Shengqian Wang

Laboratory demonstrations on a pyramid wavefront sensor without modulation for closed-loop adaptive optics system

The feasibility and performance of the pyramid wavefront sensor without modulation used in closed-loop adaptive optics system is investigated in this paper. The theory concepts and some simulation results are given to describe the detection trend and the linearity range of such a sensor with the aim to better understand its properties, and then a laboratory setup of the adaptive optics system based on this sensor and the liquid-crystal spatial light modulator is built. The correction results for the individual Zernike aberrations and the Kolmogorov phase screens are presented to demonstrate that the pyramid wavefront sensor without modulation can work as expected for closed-loop adaptive optics system.

Comparison between non-modulation four-sided and two-sided pyramid wavefront sensor

Based on the diffraction theory the paper analyzes non-modulation Pyramid wavefront sensor (PWFS, namely, four-sided pyramid) and two-sided pyramid wavefront sensor (TSPWFS), and expresses the detected signals as a function of the measured wavefront. The expressions of the detected signals show that non-modulation PWFS and TSPWFS hold the same properties of both slope and direct phase sensors. We compare both sensors working in slope and phase sensing by theory and numerical simulations. The results demonstrate that the performance of TSPWFS excels that of PWFS. Additionally, the influence of interference between adjacent pupils is discussed.

Testing the pyramid wavefront sensor without modulation used in the closed-loop adaptive optics system

The pyramid wavefront sensor is an innovative device with the special characteristics of variable gain and adjustable sampling in real time to enable an optimum match of the system performance, which make it an attractive option for next generation adaptive optics system compared with the Shack-Hartmann. At present most of the pyramid wavefront sensor are used with modulation based on oscillating optical component in order to give a linear measurement of the local tilt, but the PWFS without modulation would greatly simplify the optical and mechanical design of the adaptive optics system and also give highest sensitivity as expected to be achieved. In this paper we describe the optical setup of our adaptive optics system with nonmudulated pyramid wavefront sensor. In this system, the pyramid wavefront sensor with 8×8 sub-apertures in the pupil diameter has been designed, and the deformable mirror with 61 actuators based on the liquid-crystal spatial light modulator is used to introduce aberrations into the system, as well as to correct them afterwards. The closed-loop correction results of single order Zernike aberrations and the Kolmogorov turbulence phase screen are given to show that the PWFS without modulation can work as expected for closed-loop system.

Error analysis of the piston estimation method in dispersed fringe sensor

Dispersed fringe sensor (DFS) is an important phasing sensor of next-generation optical astronomical telescopes. The measurement errors induced by the measurement noise of three piston estimation methods for the DFS including least-squared fitting (LSF) method, frequency peak location (FPL) method and main peak position (MPP) method, are analyzed theoretically and validated experimentally in this paper. The experimental results coincide well with the theoretical analyses. The MPP, FPL, LSF are used respectively when the DFS operates with broadband light (central wavelength: 706 nm, bandwidth: 23 nm). The corresponding root mean square (RMS) value of estimated piston error can be achieved to be 1 nm, 3 nm, 26 nm, respectively. Additionally, the range of DFS with the FPL can be more than 100 μm at the same time. The FPL method can work well both in coarse and fine phasing stages with acceptable accuracy, compared with LSF method and MPP method.

Astronomical AO in Key Laboratory of Adaptive Optics, Chinese Academy of Sciences

The AO progresses for astronomy in the Key Laboratory of Adaptive Optics, Chinese Academy of Sciences are reported in this presentation. For night-time astronomical observations, the recent AO technological developments, such as Laser Guide Star, Pyramid Sensor and Deformable Secondary Mirror, are introduced. The solar AO researches are also presented for day-time astronomical observations. Furthermore, we will show the on-sky high resolution observational results in the 1.8m telescope at Gaomeigu site, Yunnan Observatory and the 1-m New Vacuum Solar Telescope (NVST) at Fuxian Lake Solar Observatory respectively.

The pyramid wavefront sensor used in the closed-loop adaptive optics system

Pyramid wavefront sensor is a promising sensor technology based on the beam splitting in the focal plane. Due to its advantages of adjustable gain and variable spatial sampling, the pyramid wavefront sensor has been successfully applied in many large telescopes. In recent years, we have carried out the related research of this sensor. Firstly we studied the adaptive optical closed-loop system based on the liquid crystal spatial light modulator and the pyramid wavefront sensor. Subsequently, the adaptive optical system based on the pyramid wavefront sensor and the deformable mirror is studied in our lab. Currently the experiment on the 1.8-m telescope at Yunnan observatory has been successfully carried out and the high resolution images of the natural stellar star have been obtained. The experiment results are present in this paper.

