Showing papers on "Wavefront sensor published in 2022"

Overview and status of the GMT wavefront control system

Abstract: The Giant Magellan Telescope (GMT) wavefront control system provides active optics control and optical turbulence correction for every instrument on the 25.4 m diameter GMT. The GMT has four first-generation wavefront control modes that balance image quality, field of view, sky coverage, and development risk: Natural Seeing, Ground-Layer AO, Natural Guide Star AO, and Laser Tomography AO. Several aspects of the GMT wavefront control design have been recently updated. The Acquisition, Guiding, and Wavefront Sensing Subsystem, used in all control modes, has completed final design and a full-scale prototype sensor is being assembled. A Holographic Dispersed Fringe Sensor has been developed to improve the segment phasing capture range and stability of the Natural Guide Star AO mode. In the Laser Tomography AO mode, a high-speed infrared imager in each instrument will measure segment phasing disturbances using phase retrieval on a faint natural guide star, replacing an inter-segment differential laser metrology truss as the primary phasing sensor. High-fidelity simulations of all wavefront control modes have been developed, and we are developing wavefront sensor prototypes on laboratory testbeds that replicate the GMT optical design. We review the performance expectations in each control mode, and describe our plan to complete the wavefront control system development.

Terahertz referenceless wavefront sensing by means of computational shear-interferometry.

Abstract: In this contribution, we demonstrate the first referenceless measurement of a THz wavefront by means of shear-interferometry. The technique makes use of a transmissive Ronchi phase grating to generate the shear. We fabricated the grating by mechanical machining of high-density polyethylene. At the camera plane, the +1 and -1 diffraction orders are coherently superimposed, generating an interferogram. We can adjust the shear by selecting the period of the grating and the focal length of the imaging system. We can also alter the direction of the shear by rotating the grating. A gradient-based iterative algorithm is used to reconstruct the wavefront from a set of shear interferograms. The results presented in this study demonstrate the first step towards wavefield sensing in the terahertz band without using a reference wave.

The optical and mechanical design for the 21,000 actuator ExAO system for the Giant Magellan Telescope: GMagAO-X

Abstract: GMagAO-X is the ExAO coronagraphic instrument for the 25.4m GMT. It is designed for a slot on the folded port of the GMT. To meet the strict ExAO fitting and servo error requirement (<90nm rms WFE), GMagAO-X must have 21,000 actuator DM capable of ≥2KHz correction speeds. To minimize wavefront/segment piston error GMagAO-X has an interferometric beam combiner on a vibration isolated table, as part of this “21,000 actuator parallel DM”. Piston errors are sensed by a Holographic Dispersed Fringe Sensor (HDFS). In addition to a coronagraph, it has a post-coronagraphic Low Order WFS (LLOWFS) to sense non-common path (NCP) errors. The LLOWFS drives a non-common path DM (NCP DM) to correct those NCP errors. GMagAO-X obtains high-contrast science and wavefront sensing in the visible and/or the NIR. Here we present our successful externally reviewed (Sept. 2021) CoDR optical-mechanical design that satisfies GMagAO-X’s top-level science requirements and is compliant with the GMT instrument requirements and only requires COTS parts.

Adaptive Optics at the European Solar Telescope: status and future developments

Abstract: The European Solar Telescope (EST) is a 4.2-m telescope which has been redesigned with a fully integrated Multi-Conjugate Adaptive Optics (MCAO) into the optical path right after the EST primary mirror. The current baseline configuration considers four altitude Deformable Mirrors (DM) conjugated to 5, 9, 12 and 20 km above the telescope entrance pupil and an Adaptive Secondary Mirror (ASM) conjugated to the entrance pupil. The wavefront sensing will be performed by a set of correlation-based Shack Hartmann wavefront sensors (WFS) combining an on-axis High-Order WFS (HOWFS) to be used either in Single Conjugate AO (SCAO) to drive the ASM as well as operating simultaneously with a Multi-Directional WFS (MDWFS) to drive the MCAO. Beyond the current baseline configuration, different alternatives are currently being investigated both in the wavefront sensing strategy by evolving from a HOWFS+MDWFS into possibly a single High Order Multi Directional WFS (HOMDWFS) and/or wavefront sensors operating at different observing bands.

