Optical sparse aperture imaging systems require a very accurate control of the phase errors between sub-apertures to achieve high-resolution. The linearized analytic phase diversity (LAPD) is a fast technique to co-phase sparse aperture imaging systems. However, it requires focal plane and out-of-focus images to achieve phase diversity, so the light path is complex. In this Letter, we propose a spatially modulated analytic phase diversity algorithm. The diversity images are obtained by the spatial modulation diversity technology (SMDT). The validity of the method was verified by simulation experiments. The method can typically detect piston errors within |π/2| rad rms in the extended target scenes. Co-phase accuracy can be achieved by iterating up to λ/60. The method does not require additional splitting device, which is conducive to system miniaturization, while the required calculations are simple and the piston detection is fast. This method can give a simple, fast and low system complexity solution for fine phasing of sparse aperture imaging systems under any imaging scenes.
In the beam control system, the correction of optical axis is an essential process of the optical system. Adjusting the installation angle of the relay mirror is an effective way to change the optical axis direction. However, due to the forced deviation of the installation angle from the theoretical design value, the supporting structure will produce greater stress, which will affect the surface accuracy of the mirror. Therefore, we present an optimization method that can reduce the sensitivity of mirror surface. First, the surface sensitivity is introduced to measure the surface accuracy under the forced deflection angle. Second, the external response function of surface sensitivity is realized by combining dynamic link library and MATLAB. At last, the topological structure is optimized with the sum of the sensitivity weights of two cases as the objective function and the dynamic resonance frequency as the constraint. The optimized structure shows that the surface sensitivity around the x axis decreases from 2.58 nm / ± 1 ″ to 0.32 nm / ± 1 ″ , and the surface sensitivity around the y axis decreases from 3.75 nm / ± 1 ″ to 0.09 nm / ± 1 ″ .
The thermal induced effect errors including the surface distortion of heated mirror and micro-thermal turbulence fluctuation at the optical surface dramatically degrade the image quality of the telescope. To address the problem, we have proposed an air-knife system consisting of an annular flushing subsystem and a central sucking subsystem and reported its simulation analysis. This paper presents the detailed experimental performance of the air-knife thermal control system. The scaling experiment is conducted in a thermo-cycling experiment room with different environmental conditions, where the temperature fluctuations and wavefront perturbation of the scaling mirror can be accurately measured. It is shown from the experimental results that the approximately laminar forced air flow at the optical surface does blow away the turbulence fluctuation and not induce novel low order wavefront aberrations. Meanwhile, the air knife system contributes to the stability of the thermal boundary layer and enhances the convective heat exchange between mirror and air around. As a result, the air-knife system significantly decreases the surface-to-air temperature difference and improves the image quality with a thermal response. Furthermore, it is found that thermal control efficiency is less significant with the increase of the air intake flow or the decrease of the surface-to-air temperature difference. The scaling experiment results demonstrate the practicability of the air-knife thermal control system for large-aperture primary mirror.
The resolution of an optical imaging system is often limited by various phase aberrations. To get the joint estimation of an object and aberrations of an imaging system with perturbations, a defocused spatial diversity technology with aperture scanning was proposed and evaluated. This technology creates spatial diversity images by scanning an aperture in the defocused plane of the aberrated imaging system. Based on these diversity images, the stochastic parallel gradient descent algorithm was used to recover the phase aberration of the imaging system by adaptively optimizing the coefficients of Zernike polynomials. Then the near-diffraction-limited image of the object can be restored using a multiframe Winner–Helstrom filter. Numerical simulations performed with different phase distributions validated the technology. The technology proposed may be widely used in aberrated imaging systems for aberration detection and image restoration.
KEYWORDS: Mirrors, Telescopes, Optical instrument design, Space telescopes, Active optics, Optical components, Control systems, Actuators, Fluctuations and noise, Cooling systems
For a better understanding and forecasting of the universe, the high resolution observations are needed. The largeaperture telescope is an integrated success with a combination of material, mechanics, optics and electronics. The telescope is a classic Cassegrain configuration with open structure, alt-azimuth mount, and retractable dome. The instrumentation has a rotating mass of approximately 52 tons and stands over 9 m tall. The 3-m aperture primary mirror is a honeycomb lightweighted mirror with fused silica material and active cooling. The paper will address preliminary design and development of the telescope mount structure, axes drive system, encoder mount and primary mirror system. The structure must have the best performance of stiffness and stability to demand an acceptable image quality. As the largest optical element of the telescope, primary mirror must be well controlled and protected both during operational and non-operational periods. An active cooling system of primary mirror is provided by a flushing subsystem at the front side and sucking subsystem on the central hole to keep the temperature of the facesheet close to that of ambient air. A two-layer mirror cover mounted on the elevation ring is proposed to protect the optical elements and inner beam tube from dust, dirt and debris. Furthermore, the latest plans for future upgrades will be also described.
We propose and demonstrate an coherent imaging technique by using defocus grating. For the imaging system, the defocus grating combined with lens with short focal length is used to realize multiplane imaging on the lens' focal plane, simultaneously. Based on these multiplane images, Gerchberg-Saxton (GS) algorithm is used to reconstruct the complex amplitude distribution of the input imaging beam. By using computational imaging and digital wavefront distortion correction with stochastic parallel gradient descent (SPGD) algorithm, this technology can be used for joint estimation of both pupil aberrations and an high resolution image of the object, successfully.