Study on application of spectral filter in detecting stars in daytime

The limitation of star detection in daytime is poor contrast of image. Compare with the strong sky background light, star signal is so weak that can’t be detected. There are some solutions to find star signal out from strong sky background, spectral filter is selected in this article. According to the different spectrum distributions of stars and sky background, different dead wavelengths is chosen to reduce the sky background light and increase the contrast between star object and sky background. Different kinds of spectral filters were placed between adaptive optical system and secondary mirror of 1.8m telescope in Lijiang observation station. The RMS value of wavefront correction residual error and the FWHM value of far-field light spot are chosen to evaluate the correction capability of AO system. Applying spectral filter, the RMS value is improved from 0.25λ to 0.17λ and the FWHM value is improved from 0.46 arc sec to 0.34 arc sec. Both of the evaluation criterions show that spectral filter is effective for detecting stars in daytime. Furthermore, it also makes it possible to apply our adaptive optics system in daytime in future.

Analysis on measured signal retrieval approaches in non-modulation pyramid wavefront sensor

Pyramid wavefront sensor (PWFS) without modulation is prevailing over one with modulation. So far how to describe measured signals of non-modulation PWFS needs deeply research. In this paper, the theory of the non-modulation PWFS is briefly presented according to wave optics. This paper analyses the existing four approaches in theory. By numerical simulation this paper further verifies the performance of four approaches under the experiment condition. The result shows that the approach with total intensity of pixels conjugate to the same spot in the pupil as signal denominator is the best choice for the non-modulation PWFS in closed-loop correction.

Application of a white-light interferometric measuring system as co-phasing the segmented primary mirrors of the high-aperture telescope

For the optical system of the telescope, with the increase in telescope size, the manufacture of monolithic primary becomes increasingly difficult. Instead, the use of segmented mirrors, where many individual mirrors (the segments) work together to provide an image quality and an aperture equivalent to that of a large monolithic mirror, is considered a more appropriate strategy. But with the introduction of the large telescope mirror comprised of many individual segments, the problem of insuring a smooth continuous mirror surface (co-phased mirrors) becomes critical. One of the main problems is the measurement of the vertical displacement between the individual segments (piston error), for such mirrors, the segment vertical misalignment (piston error) between the segments must be reduced to a small fraction of the wavelength (<100nm) of the incoming light. The measurements become especially complicated when the piston error is in order of wavelength fractions. To meet the performance capabilities, a novel method for phasing the segmented mirrors optics system is described. The phasing method is based on a high-aperture Michelson interferometer. The use of an interferometric technique allows the measuring of segment misalignment during the daytime with high accuracy, which is a major design guideline. The innovation introduced in the optical design of the interferometer is the simultaneous use of monochromatic light and multiwavelength combination white-light source in a direct method for improving the central fringe identification in the white-light interferometric phasing system. With theoretic analysis, we find that this multiwavelength combination technique can greatly increase the visibility difference between the central fringe and its adjacent side fringes, and thus it offers an increased signal resolution. So make the central fringe identification become easier, and enhance the measure precision of the segment phasing error. Consequently, it is suitable for high-precision measurement purpose and application in the segment piston error phasing system. The description about the expected interferograms and the feasibility of the phasing method are presented here.

Design of a white-light interferometric measuring system for co-phasing the primary mirror segments of the next generation of ground-based telescope

With the increase of telescope size, the manufacture of monolithic primaries becomes increasingly difficult. Instead, the use of segmented mirrors, where many individual mirrors (the segments) work together to provide good image quality and an aperture equivalent to that of a large monolithic mirror, is considered a more appropriate strategy. But, with the introduction of large telescope mirror comprised of many individual segments, the problem of insuring a smooth continuous mirror surface (co-phased mirrors) becomes critical. One of the main problems arising in the co-phasing of the segmented mirrors telescope is the problem of measurements of the vertical displacements between the individual segments (piston errors). Because of such mirrors to exhibit diffraction-limited performance, a phasing process is required in order to guarantee that the segments have to be positioned with an accuracy of a fraction of a wavelength of the incoming light.The measurements become especially complicated when the piston error is in order of wavelength fractions. To meet the performance capabilities, a novel method for phasing the segmented mirrors optical system is described. The phasing method is based on a high-aperture Michelson interferometer. The use of an interferometric technique allows the measurement of segment misalignment during daytime with high accuracy, which is a major design guideline. The innovation introduced in the optical design of the interferometer is the simultaneous use of both monochromatic and white-light sources that allows the system to measure the piston error with an uncertainty of 6nm in 50µm range. The description about the expected monochromatic and white-light illumination interferograms and the feasibility of the phasing method are presented here.