First lab results of segment/petal phasing with a pyramid wavefront sensor and a novel holographic dispersed fringe sensor (HDFS) from the Giant Magellan Telescope high contrast adaptive optics phasing testbed

Abstract: The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirror segments that are separated by large > 30 cm gaps, making them susceptible to fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. If we wish to utilize the full 25.4-m diffractionlimited aperture of the GMT for high-contrast natural guide star adaptive optics (NGSAO) science (e.g., direct imaging of habitable zone earth-like planets around late type stars), the seven mirror segments must be co-phased to well within a fraction of a wavelength. The current design of the GMT involves seven adaptive secondary mirrors, a dispersed fringe sensor, and a pyramid wavefront sensor (PyWFS) to measure and correct the total path length between segment pairs, but these methods need to be tested “end-to-end” in a lab environment if we hope to officially retire the GMT high risk item of phasing performance. We present the design and working prototype of a “GMT High-Contrast Adaptive Optics phasing Testbed” (p-HCAT) which leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT NGSAO science. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS and a novel Holographic Dispersed Fringe Sensor (HDFS) with and without simulated atmospheric turbulence. We show that the PyWFS alone was able to successfully control piston without turbulence within 12-33 nm RMS for 0 λ/D – 5 λ/D modulation, but was unsuccessful at controlling segmented piston with generated ∼ 0.6 arcsec and ∼ 1.2 arcsec seeing turbulence due to non-linear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. We report the success of an alternate solution to control segmented piston using the novel HDFS while controlling all other modes with the PyWFS purely as a slope sensor (piston mode removed). This “second channel” WFS method worked well to control piston to within 50 nm RMS and ± 10 μm dynamic range under simulated 0.6 arcsec and 1.2 arcsec atmospheric seeing conditions. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT and likely other Giant Segmented Mirror Telescopes (GSMTs) as well. Therefore, an additional “second channel” piston sensor, such as the novel HDFS, is strongly suggested.

Wide-Aperture Bimorph Deformable Mirror for Beam Focusing in 4.2 PW Ti:Sa Laser

Abstract: The bimorph deformable mirror with a diameter of 320 mm, including 127 control electrodes, has been developed and tested. The flatness of the initial mirror surface of about 1 μm (P-V) was achieved by mechanically adjusting the mirror substrate fixed in the metal mount. To correct for the aberrations and improve the beam focusing in the petawatt Ti:Sa laser, the wide-aperture adaptive optical system with the deformable mirror and Shack–Hartmann wavefront sensor was developed. Correction of the wavefront aberrations in the 4.2 PW Ti:Sa laser using the adaptive system provided increases the intensity in the focusing plane to a value of 1.1 × 1023 W/cm2

The Giant Magellan Telescope natural guidestar adaptive optics mode: improving the robustness of segment piston control

Abstract: The Natural Guidestar Adaptive Optics (NGAO) system of the Giant Magellan Telescope (GMT) uses a visible-light modulated pyramid wavefront sensor (PWFS) to measure the wavefront aberrations, including residual segment piston errors. This paper presents an updated wavefront sensing and control architecture that increases the robustness of segment piston control. In the first place, the Natural Guide star Wavefront Sensor (NGWS) design has been updated to incorporate the recently proposed Holographic Dispersed Fringe Sensor (HDFS) as a second wavefront sensing channel measuring unambiguously differential segment piston errors as large as ~14 microns. In the second place, an optimized modal control strategy has been developed to compensate mode-by-mode for the optical gain of the PWFS. We report on the simulated NGAO performance results obtained when using a R=12 guide star, and with a turbulence strength of 𝑟0 =12.8 cm.

