Imaging performance of microscopy adaptive-optics system using scene-based wavefront sensing

The paper deals with with the on-sky performance of the pyramid wavefront sensor-based Adaptive Optics (AO) systems. These wavefront sensors are of great importance, being used in all first light AO systems of the ELTs (E-ELT, GMT, and TMT), currently in design phase. In particular, non-common path aberrations (NCPAs) are a critical issue encountered when using an AO system to produce corrected images in an associated astronomical instrument. The AO wavefront sensor (WFS) and the supported scientific instrument typically use a series of different optical elements, thus experiencing different aberrations. The usual way to correct for such NCPAs is to introduce a static offset in the WFS signals. In this way, when the AO loop is closed the sensor offsets are zeroed and the deformable mirror converges to the shape required to null the NCPA. The method assumes that the WFS operation is linear and completely described by some pre-calibrated interaction matrix. This is not the case for some frequently used wavefront sensors like the Pyramid sensor or a quad-cell Shack-Hartmann sensor. Here we present a method to work in closed-loop with a pyramid wavefront sensor, or more generally a non-linear WFS, introducing a wavefront offset that remains stable when AO correction quality changes due to variations in external conditions like star brightness, seeing, and wind speed. The paper details the methods with analytical and numerical considerations. Then we present results of tests executed at the LBT telescope, in daytime and on sky, using the FLAO system and LUCI2 facility instrument. The on-sky results clearly show the successful operation of the method that completely nulls NCPA, recovering diffraction-limited images with about 70% Strehl ratio in H band in variable seeing conditions. The proposed method is suitable for application to the above-mentioned ELT AO systems.

This paper presents the design of an Adaptive Optics (AO) system for retinal imaging applications. The development of retinal imaging systems allows for early diagnosis of eye diseases. Such systems can increase the quality of life of patients as well as curtail increasing health care costs through early eye disease detection and treatment. Until recently, AO systems have been prohibitively expensive and cumbersome. This has been mainly due to the size and cost of flexible membrane mirrors normally used as the aberration correction device. Recent developments in the technology of Microelectromechanical System (MEMS) based actuators allow the implementation of AO systems which would have been difficult to implement a few years ago due to exorbitant costs. The aim of this paper is to present the design of a compact and flexible low cost AO system using off the shelf components to measure and compensate for the aberrations of the eye. The design is based around the system’s main components which include a 52 channel magnetically actuated deformable membrane mirror, a Shack Hartmann wavefront sensor and a control system which runs on a single processor personal computer. All the components are commercially available. The use of the MEMS-based magnetically actuated mirror allows for increased resolution and force compared to conventional membrane mirrors designed mainly for use in astronomical applications. The performance of the closed-loop system is evaluated through experiments. Although designed as a diagnostic tool for eye diseases, such a system will find a number of applications in basic research in the visual sciences, including the study of microscopic structures in the living retina that could not be seen before. Optometrists, retinal surgeons, and ophthalmologists will also benefit from using such a system, through potential improvements on commonly used instruments such as phoropters and fundus cameras.

For performance evaluation of an adaptive optics (AO) system, the probability of the system residual wavefront variance can provide more information than the wavefront variance average. By studying the Zernike coefficients of an AO system residual wavefront, we derived the exact expressions for the probability density functions of the wavefront variance and the Strehl ratio, for instantaneous and long-term exposures owing to the insufficient control loop bandwidth of the AO system. Our calculations agree with the residual wavefront data of a closed loop AO system. Using these functions, we investigated the relationship between the AO system bandwidth and the distribution of the residual wavefront variance. Additionally, we analyzed the availability of an AO system for evaluating the AO performance. These results will assist in designing and probabilistic analysis of AO systems.

