Adaptive Optics System Research Articles

Design and fabrication of piezoelectrically driven deformable reflective micromirrors based on MEMS technology

As the core component of the adaptive optics system, the deformable mirror is developing towards miniaturization and integration through the combination of MEMS technology. In this paper, based on the inverse piezoelectric effect of piezoelectric materials, a piezoelectrically driven deformable reflective micromirror is designed and fabricated by physical vapor deposition, UV lithography, eutectic bonding, mechanical cutting, deep reactive ion etching (RIE) and other processes to complete the preparation of micromirrors. The problems related to the eutectic bonding process of PZT and Si in the fabrication of micromirrors are emphatically studied through tensile experiments. The micromirror was tested with a laser interferometer. The test results showed that the prepared micromirror could achieve a maximum deformation of 0.6 μm in the positive direction and 1.7 μm in the negative direction when a voltage of ±1.5 kV was applied.

Companion mass limits for 17 binary systems obtained with binary differential imaging and MagAO/Clio

ABSTRACT Improving direct detection capability close to the star through improved star subtraction and post-processing techniques is vital for discovering new low-mass companions and characterizing known ones at longer wavelengths. We present results of 17 binary star systems observed with the Magellan adaptive optics system (MagAO) and the Clio infrared camera on the Magellan Clay Telescope using binary differential imaging (BDI). BDI is an application of reference differential imaging (RDI) and angular differential imaging (ADI) applied to wide binary star systems (2 arcsec <Δρ < 10 arcsec) within the isoplanatic patch in the infrared. Each star serves as the point spread function (PSF) reference for the other, and we performed PSF estimation and subtraction using principal component analysis. We report contrast and mass limits for the 35 stars in our initial survey using BDI with MagAO/Clio in L′ and 3.95 µm bands. Our achieved contrasts varied between systems, and spanned a range of contrasts from 3.0 to 7.5 magnitudes and a range of separations from 0.2 to 2 arcsec. Stars in our survey span a range of masses, and our achieved contrasts correspond to late-type M-dwarf masses down to ∼10 MJup. We also report detection of a candidate companion signal at 0.2 arcsec (18 au) around HIP 67506 A (SpT G5V, mass ∼1.2 M⊙), which we estimate to be $\sim 60-90 \, \rm{M_{Jup}}$. We found that the effectiveness of BDI is highest for approximately equal brightness binaries in high-Strehl conditions.

Derivation of Ambiguity in Wavefront Aberration and Quantitative Analysis in AO System

Adaptive optics (AO) can effectively correct wavefront aberration, with many works qualitatively utilizing the defocus preconditioner to improve the aberration correction accuracy, but no work has yet quantitatively optimized the defocus distance to obtain higher accuracy. Spurred by the theoretical gap, this paper provides the derivation revealing the mechanism of the above phenomenon and proposes a theoretical method for determining the optimal defocus distance (OPDD) according to AO system parameters instead of numerous tests. Given that the complex conjugate amplitude can generate the same point spread function (PSF), we define the dual wavefront and present the Zernike coefficients expression with additional aberration due to the PSF variation. Then we derive the probability of the dual wavefront (PDWF) with the relevant conclusions of label noise. We suppose that the defocus preconditioner reduces the PDWF, thus effectively improving the accuracy, which is verified through the quantitative analysis where a deep neural network is utilized to correct aberration. Furthermore, the correction accuracy at the theoretical OPDD is no less than 98% of the highest accuracy in experiments, which quantitatively demonstrates the effectiveness of the method. Overall, this work significantly improves the efficiency of the AO system setup.

The Effects of Optical Aberrations to Illumination Beam Thickness in Two-Photon Excitation Microscopes

When performing in vivo imaging of live samples, it is a big challenge to penetrate thick tissues while still maintaining high resolution and a large field of view because of the sample-induced aberrations. These requirements can be met by combining the benefits of two-photon excitation, beam modulation and adaptive optics in an illumination path. However, the relationship between aberrations and the performance of such a microscopy system has never been systematically and comprehensively assessed. Here, two-photon Gaussian and Bessel beams are modulated as illumination beams, and how aberrations affect the thickness of the illumination beams is evaluated. It is found that the thickness variation is highly related to the azimuthal order of Zernike modes. The thickness of the two-photon Gaussian beam is more sensitive to Zernike modes with lower azimuthal order, while the thickness of the two-photon Bessel beam is more sensitive to the higher-azimuthal-order Zernike modes. So, it is necessary to design a new strategy to correct aberrations according to the effects of different Zernike modes in order to maximize the correction capability of correctors and reduce the correction errors for those insensitive Zernike modes. These results may provide important guidance for the design and evaluation of adaptive optical systems in a two-photon excitation microscope.