AO3000 at Subaru: combining for the first time a NIR WFS using First Light’s C-RED ONE and ALPAO’s 64x64 DM

Abstract: After 16 years of on-sky operation, Subaru Telescope’s facility adaptive optics AO188 is getting several major upgrades to become the extreme-AO AO3000 (3000 actuators in the pupil compared to 188 previously). AO3000 will provide high-Strehl images for several instruments from visible to mid-infrared, notably the Infrared Camera and Spectrograph (IRCS), and the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO). For this upgrade, the original 188-element deformable mirror (DM) will be replaced with ALPAO’s 64 × 64 DM. The visible wavefront sensor will also be upgraded at a later date, but in the meantime we are adding a near-infrared Wavefront Sensor (NIR WFS), using either a double roof prism pyramid mode or a focal plane WFS mode. This new wavefront sensor will use for the first time First Light’s C-RED ONE camera, allowing for a full control of the 64 × 64 DM at up to 1.6 kHz. One of the challenges is the use of non-destructive reads and a rolling shutter with the modulated pyramid. This upgrade will be particularly exciting for SCExAO, since the extreme-AO loop will focus more on creating high-contrast dark zones instead of correcting large atmospheric residuals. It will be the first time two extreme-AO loops will be combined on the same telescope. Finally, the setup AO3000+SCExAO+IRCS will serve as a perfect demonstrator for the Thirty Meter Telescope’s Planetary Systems Imager (TMT-PSI). We will present here the design, integration and testing of AO3000, and show the first on-sky results.

Towards on-sky adaptive optics control using reinforcement learning

Abstract: The direct imaging of potentially habitable Exoplanets is one prime science case for the next generation of high contrast imaging instruments on ground-based extremely large telescopes. To reach this demanding science goal, the instruments are equipped with eXtreme Adaptive Optics (XAO) systems which will control thousands of actuators at a framerate of kilohertz to several kilohertz. Most of the habitable exoplanets are located at small angular separations from their host stars, where the current XAO systems' control laws leave strong residuals.Current AO control strategies like static matrix-based wavefront reconstruction and integrator control suffer from temporal delay error and are sensitive to mis-registration, i.e., to dynamic variations of the control system geometry. We aim to produce control methods that cope with these limitations, provide a significantly improved AO correction and, therefore, reduce the residual flux in the coronagraphic point spread function. We extend previous work in Reinforcement Learning for AO. The improved method, called PO4AO, learns a dynamics model and optimizes a control neural network, called a policy. We introduce the method and study it through numerical simulations of XAO with Pyramid wavefront sensing for the 8-m and 40-m telescope aperture cases. We further implemented PO4AO and carried out experiments in a laboratory environment using MagAO-X at the Steward laboratory. PO4AO provides the desired performance by improving the coronagraphic contrast in numerical simulations by factors 3-5 within the control region of DM and Pyramid WFS, in simulation and in the laboratory. The presented method is also quick to train, i.e., on timescales of typically 5-10 seconds, and the inference time is sufficiently small (

HARMONI at ELT: designing a laser guide star wavefront sensors for the ELT

Abstract: HARMONI is the first light visible and near-IR integral field spectrograph for the ELT covering a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO and LTAO - or with no AO. The project is preparing for Final Design Reviews. The laser Tomographic AO (LTAO) system provides AO correction with very high sky-coverage thanks to two systems: the Laser Guide Star Sensors (LGSS) and the Natural Guide Star Sensors (NGSS). LGSS is dedicated to the analysis of the wavefront coming from 6 laser guide stars created by the ELT. It is made of 6 independent wavefront sensor (WFS) modules mounted on a rotator of 600mm diameter to stabilise the pupil onto the microlens array in front of the detector. The optical design accepts elongated spots of up to 16 arcsec with no truncation using a CMOS detector from SONY. We will present the final optical and mechanical design of the LGSS based on freeform lenses to minimize the numbers of optical components and to accommodate for the diversity of sodium layer configurations. We will focus on rotator design, illustrating how we will move 1 tons with 90” accuracy in restrictive environment. Finally, we will present the strategy to verify the system in HARMONI context. The main challenge for the verification being how to test an AO system without access to the deformable mirror, part of the ELT.