An Adaptive Optic (AO) system is a fundamental requirement of 8m-class telescopes. We know that in order to obtain the maximum possible resolution allowed by these telescopes we need to correct the atmospheric turbulence. Thanks to adaptive optic systems we are able to use all the effective potential of these instruments, drawing all the information from the universe sources as best as possible. In an AO system there are two main components: the wavefront sensor (WFS) that is able to measure the aberrations on the incoming wavefront in the telescope, and the deformable mirror (DM) that is able to assume a shape opposite to the one measured by the sensor. The two subsystem are connected by the reconstructor (REC). In order to do this, the REC requires a “common language" between these two main AO components. It means that it needs a mapping between the sensor-space and the mirror-space, called an interaction matrix (IM). Therefore, in order to operate correctly, an AO system has a main requirement: the measure of an IM in order to obtain a calibration of the whole AO system. The IM measurement is a 'mile stone' for an AO system and must be done regardless of the telescope size or class. Usually, this calibration step is done adding to the telescope system an auxiliary artificial source of light (i.e a fiber) that illuminates both the deformable mirror and the sensor, permitting the calibration of the AO system. For large telescope (more than 8m, like Extremely Large Telescopes, ELTs) the fiber based IM measurement requires challenging optical setups that in some cases are also impractical to build. In these cases, new techniques to measure the IM are needed. In this PhD work we want to check the possibility of a different method of calibration that can be applied directly on sky, at the telescope, without any auxiliary source. Such a technique can be used to calibrate AO system on a telescope of any size. We want to test the new calibration technique, called “sinusoidal modulation technique”, on the Large Binocular Telescope (LBT) AO system, which is already a complete AO system with the two main components: a secondary deformable mirror with by 672 actuators, and a pyramid wavefront sensor. My first phase of PhD work was helping to implement the WFS board (containing the pyramid sensor and all the auxiliary optical components) working both optical alignments and tests of some optical components. Thanks to the “solar tower” facility of the Astrophysical Observatory of Arcetri (Firenze), we have been able to reproduce an environment very similar to the telescope one, testing the main LBT AO components: the pyramid sensor and the secondary deformable mirror. Thanks to this the second phase of my PhD thesis: the measure of IM applying the sinusoidal modulation technique. At first we have measured the IM using a fiber auxiliary source to calibrate the system, without any kind of disturbance injected. After that, we have tried to use this calibration technique in order to measure the IM directly “on sky”, so adding an atmospheric disturbance to the AO system. The results obtained in this PhD work measuring the IM directly in the Arcetri solar tower system are crucial for the future development: the possibility of the acquisition of IM directly on sky means that we are able to calibrate an AO system also for extremely large telescope class where classic IM measurements technique are problematic and, sometimes, impossible. Finally we have not to forget the reason why we need this: the main aim is to observe the universe. Thanks to these new big class of telescopes and only using their full capabilities, we will be able to increase our knowledge of the universe objects observed, because we will be able to resolve more detailed characteristics, discovering, analyzing and understanding the behavior of the universe components.

For a non-cooperative target, a laser beacon is created by illuminating the target with a beacon beam. When a beacon beam propagates though deep turbulence, turbulence spreads the beam. A conventional phase conjugate adaptive optics (AO) system is not efficient in the presence of Beacon Anisoplanatism when the beacon beam spot size at the target includes many isoplanatic patch sizes. We introduce a concept of the wavefront-based stochastic parallel gradient decent (WSPGD) AO system, which uses an off-axis wavefront sensor to provide feedback for the beam control algorithm. This concept is based on the finding that the phase aberrations of laser return from the target contain information about beam spot size at the target, and that correction of a limited number of low-order Zernike modes increases on-axis intensity and power in the bucket at the target. We evaluated the WSPGD AO system performance in simulation for two tactical engagement scenarios in the presence of strong turbulence. We found that that the WSPGD AO system can efficiently compensate the effects of strong turbulence including Beacon Anisoplanatism, even when the beam spot size at the target includes up to 20 isoplanatic patch sizes and the isoplanatic angle is by a factor of 2.6 less than the diffraction limit. The Strehl ratio gain for this scenario is 1.6 - 2.5, and the maximum Strehl ratio is achieved after 15-20 iterations. A laboratory demonstration performed under a separate program confirmed our theoretical predictions.