The sparse-optical-synthetic-aperture systems enlarge the aperture and increase the spatial resolution of telescope system via several sub-apertures distributed in specific way. The difficulty of its realization lies in detecting and correcting co-phase errors of the sub-apertures. This paper proposed the method of multi-spectral modulation detection of co-phasing errors for sparse-optical-synthetic-aperture systems. The method can detect the errors via phase modulation on a sub-aperture in the situation of different wavelengths. Firstly, this paper introduced the theory and implementation process of the method in detail. Then the paper analyzed the detection performance of the method and the influence of the sub-apertures structure on detection performance based on a three-sub-aperture system. These results show that the method can accurately detect the sub-apertures' co-phasing errors of the sparse-optical-synthetic-aperture systems. Compared with the current methods, the method proposed in this paper has many advantages, such as faster detection speed and wider detection range.
In order to improve the dynamic stiffness of telescope mount, the accuracy of aiming and stability of optical system, a topology optimization method based on the theory of variable density and taking maximum stiffness as objective function is studied. In the topology optimization analysis of elevation ring, one of the most important members of the telescope mount, two kinds of structural are designed: one is a traditional plate welding structure and the other is a combination of plate welding and truss welding. Furthermore, the stiffness and modal performance of the elevation ring in different performances are analyzed and compared. The results show that in meeting the strength and stiffness of the premise, the mass of elevation ring with plate welding and truss welding is 7.00T and the moment of inertia is 11.94 t•m2. What`s more, the total deformation in the horizontal direction and the zenith direction are 6.70μm and 55.86μm, respectively; the stress is within stress range of material's promise; the modal is 105.9Hz.Compared with the traditional structural with plate welding, this new structural design approached to ensure the dynamic stiffness while effectively reducing its own weight with reduction rate 10.7% and moments of inertia with reduction rate 12.3%. This new structural of plate welding and truss welding has obvious advantages in lightweight design. This new design method based on topology optimization will provide efficient help to later components design of the telescope mount.
We propose and demonstrate the adaptive conversion of input beam with unsymmetrical intensity distribution into a near-diffraction-limited flattop beam with desired parameters in the near field based on a combination of dual phase-only liquid crystal spatial light modulators (LC-SLMs) and the stochastic parallel gradient descent (SPGD) algorithm. One phase-only LC-SLM redistributes the intensity of the input beam to the desired distributions at the other phase-only LCSLM plane, and the other phase-only LC-SLM compensates the wavefront of the output beam. The SPGD algorithm adaptively optimizes the phase distributions of dual phase-only LC-SLMs to reduce the variance between the actual beam shape and the target beam shape. The experimental results on a prefabricated unsymmetrical input beam show that the technique is capable of adaptively creating near-diffraction-limited flattop beams with desired parameters.
KEYWORDS: Coherence imaging, Imaging systems, Super resolution, Diffraction, Image resolution, James Webb Space Telescope, Telescopes, Astronomy, Space telescopes, Numerical simulations
The high resolution is what the synthetic aperture technique quests for. In this paper, we propose an approach of coherent synthetic imaging with sparse aperture systems using multi-aperture scanning Fourier ptychography algorithm, which can further improve the resolution of sparse aperture systems. The reported technique first acquires a series of raw images by scanning a sparse aperture system and then the captured images are used to synthesize a larger spectrum in the frequency domain using aperture-scanning Fourier ptychography algorithm. The system’s traveling circumvent its diffraction limit so that a super-resolution image can be obtained. Numerical simulation demonstrates the validity. The technique proposed in this paper may find wide applications in synthetic aperture imaging and astronomy.
The incoherent digital holography makes it possible to record holograms under incoherent illumination, which lowers requirement for the coherence of light sources and results in expanding its application to white-light and fluorescence illuminating circumstances. The Fresnel Incoherent Correlation Holography (FINCH) technology achieves diverging the incident beam and shifting phase by mounting phase masks on the phase modulator. Then it obtains holograms with phase difference and reconstructs the image. In this paper, we explain the principles of the FINCH technology, and introduce the n-step phase-shifting method which is utilized to eliminate the twin image and bias term in holograms. During the research, we studied what impact the term n may have on imaging performance, compared imaging performances when different phase masks are mounted on SLM, and established simulation system on imaging with which imaging performances are deeply inspected. At last, it is shown in the research that the FINCH technology could record holograms of objects, from which clear images could be reconstructed digitally.
We present a coherent approach of complex amplitude reconstruction, termed aperture-resizing Fourier ptychography (FP). The reported approach resizes the pupil size of an imaging system and captures the corresponding intensity images. The acquired images are then synthesized in the frequency domain via iteration computation to reconstruct a complex sample wavefront without known phase information and interferential optics. Like holography, it is capable of reconstructing all information of the object. The reported aperture-resizing FP may find wide applications in 3D refocusing, 3D object tracking, remote sensing and microscopy.
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