Differential piston sensing with LIFT: application to the GMT

Abstract: The Giant Magellan Telescope’s primary and deformable secondary mirror are each composed of 7 segments. The Natural Guide Star (NGS) wavefront sensor has the critical task to keep these 7 segments in phase in addition to the classical Adaptive Optics (AO) correction. The baseline defined several years ago has two pyramid wavefront sensors working in the visible. The first one is used to close the AO loop (main channel), but it is not sensitive to differential pistons that are multiples of its central wavelength (λ1), leading to segment ejections. The second pyramid, sensing at a slightly higher wavelength, is then used as a slow ”truth sensor” (2nd channel) to derive the sign of a segment ejection and correct it by steps of λ1. However, the robustness of this solution with respect to noise and turbulence conditions is not satisfying. We are now in a prototyping phase, for which the first step is to improve the baseline or find an alternative design for the 2nd channel in order to gain robustness. One of the potential solutions is LIFT, a focal-plane wavefront sensor. By making use of the sensor at two different wavelengths, it is possible to derive an unambiguous differential piston measurement. In this work, we describe our piston control strategy and show the results of end-to-end simulations comprising the full AO loop and the 2nd channel correction at the faint end of the NGS mode.

HARMONI at ELT: towards a final design for the Natural Guide Star Sensors system

Abstract: HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for final design review (FDR). The Natural Guide Star Sensors (NGSS) system of HARMONI provides wavefront and image stabilization sensing for each of the four observing modes of the instrument, LTAO, SCAO, HCAO, and NOAO. It consists of five subsystems, three of which provide wavefront sensing (LOWFS, SCAOS and HCM), the remaining two (ESE and ISB) providing thermal and mechanical functions. To limit thermal background and to ensure the required stability, the sensors operate in a cold, thermally stabilized, dry gas environment. This paper presents the overall design of the system with emphasis on system analysis, assembly and test, and maintenance.

Keck All sky Precision Adaptive optics program overview

Abstract: We present the status and plans for the Keck All sky Precision Adaptive optics (KAPA) program. KAPA includes (1) an upgrade to the Keck I laser guide star adaptive optics (AO) facility to improve image quality and sky coverage, (2) the inclusion of AO telemetry-based point spread function estimates with all science exposures, (3) four key science programs, and (4) an educational component focused on broadening the participation of women and underrepresented groups in instrumentation. For this conference we focus on the KAPA upgrades since the 2020 SPIE proceedings1 including implementation of a laser asterism generator, wavefront sensor, real-time controller, asterism and turbulence simulators, the laser tomography system itself along with new operations software and science tools, and modifications to an existing near-infrared tip-tilt sensor to support multiple natural guide star and focus measurements. We will also report on the results of daytime and on-sky calibrations and testing.

Keck adaptive optics facility: real time controller upgrade

Abstract: The W. M. Keck Observatory Adaptive Optics (AO) facilities have been operating with a Field Programmable Gate Array (FPGA) based real time controller (RTC) since 2007. The RTC inputs data from various AO wavefront and tip/tilt sensors; and corrects image blurring from atmospheric turbulence via deformable and tip/tilt mirrors. Since its commissioning, the Keck I and Keck II RTCs have been upgraded to support new hardware such as pyramid wavefront and infrared tip-tilt sensors. However, they are reaching the limits of their capabilities in terms of processing bandwidth and the ability to interface with new hardware. Together with the Keck All-sky Precision Adaptive optics (KAPA) project, a higher performance and a more reliable RTC is needed to support next generation capabilities such as laser tomography and sensor fusion. This paper provides an overview of the new RTC system, developed with our contractor/collaborators (Microgate, Swinburne University of Technology and Australian National University), and the initial on-sky performance. The upgrade includes an Interface Module to interface with the wavefront sensors and controlled hardware, and a Graphical Processing Unit (GPU) based computational engine to meet the system’s control requirements and to provide a flexible software architecture to allow future algorithms development and capabilities. The system saw first light in 2021 and is being commissioned in 2022 to support single conjugate laser guide star (LGS) AO, along with a more sensitive EMCCD camera. Initial results are provided to demonstrate single NGS & LGS performance, system reliability, and the planned upgrade for four LGS to support laser tomography.