Distortion Correction In Particle Image Velocimetry For Measurements Through Fluctuating Interfaces Using A Deep Neural Network And Wavefront Sensing

Imaging-based flow measurements like Particle Image or Particle Tracking Velocimetry re-quire an undistorted optical access to the measurement location. Changes of the refractive index in the light path can cause image deterioration and lead to severe measurement errors. While static aberration as e.g. from curved walls can be corrected by of means static correction optics or dedicated light ray models, this is hardly possible if the aberration change with time at high bandwidth. Many cases of this kind exists in fluid mechanics as e.g. free liquid jets, bubbles, droplets or film flows. These interfaces are related to several open research questions that concern the interaction between the media on both sides of the interface. Hardware-based corrections adapted from astronomy that employs a deformable membrane mirror, a wavefront sensor and a fast control electronics are a viable approach for solving this measurement problem. However, these closed-loop control schemes require extensive and dedicated hardware and are limited by the technical specifications of the adaptive optical mirror in terms of correction bandwidth, stroke and spatial resolution. Here we present an approach based on Multiple-Input deep convolutional neural network that works with a wavefront sensor only and spares the adaptive optical component. Based on a model experiment and a PIV measurement, we demonstrate that this system is able to correct for the errors that are induced by an oscillating water surface by 82 %. The technique can be an alternative to classical closed-loop adaptive optical systems where the performance of the actuators is not sufficient. This can open a perspective towards measurement of interface-related flows that were not accessible before.

Particle Tracking Velocimetry With Dynamic Aberration Correction For 3D Flow Measurements Through Fluctuating Phase Boundaries

Imaging based flow measurements allow the observation of complex three-dimensional flows with a high temporal resolution. However, at least one optical access with a good imaging quality is needed. In fluids with a dynamic phase boundary, often the only optical access is the dynamic surface (e. g. droplet on opaque surface). In this case, temporally varying aberrations are introduced that can increase the measurement uncertainty significantly. Additionally, many flows in technical systems are three-dimensional and turbulent so that volumetric measurements are desirable. In this contribution, we present the first 3D-3C flow measurement system with dynamic aberration correction and a single optical access. By using a deformable mirror with 69 actuators, a laser guide star and a wavefront sensor, the dynamic aberrations that are introduced by the fluctuating phase boundary can be corrected. By introducing a double-helix point spread function (DH-PSF) with a spatial light modulator (SLM) we achieve three-dimensional flow measurements with only one camera. For demonstration purposes, the flow in a microchannel was measured through an oscillating puddle-like droplet. By applying the adaptive-optics system, the measurement uncertainty could be lowered by 58 % relative to measurements through a static droplet. The technique has the potential to allow three-dimensional flow measurements in droplets on opaque surfaces (e. g. membrane of fuel cell). Thus, new insights about the movement of droplets (e. g. in fuel cells) are made possible.

Wavefront-corrected post-compression of a 100-TW Ti:sapphire laser.

We analyzed and corrected the wavefront distortion induced during the post-compression of a 100-TW Ti:Sapphire laser and achieved the intensity enhancement. In the post-compression, the spectral broadening of the laser was obtained by propagating through three 0.5 mm-thick fused silica plates and the laser pulse duration was post-compressed from 24 fs to 11 fs using a set of chirped mirrors. We measured the wavefront aberrations due to the intensity-dependent nonlinear process during the post-compression of femtosecond high-power laser pulses. By compensating for the wavefront aberrations with an adaptive optics system, the Strehl ratio of the post-compressed beam was improved from 0.37 to 0.52 and the focused intensity of the post-compressed beam could be enhanced by a factor of 1.5, while the enhancement without the wavefront correction was only a factor of 1.1 in spite of the peak-power enhancement by a factor of 1.8.

