Astronomical extended object image reconstruction after primary correction by Yunnan Observatory 1.2m adaptive optics system

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.

The Gemini 8 meter telescopes performance with active and adaptive optics as a system is given. The telescopes are being designed to deliver near diffraction limited images at infrared wavelengths to the focal plane. This is achieved with a combination of innovative telescope design, a fully active control system and a natural guide star adaptive optics (AO) system for the Mauna Kea Telescope. The predicted delivered performance while under full active control is given at 2.2 microns. The top level AO system error budget is presented including the effects of instrumentation. The Gemini telescopes have been designed from the outset to be fully active; from control of the primary mirror surfaces and positioning of the secondary to ventilation of the enclosure by control over the ventilation gates. Descriptions of the concepts used in the various subsystems have been published previously. Here, we emphasize the system level interactions between the Gemini adaptive optics system and the telescope and instruments. This includes a performance summary of how the telescope operates with and without AO. First, the current system concept is outlined, which includes wavefront sensors/guiders in the following areas: (1) acquisition and guiding system, peripheral wavefront sensors; (2) scientific instruments, on instrument wavefront sensors; (3) adaptive optics system, facility wavefront sensor. The system performance depends upon the interactions of these three key sensor areas. For non-AO use, both peripheral and on instrument wavefront sensor may be used to support fast and slow guiding and active control of the telescope alignment and wavefront. For AO use, combinations of all three types of wavefront sensors may be used for adaptive atmospheric compensation in addition to the functions listed above. The system is designed to quickly change between modes of operation (AO to non-AO and back) under remote control.

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.

A general approach is presented for evaluating and optimizing the degree of atmospheric turbulence compensation achieved by an adaptive optics system employed as part of a extended field-of-view astronomical telescope. This method applies to an adaptive optics system incorporating either one or multiple natural and/or laser guide stars sensed by wave-front slope sensors, a wave-front reconstruction algorithm equivalent to a matrix multiply, a servo control law to temporally filter the output of the wave-front reconstructor, and one or several deformable mirrors optically conjugate to different ranges along the telescope’s line-of-sight. The degree of atmospheric turbulence compensation achieved by such a configuration is evaluated in terms of the residual mean-square phase distortion and turbulence-induced optical transfer function at each point in the telescope’s field-of-view. This evaluation accounts for the combined, interacting effects of the atmospheric turbulence C n 2 ( h ) profile and wind velocity profile, wave-front sensor and deformable mirror fitting error, wave-front sensor noise, finite control system bandwidth, and the net anisoplanatism associated with a given constellation of natural and/or laser guide stars. Optimal wave-front reconstruction algorithms are derived which minimize the telescope’s field-of-view-averaged residual mean-square phase distortion. These evaluation and optimization results are first-order in the sense that they neglect diffraction effects and assume that all adaptive optics components are strictly linear. Results for the case of a finite servo bandwidth also require the wave-front reconstruction algorithm coefficients to satisfy a set of linear constraints; the reconstructor must precisely predict the deformable mirror actuator command vector in the absense of sensor noise and atmospheric turbulence. Numerical results are presented for a variety of adaptive optics configurations incorporating either a single guide star, multiple guide stars and a single deformable mirror, or multiple guide stars and two deformable mirrors.

In conventional Adaptive optics (AO) system, wavefront aberration is measured by wavefront sensor. However, a point source is usually not available in most imaging systems. Therefore development of deformable mirror control scheme in adaptive optics system based on vision information will extend the application area of AO technique. A focus measure (FM) is a measure of image quality and can be calculated from image data obtained from the image sensor. It can be used to find the optimal shape of the deformable mirror that compensates the wavefront aberration. In this paper, we analyze the performance of focus measures under different types of wavefront aberrations. The relation of FM vs. the aberration strength is analyzed systematically for different aberration modes and different FM functions. The FM performance under combination of aberrations is also investigated. This research supplies a guide for selection of the suitable FM function and strategy for control of deformable mirror in the AO system.

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.

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.

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.

The Subaru laser guide star adaptive optics (AO) system was installed at the Nasmyth focus of the Subaru Telescope, and had the first light with natural guide star on October 2006. The AO system has a 188-element curvature based wavefront sensor with photon-counting avalanche photodiode (APD) modules. It measures high-order terms of wavefront using either of a single laser (LGS) or natural guide star (NGS) within a 2' diameter field. The AO system has also a source simulator. It simulates LGS and NGS beams, simultaneously, with and without atmospheric turbulence by two turbulent layer at about 0 and 6 km altitudes, and reproduces the cone effect for the LGS beam. We describe the design, construction, and integration of the curvature wavefront sensor and calibration source unit.

