perturbation modifies the electrostatic field and thus the electrostatic force. The equivalent stiffness matrix corresponding to the perturbation and the change in the electrostatic force is then added to stiffness matrix in order to explain natural frequency shifting. The numerical examples are presented and compared with the experiments in the literatures.