Improved Adaptive Optics (AO) systems continue to revolutionize ground-based astronomy. Key to understanding a new AO systems capability is a quantitative measurement of performance, such as the Strehl Ratio (SR). At the Large Binocular Telescope (LBT) we have undertaken a program to monitor the performance of our AO system in regular use for science operations. Input to our analysis includes data taken specifically for this purpose during twilight and on engineering nights. We report our findings thus far and in particular discuss the challenges of collecting consistent data sets in twilight, the methods we used to overcome those challenges, and preliminary results from data collected so far.

We discuss the chosen concepts, detailed design, implementation and calibration of the 85-electrode adaptive optics (AO) system of the Swedish 1-meter Solar Telescope (SST), which was installed in 2013. The AO system is unusual in that it uses a combination of a monomorph mirror with a Shack-Hartmann (SH) wavefront sensor (WFS) and a second high-resolution SH microlens array to aid the characterization, calibration, and modal control of the deformable mirror. An Intel PC workstation performs the heavy image processing associated with cross-correlations and real-time control at a 2 kHz update rate with very low latency. The computer and software continue the successful implementation since 1995 of earlier generations of correlation tracker and AO systems at SST and its predecessor, the 50-cm Swedish Vacuum Solar Telescope, by relying entirely on work-station technology and an extremely efficient algorithm for implementing cross-correlations with the large field of view of the WFS. We describe critical aspects of the design, calibrations, software, and functioning of the AO system. The exceptionally high performance is testified through the highest Strehl ratio (inferred from the measured granulation contrast) of existing meter-class solar telescopes, as demonstrated here at wavelengths shorter than 400 nm and discussed in more detail in a previous separate publication We expect that some aspects of this AO system may also be of interest outside the solar community.

Working on low-contrast, extended, time-varying objects such as the solar granulation, solar adaptive optics (AO) system uses correlation algorithms to detect image shift of the Shack-Hartmann (SH) wavefront sensor (WFS) instead of centroid algorithm in night-time adaptive optics system. An real-time image shift detection processor, which consists of a Xilinx FPGA and a TI DSP, has been developed for a low-order solar AO system based on cross correlation coefficient algorithm. Image shift of integer pixels can be calculated in the FPGA and DSP is responsible for parabolic interpolation to obtain subpixel accuracy. The experimental results show that the processor can obtain correct image shift and satisfy the time latency requirement of the AO system.

In this paper we discuss the performance of low order adaptive optics (AO) systems. We present improved calculations of Strehl ratio achievable at various D/r<SUB>o</SUB> ratios for AO systems up to 7<SUP>th</SUP> order. Additionally we present calculations concerning the throughput of low order AO systems used in conjunction with spectrometers. We then show the results of a detailed simulation of the expected interaction of the University of Hawaii prototype AO system with spectrometers through the V,I,J,H,K optical bands. This simulation shows that AO spectroscopy performance should always be acceptable in the near IR (J,H,K), and often in the visible (V,I). We show the results of our initial efforts at deconvolving the AO PSF. These results indicate that deconvolution is likely to be quite widely applicable to AO images. Finally we discuss how it might be possible to extend the applicability and accuracy of deconvolution using the AO drive and wavefront error signals. The following results are for on-axis system, the effects of atmospheric anisoplanitism have not been addressed in this paper.

It is well known that using two or more deformable mirrors (DMs) can improve the compensation ability of an adaptive optics (AO) system. However, to keep the stability of an AO system, the correlation between the multiple DMs must be suppressed during the correction. In this paper, we proposed a slope-based decoupling algorithm to simultaneous control the multiple DMs. In order to examine the validity and practicality of this algorithm, a typical woofer-tweeter (W-T) AO system was set up. For the W-T system, a theory model was simulated and the results indicated in theory that the algorithm we presented can selectively make woofer and tweeter correct different spatial frequency aberration and suppress the cross coupling between the dual DMs. At the same time, the experimental results for the W-T AO system were consistent with the results of the simulation, which demonstrated in practice that this algorithm is practical for the AO system with dual DMs.

Liquid crystal (LC) based spatial light modulators (SLM) offer a low cost alternative to the expensive deformable mirrors used in most adaptive optics systems. Real time correction of simulated atmospheric turbulence is demonstrated. The corrective element is a 10 by 10 pixel ferroelectric liquid crystal (FLC) SLM. An improvement in the Strehl ratio from 23% to 33% with a frame rate of 6Hz was demonstrated. A nematic liquid crystal (NLC) SLM was used to generate realistic atmospheric turbulence.