Application of the General Extended-Object Wavefront Sensor into Aberration Correction of Lasers

Thermal aberrations are the main impediments for high-energy transverse multimode lasers to achieve high brightness and beam qualities. To address this issue using adaptive optics, traditional wavefront sensing methods have great limitations for aberration correction in high-energy transverse multimode lasers, because these lasers are no longer point sources. The general extended-object wavefront sensor (GEWFS) invented by the authors can work with extended sources in real time, and it is highly suitable to solve this problem. The application of the GEWFS into the compensation of thermal aberrations in a high-energy laser is preliminarily investigated by means of a simulation experiment. In this simulation experiment, a transverse multimode laser diode simulates a high-energy laser, a and phase plate thermal aberrations, which is compensated by a closed-loop adaptive optics system mainly composed of the GEWFS and a membrane deformable mirror. Simulation experimental results show that the peak intensity of the laser beam image is improved about 2.35 times, and the approximate Strehl ratio of the laser system attained 0.90. The results indicate that the GEWFS is simple and accurate enough for wavefront sensing from extended sources, and it will greatly expand the application fields of adaptive optics.

Radial distribution of the carbonaceous nano-grains in the protoplanetary disk around HD 169142

Context. HD 169142 is part of the class of (pre-)transitional protoplanetary disks showing multiple carbon nanodust spectroscopic signatures (aromatic, aliphatic) dominating the infrared spectrum. Precise constraints on the spatial distribution and properties of carbonaceous dust particles are essential to understanding the physics, radiative transfer processes, and chemistry of the disk. The HD 169142 disk is seen almost face-on and thus it offers a unique opportunity to study the dust radial evolution in disks. Aims. We investigate the spatial distribution of the carriers of several dust aromatic emission features of the disk across a broad spatial range (10–200 AU) as well as their properties. Methods. We analysed imaging and spectroscopic observations in the 8–12 µm range from the VLT Imager and Spectrometer for mid-Infrared (VISIR) at the Very Large Telescope (VLT), as well as adaptive optics spectroscopic observations in the 3–4 µm range from the Nasmyth Adaptive Optics System – Near-Infrared Imager and Spectrograph (NACO) at VLT. The data probe the spatial variation of the flux in the 3.3 µm, 8.6 µm, and 11.3 µm aromatic bands. To constrain the radial distribution of carbonaceous nano-grains, the observations were compared to model predictions using The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), which is integrated into the POLARIS radiative transfer code by calculating the thermal and stochastic heating of micro-and nanometer-sized dust grains for a given disk structure. Results. Our data show predominant nano-particle emission at all radii (accessible with our resolution of about 0.1″ or ~12 AU at 3 µm and ~0.3″, 35 AU at 10 µm) in the HD 169142 disk. This unambiguously shows that carbonaceous nano-grains dominate radiatively the infrared spectrum in most of the disk, a finding that has been suggested in previous studies. In order to account for both VISIR and NACO emission maps, we show the need for aromatic particles distributed within the disk from the outermost regions to a radius of 20 AU, corresponding to the outer limit of the inner cavity derived from previous observations. In the inner cavity, these aromatic particles might be present but their abundance would then be significantly decreased.