Optimizing directly the system performance metric is an important method for correcting wave-front distortions in adaptive optics (AO) systems. In extended object imaging, some classical image quality metrics are often used as system performance evaluation function, which is the optimized object of the control algorithm. But those metrics do not consider the existence of imaging noise. Practically, the observed object images are degraded not only by the atmospheric turbulence but also imaging system noise. The noise in image will affect the value of image quality criteria and further affect the correction capability of AO system. An AO system with Stochastic Parallel Gradient Descent (SPGD) algorithm and a 61-element deformable mirror is simulated to restore the image of a turbulence-degraded extended object and the gray level variance function acted the optimized object by control algorithm. The analysis results show that the correction capability is affected severely when the turbulence strength is relatively big (D/r0<10). This paper presents a method that the observed image is processed by the bilateral filter before the system performance metric is calculated, so that the effect of noise on the system performance metric can be mitigated. Numerical simulation results verify the method is effective and the image quality based on AO technique and the bilateral filter is much better than that of only using AO technique.

The technology and components required to implement discrete adaptive optics systems capable of compensating wavefront errors caused by atmospheric turbulence in ground-based astronomical telescopes are reviewed. Characteristics of the major types of deformable mirrors, wavefront sensors and wavefront reconstructors are described. The effects of device limitations such as the size of the compensation subapertures and the signal to noise ratio of the wavefront sensor detector on the overall performance of adaptive optics systems are discussed. This review indicates that the technology exists to enable conventional adaptive optics systems to perform close to their inherent performance limits, the major impediment being the high cost of the components required. However, a larger problem exists in that the usefulness of adaptive optics for ground-based astronomy is severely limited by external factors such as the small size of the isoplanatic patch and the small photon flux available from most astronomical objects. The conclusion is that new system concepts are needed to overcome these external limitations and to make adaptive optics a useful technique for ground-based astronomy. Among the new approaches that have already been proposed are laser guide stars and multiple wavefront correctors.

Some computational problems arising in adaptive optics imaging systems

Some classical image quality metrics are often used as system performance evaluation function, which is the optimized object of the control algorithm, when the Adaptive Optics (AO) system without a wave-front sensor is used to correct extended object imaging. However those metrics do not consider the existence of imaging noise. Practically, the observed object images are degraded not only by the atmospheric turbulence but also imaging system noise. The noise in image will affect the value of image quality metric and further affect the correction capability of AO system. An AO system with Stochastic Parallel Gradient Descent (SPGD) algorithm and a 61-element deformable mirror is simulated to restore the image of a turbulence-degraded extended object and the metric based on the frequency spectrum entropy is acted as the optimized object by control algorithm. Based on the simulation mode, the correction capability of the AO system is investigated through wave-front aberrations under different turbulence strength with different noise. Numerical simulation results verify the metric based on the Frequency Spectrum Entropy (FSE) is effective when the noise of imaging system is considered and the correction capability of the AO system is improved obviously.

METIS, the Mid-nfrared E-ELT Imager and Spectrometer, will be providing high-sensitivity imaging and high-resolution spectroscopy in the mid-infrared (3-19 micrometer) to the E-ELT. In order to achieve the exceptional performance required by its driving science cases, exoplanets and proto-planetary disks, METIS will be featuring two Adaptive Optics (AO) systems — a first-light Single Conjugate Adaptive Optics (SCAO) system, complemented by a Laser Tomographic Adaptive Optics (LTAO) system, most likely, a few years after first light. METIS, being one of the three first light science instruments on the European Extremely Large Telescope (E-ELT), will be one of the first instruments using the integrated deformable mirror of the E-ELT for its Adaptive Optics (AO) correction. The internal SCAO system designed to maximize the performance for bright targets and has its wavefront sensors (WFSs) build inside the METIS cryostat to minimize the number of warm surfaces towards the science detectors. Although the internal dichroic will reflect all light short wards of 3 micrometers towards the WFS, only the IR light will most likely be used, mainly due to the expected improved performance at longer wavelengths for the WFS. A trade-off has been made between both visible versus infrared wave front sensing as well as Pyramid versus Shack-Hartmann, under various observing conditions and target geometries, taking into account performance, target availability, reliability and technology readiness level. The base line for the SCAO system is to minimize system complexity, thereby ensuring system availability and reliability even under first-light conditions. Since the SCAO system will require a bright guide star near the science target, it can only be used for a limited number of targets. The LTAO system, consisting of up to 6 LGS and up to 3 low-order NGS WFS and located outside the cryostat, is designed to increase the sky coverage on arbitrary targets to >80%. Investigations are ongoing if the internal SCAO system can be used as either a Low-Order WFS or metrology 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.