Wavefront-controllable all-silicon terahertz meta-polarizer

Abstract: Metamaterial devices that can directly generate polarized waves from unpolarized waves and simultaneously manipulate the wavefront are needed for advanced optical applications. However, conventional wavefront-controllable polarization conversion metasurfaces (PCMs) rely on an incident wave with a certain polarization state rather than an unpolarized wave. A few specially designed meta-polarizers had obtained polarized waves from unpolarized waves, but the simultaneous wavefront manipulation was not achieved. In this work, we report wavefront-controllable terahertz (THz) meta-polarizers based on all-silicon materials, which have higher integration characteristics than conventional meta-polarizers and wavefront-controllable PCMs. Such THz meta-polarizers merge the desired phase profile into meta-atoms’ beam interference, and will perform direct generation of polarized waves from unpolarized waves and realize simultaneous wavefront manipulations. This work is expected to provide a new impact for THz wave manipulations.

Prediction of wavefront distortion for wavefront sensorless adaptive optics based on deep learning.

Abstract: Aimed at the slow detection speed and low measurement accuracy of wavefront aberration in current wavefront sensorless adaptive optic technology, different convolution neural networks (CNNs) are established to detect the turbulence wavefront, including an ordinary convolutional neural network, a ResNet network, and an EfficientNet-B0 network. By using the nonlinear fitting ability of deep neural networks, the mapping relationship between Zernike coefficients and focal degraded image can be established. The simulation results show that the optimal network model after training can quickly and efficiently predict the Zernike coefficients directly from a single focal degraded image. The root-mean-square errors of the wavefront detection accuracy of the three networks are 0.075λ, 0.058λ, and 0.013λ, and the time consumed for predicting the wavefront from the single degraded image are 2.3, 4.6, and 3.4 ms, respectively. Among the three networks presented, the EfficientNet-B0 CNN has obvious advantages in wavefront detection accuracy and speed under different turbulence intensities than the ordinary CNN and ResNet networks. Compared with the traditional method, the deep learning method has the advantages of high precision and fast speed, without iteration and the local minimum problem, when solving wavefront aberration.

Large-aperture adaptive optical system for correcting wavefront distortions of a petawatt Ti : sapphire laser beam

Abstract: Abstract This paper reports a large-aperture adaptive optical system with a bimorph deformable mirror and Shack – Hartmann wavefront sensor for aberration correction and beam focusing improvement in state-of-the-art petawatt Ti : sapphire lasers. We consider methods for providing feedback to the wavefront sensor and obtaining an objective wavefront that optimises beam focusing onto a target. The use of an adaptive system with a controlled 127-channel 320-mm-aperture mirror in a Ti : sapphire laser with an output power of 4.2 PW has made it possible to obtain a record high laser beam intensity: 1.1 × 10 23 W cm −2 .

Application of Adaptive Optics in Ophthalmology

Abstract: The eye, the photoreceptive organ used to perceive the external environment, is of great importance to humans. It has been proven that some diseases in humans are accompanied by fundus changes; therefore, the health status of people may be interpreted from retinal images. However, the human eye is not a perfect refractive system for the existence of ocular aberrations. These aberrations not only affect the ability of human visual discrimination and recognition, but restrict the observation of the fine structures of human eye and reduce the possibility of exploring the mechanisms of eye disease. Adaptive optics (AO) is a technique that corrects optical wavefront aberrations. Once integrated into ophthalmoscopes, AO enables retinal imaging at the cellular level. This paper illustrates the principle of AO in correcting wavefront aberrations in human eyes, and then reviews the applications and advances of AO in ophthalmology, including the adaptive optics fundus camera (AO-FC), the adaptive optics scanning laser ophthalmoscope (AO-SLO), the adaptive optics optical coherence tomography (AO-OCT), and their combined multimodal imaging technologies. The future development trend of AO in ophthalmology is also prospected.