ISPY – NaCo Imaging Survey for Planets around Young stars

Context.Direct imaging has made significant progress over the past decade, in part thanks to a new generation of instruments and excellent adaptive optic systems, but also thanks to advanced post-processing techniques. The combination of these two factors allowed the detection of several giant planets with separations as close as 0.2 arcsec with contrasts typically reaching 9–10 magnitudes at nearinfrared wavelengths. Observing strategies and data rates vary depending on the instrument and the wavelength, withL-andM-band observations yielding tens of thousands of images to be combined.Aims.We present a new approach, tailored for VLT/NaCo observations performed with the Annular Groove Phase Mask (AGPM) coronagraph, but that can be applied to other instruments using similar coronagraphs. Our pipeline aims to improve the post-processing of the observations on two fronts: identifying the location of the star behind the AGPM to better align the science frames and performing frame selection.Methods.Our method relies on finding the position of the AGPM in the sky frame observations, and correlating it with the circular aperture of the coronagraphic mask. This relationship allows us to retrieve the location of the AGPM in the science frames. We are then able to model the torus shape visible in the sky-subtracted science frames, as a combination of negative and positive 2D Gaussian functions. The model provides additional information that is useful to design our frame selection criteria.Results.We tested our pipeline on three targets (βPictoris, R CrA, and HD 34282), two of which have companions at intermediate and close separations, and the third hosts a bright circumstellar disk. We find that the centering of the science frames has a significant impact on the signal-to-noise ratio (S/N) of the companions. Our results suggest that the best reduction is achieved when performing the principal component analysis centered on the location of the AGPM and derotating the frames centered at the location of the star before collapsing the final datacube. We improved the S/N of companions aroundβPictoris and R CrA by 24 ± 3% and 117 ± 11% respectively, compared to other state-of-the-art reductions. We find that the companion position for all the centering strategies are consistent within 3σ. Finally, we find that even for NaCo observations with tens of thousands of frames, frame selection yields just marginal improvement for point sources, but may improve the final images for objects with extended emission such as disks.Conclusions.We propose a novel approach to identify the location of the star behind a coronagraph even when it cannot easily be determined by other methods. We led a thorough study on the importance of frame selection, concluding that the improvements are marginal in most cases, but may yield better contrast in some specific cases. Our approach can be applied to the wealth of archival NaCo data and, assuming that the field of view includes the edges of the coronagraphic mask, its implementation can be adapted to other instruments with coronagraphs similar to the AGPM used on NaCo (e.g., Keck/NIRC2, LBT/LMIRCam).

Adaptive optics wavefront correction using a damped transpose matrix of the influence function.

To assess the performance of adaptive optics and predict an optimal wavefront correction, we built a wavefront reconstructor with a damped transpose matrix of the influence function. Using an integral control strategy, we tested this reconstructor with four deformable mirrors in an experimental system, an adaptive optics scanning laser ophthalmoscope, and an adaptive optics near-confocal ophthalmoscope. Testing results proved that this reconstructor could ensure a stable and precise correction for wavefront aberration compared to a conventional optimal reconstructor formed by the inverse matrix of the influence function. This method may provide a helpful tool for testing, evaluating, and optimizing adaptive optics systems.

Analysis of the Optical Turbulence Model Using Meteorological Data

The model of atmosphere optical turbulence is important in the research field of laser atmospheric transmission, and plays a key role in astronomical site selection. In this paper, the single and overall statistical analysis between different outer scale models (HMNSP99 and the Dewan model) were conducted and the results show that the HMNSP99 model has better performance with the lowest bias, root mean square error, and center root mean square error. The results of the statistical analysis of three turbulence parameters revealed that there is a correlation between turbulence parameters and statistical operators, where statistical operators increase significantly when wind shear and temperature gradient respectively exceed 0.016 s−1, 0 K/m, and the outer scale is within 2.5 m. Furthermore, a new statistical outer-scale model, the WSTG model, is proposed and the results of statistical analysis present that the WSTG model is more reliable than the HMNSP99 model in reconstructing optical turbulence strength. These results acquired from this paper add substantially to our understanding of atmosphere optical turbulence and the conclusions can be applied to improve the performance of an adaptive optics system and astronomical site selection.

Comparative study of Shack-Hartmann configurations for atmospheric turbulence reconstructions in solar adaptive optics

The correction of the phase variations induced by the atmospheric turbulence is one of the most complex problems that an Adaptive Optics (AO) System must deal with, as it must calculate the properties of all the atmosphere traversed by the light from several measures taken by ground-based telescopes. Traditional reconstructors systems used in AO are based on computational algorithms where its reconstruction quality improves with the number of measures made by the telescopes’ sensors. That means that sensors are getting greater and greater with their corresponding higher financial expense. Artificial Intelligence (IA) has become in recent years a real alternative to traditional computational methods as reconstructors for AO systems. Fully-convolutional neural networks (FCNs) specifically have shown great performances working in Solar AO, demonstrating their ability to obtain a lot of valuable information from the recorded images for the wavefront phase evaluation. Along this research, the influence of the properties of the telescope's sensors and of the observations in the reconstructions made by the FCNs’ is measured, to obtain the configuration that best suits the performance of artificial neural networks (ANN). The presented results determine the way forward for the future sensors for telescopes with reconstruction systems based on ANNs, to obtain higher quality reconstructions employing fewer economic resources.