The curvature‐wavefront‐based adaptive optics (AO) system with a laser guide star is under commissioning at Subaru Telescope. The AO system has been in use since September 2008 using a natural guide star. The testing of laser projection on the sky resumed on February 13, 2009. We have obtained around 0.55 of Strehl ratio in the K band under seeing conditions of 0.4 arcsecond (at K band) using an 8th‐magnitude single star. The Strehl ratio was about 0.2 in the K band using a faint guide star around 16th magnitude. Even though the seeing condition was 1 arcsecond, we obtained an almost diffraction‐limited image. The total equivalent magnitude of the laser guide star was estimated at 10.7 magnitude. However, the full width half maximum is around 2 arcseconds under the seeing condition of 1 arcsecond in K‐band.

Solar Adaptive Optics (AO) shares many issues with night-time AO, but it also has its own particularities. The wavefront sensing is performed using correlations to efficiently work on the solar granulation as a reference. The field of view for that measurement usually is around 10". A sensor collecting such a wide field of view averages wavefront information from different sky directions, and the anisoplanatism thus has a peculiar impact on the performance of solar AO and MCAO systems. Since we are entering the era of large solar telescopes (European Solar Telescope, Advanced Technology Solar Telescope) understanding this issue is crucial to evaluate its impact on the performance of future AO systems. In this paper we model the correlating wide field sensor and the way it senses the high altitude turbulence. Thanks to this improved modelling, we present an analysis of the influence of this sensing on the performance of each AO configuration, conventional AO and MCAO. In addition to the analytical study, simulations similar to the case of the EST AO systems with FRiM-3D (the Fractal Iterative Method for Atmospheric tomography) are used in order to highlight the relative influence of design parameters. In particular, results show the performance evolution when increasing the telescope diameter. We analyse the effect of high altitude turbulence correlation showing that increasing the diameter of the telescope does not degrade the performance when correcting on the same spatial and temporal scales.

: The end-to-end performance achieved by an adaptive optical (AO) imaging system is determined by a combination of the residual time-varying phase distortions associated with atmospheric turbulence, and the quasi-static unsensed and uncorrectable aberrations in the optical system itself. Although the effects of these two errors on the time averaged Strehl ratio and the time averaged optical transfer function (OTF) of the AO system are not formally separable, such an approximation is found to be accurate to within a few per cent for a range of representative residual wave-front errors. In these calculations, we have combined static optical system aberrations and time-varying residual phase distortion characteristic of deformable mirror (DM) fitting error, wave-front sensor (WFS) noise, and anisoplanatism. The static aberrations consist of focus errors of varying magnitudes as well as a combination of unsensed and uncorrectable mirror figure errors derived from modeling by the Gemini beta-meter Telescopes project. The overall Strehl ratios and OTF's due to the combined effect of these error sources are well approximated as products of separate factors for the static and time-varying aberrations, as long as the overall Strehl ratio due to both errors is greater than about 0.1. For lower Strehl ratios the products provide lower bounds on the actual values of the Strehl ratio and OTF. The speckle transfer function (STF) is also well approximated by a product of two functions, but only where AO compensation is sufficiently good that speckle imaging techniques are usually not required.

In this paper, we study a method of simulating optical wave propagation from a sodium laser beacon, specifically applied to Shack-Hartmann wavefront sensing. A Gaussian laser is propagated from the ground to the three-dimensional scattering volume in the mesosphere (80-100km). The resonant backscatter acts as the laser beacon for the adaptive optics (AO) system. This volumetric aspect is modeled by using multiple slices at different altitudes in the sodium layer. Weak turbulence is used in the experiment and the AO system is operated in the open-loop regime only. This paper investigates the number of slices required to adequately reproduce perspective elongation in the Shack-Hartmann spots. The study showed that the most stringent case in the simulations required nine evenly-spaced slices. Other ways of distributing the slices may lead to different results.