Pupil-plane LLOWFS simulation and laboratory results from NEW-EARTH’s high-contrast imaging testbed

Abstract: Direct imaging of exoplanets can be used to characterize exoplanets by spectroscopy of their atmospheres. Coronagraphs are required to suppress the diffraction effects by blocking the starlight, however, residual wavefront error scatters starlight in the science images, losing faint exoplanet photons in stellar noise. The performance of a coronagraphic system is thus contingent upon how efficiently the wavefront aberrations are minimized. Lyot-stop low-order wavefront sensor (LLOWFS) is a well-established sensor that senses the light rejected by the focal plane mask and corrects low-order aberrations upstream of the coronagraph. Previous versions of the LLOWFS sensed the residual starlight at the defocused focal plane. However, on the NRC's NEW-EARTH high-contrast imaging testbed, pupil-plane images of LLOWFS have been used to address both Zernike and Fourier modes. The goal of the testbed is to develop SPIDERS/Subaru which is the technology demonstrator of the CAL2 unit of the upcoming Gemini Planet Imager 2.0 (GPI 2.0). Both SPIDERS and CAL2 will address the low-order modes for stabilizing speckles, and demonstrate an active suppression of speckles using the Fast Atmospheric Self-Coherent Camera Technique (FAST) by creating a region of up to 10-7 contrast at small angles. Thus, obtaining sub-nanometric pointing stability using the LLOWFS is crucial for achieving stable contrast results on the bench and on-sky. Here, we present LLOWFS closed-loop laboratory results under simulated post-Adaptive Optics residuals of GPI 2.0 and simulations of the LLOWFS and FAST sensors for SPIDERS.

Centroid-Predicted Deep Neural Network in Shack-Hartmann Sensors

Abstract: The Shack-Hartmann wavefront sensor produces incorrect wavefront measurements when some sub-spots are weak and missing. In this paper, a method is proposed to predict the centroids of these sub-spots for the Shack-Hartmann wavefront sensor based on the deep neural network. Using the centroid information of present sub-spots, the method is able to predict the absent sub-spots’ positions. The feasibility and effectiveness of this method are verified by a large number of numerical simulations. The method is applied to wavefront measurement of light with non-uniform near-field intensity. The simulation results show that the proposed method is of great help to improve the measurement accuracy and the Strehl ratio of the focal spot. For wavefronts outside of the training sample, the proposed method shows good generalization and adaptability. In addition, the experiment results demonstrate that the proposed method can predict the missing sub-spots’ centroid displacements accurately even though a large proportion of sub-spot is lost randomly.

Efficient wavefront sensorless adaptive optics based on large dynamic crosstalk-free holographic modal wavefront sensing.

Abstract: The correction of wavefront sensorless adaptive optics (WFSless AO) can be significantly accelerated by using a holographic modal wavefront sensor (HMWFS). The HMWFS is realized by a computer-generated hologram (CGH) into which all aberration modes to be detected are encoded and only a single-shot image is required for simultaneous measurement of multiple modes. The conventional HMWFS suffers from a quite limited dynamic range and severe inter-modal crosstalk which deteriorates the sensing accuracy. We proposed a novel HMWFS with a large dynamic range and no crosstalk and validated its performance by simulation and experiment. In the improved HMWFS scheme, the aberration is represented by Lukosz modes whose gradients are orthogonal and the modal coefficients can be estimated independently. Instead of using a binary CGH in conventional HMWFS, a kinoform CGH with high diffraction efficiency is adopted in the improved HMWFS. The kinoform CGH is produced by a phase-only liquid-crystal spatial light modulator (LCSLM) which also serves as a wavefront corrector in our WFSless AO system.

Real-Time Correction of a Laser Beam Wavefront Distorted by an Artificial Turbulent Heated Airflow

Abstract: This paper presents a FPGA-based closed-loop adaptive optical system with a bimorph deformable mirror for correction of the phase perturbation caused by artificial turbulence. The system’s operating frequency of about 2000 Hz is, in many cases, sufficient to provide the real-time mode. The results of the correction of the wavefront of laser radiation distorted by the airflow formed in the laboratory conditions with the help of a fan heater are presented. For detailed consideration, the expansion of the wavefront by Zernike polynomials is used with further statistical analysis based on the discrete Fourier transform. The result of the work is an estimation of the correction efficiency of the wavefront distorted by the turbulent phase fluctuations. The ability of the bimorph adaptive mirror to correct for certain aberrations is also determined. As a result, it was concluded that the adaptive bimorph mirrors, together with a fast adaptive optical system based on FPGA, can be used to compensate wavefront distortions caused by atmospheric turbulence in the real-time mode.

NGWS-P: the natural guide star wavefront sensor prototype of GMT single conjugate AO system NGAO

Abstract: The Giant Magellan Telescope (GMT) Adaptive Optics (AO) systems feature a single conjugate natural guide star based AO system using the 7 deformable secondaries and a post focal wavefront sensor named NGWS (Natural Guide star Wavefront Sensor). The NGWS has two different channels: one featuring a high spatial sampling pyramid sensor dedicated to the fast frame rate correction of atmospheric turbulence and a second dedicated to the correct phasing of the 7 segments of the GMT telescope. The Arcetri AO group in collaboration with the GMT Organization (GMTO) and the University of Arizona (UA) is in charge of providing the design, fabrication and test of a prototype of the NGWS system that shall replicate all aspects of optical sensitivity including optical design, camera selection and data reduction. The prototype design starts from the baseline design for the NGWS that was provided by the Arcetri group in 2013. The prototype project Kick-off Meeting was held on April 16th 2021 and is foreseen to reach completion 34 months later. A first set of performance tests will be performed locally in Arcetri and the final prototype performance verification will happen at UA laboratories after installation of the unit on the High Contrast AO test bench developed by the AO group of UA. This final verification is scheduled for the summer of 2023. The paper reports about the prototype development work summarizing results of numerical simulation that lead to the chosen opto-mechanical design, main features and challenges of optical design for the two sensing channels.

Characterizing turbulence profile layers through celestial single-source observations

Abstract: Future spacecraft missions aim to communicate with the Earth using near-infrared lasers. The possible bit rate of free-space optical communication (FSOC) is orders of magnitude greater when compared to current radio frequency transmissions. The challenge of ground-space FSOC is that atmospheric turbulence perturbs optical wavefront propagation. These wavefront aberrations can be measured using a Shack-Hartmann wavefront sensor (SHWFS). A ground-based adaptive optics (AO) system can mitigate these aberrations along the optical path by translating wavefront measurements into deformable mirror commands. However, errors result from atmospheric turbulence continuously evolving, and there are unavoidable delays during AO wavefront correction. The length of an acceptable delay is referred to as the coherence time-a parameter dependent on the strength of turbulence profile layers and their corresponding wind-driven velocity. This study introduces a novel technique, to the best of our knowledge, for using SHWFS single-source observations, e.g., the downlink signal from a geostationary satellite, to measure the strength and velocity of turbulence profile layers. This work builds upon previous research and demonstrates that single-source observations can disentangle turbulence profile layers through studying the cross-covariance of temporally offset SHWFS centroid measurements. Simulated data are used to verify that the technique can recover the coherence time. The expected and measured results have a correlation coefficient of 0.95.

ULTIMATE-START: current status of the Subaru Tomography Adaptive optics Research experimenT

Abstract: We present the current status of the ULTIMATE-Subaru Tomography Adaptive optics Research experimenT (ULTIMATE-START) project, an upcoming laser tomography adaptive optics (LTAO) system on the Subaru telescope. The main goal of this project is to realize high Strehl ratio AO correction not only in near-infrared bands but also in visible bands above 600 nm. Our LTAO system will be operated with four 32 × 32 Shack Hartmann wavefront sensors (SH-WFSs) and four laser guide stars (LGSs). The LTAO WFSs will be installed behind AO188, which is the current AO system on the Nasmyth platform of the Subaru telescope. We will use the low-order WFS and DM of AO188 for Tip-Tilt measurements with a natural guide star (NGS) and wavefront correction. The DM of AO188 will be upgraded to a 3228 element DM. Assembling of the LTAO WFS system has completed in 2022. Currently WFS data acquisition and tomographic wavefront (WF) estimation testing are underway. We also performed test observations of a prototype single SH-WFS unit with a NGS and LGS with the Subaru telescope. A new laser launching system has been installed. A single LGS is under on-sky performance verification for the open-use observations, and four LGS system, which can make an asterism with 10-40 arcsec diameter, will be installed in 2022. The first light of the entire LTAO system is planned in early 2023.

Visible extreme adaptive optics for GMagAO-X with the triple-stage AO architecture (TSAO)

Abstract: The Extremely Large Telescopes will require hundreds of actuators across the pupil for high Strehl in the visible. We envision a triple-stage AO (TSAO) system for GMT/GMagAO-X to achieve this. The first stage is a 4K DM controlled by an IR pyramid wavefront sensor that provides the first order correction. The second stage contains the high-order parallel DM of GMagAO-X that has 21000 actuators and contains an interferometric delay line for phasing of each mirror segment. This stage uses a Zernike wavefront sensor for high-order modes and a Holographic Dispersed Fringe Sensor for segment piston control. Finally, the third stage uses a dedicated 3K dm for non-common path aberration control and the coronagraphic wavefront control by using focal plane wavefront sensing and control. The triple stage architecture has been chosen to create simpler decoupled control loops. This work describes the performance of the proposed triple-stage AO architecture for ExAO with GMagAO-X.

On-sky Reconstruction of Keck Primary Mirror Piston Offsets Using a Zernike Wavefront Sensor

Abstract: The next generation of large ground- and space-based optical telescopes will have segmented primary mirrors. Co-phasing the segments requires a sensitive wavefront sensor capable of measuring phase discontinuities. The Zernike wavefront sensor (ZWFS) is a passive wavefront sensor that has been demonstrated to sense segmented-mirror piston, tip, and tilt with picometer precision in laboratory settings. We present the first on-sky results of an adaptive optics fed ZWFS on a segmented aperture telescope, W.M. Keck Observatory's Keck II. Within the Keck Planet Imager and Characterizer light path, the ZWFS mask operates in the H band using an InGaAs detector (CRED2). We piston segments of the primary mirror by a known amount and measure the mirror's shape using both the ZWFS and a phase retrieval method on data acquired with the facility infrared imager, NIRC2. In the latter case, we employ slightly defocused NIRC2 images and a modified Gerchberg–Saxton phase retrieval algorithm to estimate the applied wavefront error. We find good agreement when comparing the phase retrieval and ZWFS reconstructions, with average measurements of 408 ± 23 and 394 ± 46 nm, respectively, for three segments pistoned by 400 nm of optical path difference (OPD). Applying various OPDs, we find that we are limited to ∼100 nm OPD of applied piston, due to insufficient averaging of the adaptive optics residuals of our observations. We also present simulations of the ZWFS that help to explain the systematic offset observed in the ZWFS reconstructed data.

Extended scene deep learning wavefront sensing.

Abstract: We have applied a combination of blind deconvolution and deep learning to the processing of Shack-Hartmann images. By using the intensity information contained in spot positions, and the fine structure of the separate images created by the lenslets, we have increased the sensitivity and resolution of the sensor over the limit defined by standard processing of spot displacements only. We also have demonstrated the applicability of the method to wavefront sensing using extended objects as a reference.

Phase response measurement of spatial light modulators based on a Shack-Hartmann wavefront sensor.

Abstract: It is known that the phase response of spatial light modulators (SLMs) measured by double-beam interferometers is sensitive to mechanical and environmental disturbances. This paper proposes a Shack-Hartmann wavefront sensor (SHWS) method to measure the phase response characteristics of the SLM. The results show that the phase modulation depth measured by the proposed method is 1.7581λ, and 1.7993λ by the Twyman-Green interferometer method. The difference in the phase modulation depth between the two methods is only 0.0412λ, and its relative error rate is 2.29%. It proves that the phase modulation accuracy obtained by the SHWS with lenslets of 73*73 used in this paper is equivalent to that of the Twyman-Green interferometer. Compared with the interferometer method, the SHWS method is simple, compact, and robust, has good real-time performance, and is relatively vibration insensitive. In the future, the SHWS method will play a more important role in the detection of the SLM's phase response.

All-fibre wavefront sensor

Abstract: We report on a tapered three-core optical fibre that can be used as a tip-tilt wavefront sensor. In this device, a coupled region of a few millimetres at the sensing tip of the fibre converts fragile phase information from an incoming wavefront into robust intensity information within each of the cores. The intensity information can be easily converted to linear wavefront error over small ranges, making it ideal for closed loop systems. The sensor uses minimal information to infer tip-tilt and is compatible with remote detector arrays. We explore its application within adaptive optics and present a validation case to show its applicability to astronomy.