Lab tests of segment/petal phasing with a pyramid wavefront sensor and a holographic dispersed fringe sensor in turbulence with the Giant Magellan Telescope high contrast adaptive optics phasing testbed

The Giant Magellan Telescope (GMT) design consists of seven circular 8.4-m diameter mirrors, together forming a single 25.4-m diameter primary mirror. This large aperture and collecting area can help extreme adaptive optics (ExAO) systems such as GMT’s GMagAO-X achieve the small angular resolutions and contrasts required to image habitable zone earth-like planets around late type stars and possibly lead to the discovery of life outside of our solar system. However, the GMT primary mirror segments are separated by large >30 cm gaps, creating the possibility of fluctuations in optical path differences (piston) due to flexure, segment vibrations, wind buffeting, temperature effects, and atmospheric seeing. To utilize the full diffraction-limited aperture of the GMT for high-contrast, natural guide star-adaptive optics science, 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 slow (∼0.03 Hz) off-axis dispersed fringe sensor (part of the acquisition guiding and wavefront sensing system’s active optics off-axis guider), and a pyramid wavefront sensor [PyWFS; part of the natural guide star wavefront sensor (NGWS) adaptive optics] to measure and correct the total path length between segment pairs, but these methods have yet to be tested “end-to-end” in a lab environment. We present the design and working prototype of a “GMT high contrast adaptive optics phasing testbed” that leverages the existing MagAO-X ExAO instrument to demonstrate segment phase sensing and simultaneous AO-control for high-contrast GMT natural guide star science [i.e., testing the NGWS wavefront sensor (WFS) architecture]. We present the first test results of closed-loop piston control with one GMT segment using MagAO-X’s PyWFS with and without simulated atmospheric turbulence. We show that the PyWFS was able to successfully control segment piston without turbulence within 12- to 33-nm RMS for 0 λ / D to 5 λ / D modulation, but was unsuccessful at controlling segment piston with generated ∼0.6 arcsec (median seeing conditions at the GMT site) and ∼1.2 arcsec seeing turbulence due to nonlinear modal cross-talk and poor pixel sampling of the segment gaps on the PyWFS detector. These results suggest that a PyWFS alone is not an ideal piston sensor for the GMT (and likely the TMT and ELT). Hence, a dedicated “second channel” piston sensor is required. We report the success of an alternate solution to control piston using a holographic dispersed fringe sensor (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 segment piston to within 50 nm RMS and ±10 μm dynamic range under simulated 0.6 arcsec atmospheric seeing (median seeing conditions at the GMT site). These results led to the inclusion of the HDFS as the official second channel piston sensor for the GMT NGWS WFS. This HDFS + PyWFS architecture should also work well to control piston petal modes on the ELT and TMT telescopes.

Key wavefront sensors features for laser-assisted tomographic adaptive optics systems on the Extremely Large Telescope

Laser guide star (LGS) wave-front sensing (LGSWFS) is a key element of tomographic adaptive optics system. However, when considering Extremely Large Telescope (ELT) scales, the LGS spot elongation becomes so large that it challenges the standard recipes to design LGSWFS. For classical Shack-Hartmann wave-front sensor (SHWFS), which is the current baseline for all ELT LGS-assisted instruments, a trade-off between the pupil spatial sampling [number of sub-apertures (SAs)], the SA field-of-view (FoV) and the pixel sampling within each SA is required. For ELT scales, this trade-off is also driven by strong technical constraints, especially concerning the available detectors and in particular their number of pixels. For SHWFS, a larger field of view per SA allows mitigating the LGS spot truncation, which represents a severe loss of performance due to measurement biases. For a given number of available detectors pixels, the SA FoV is competing with the proper sampling of the LGS spots, and/or the total number of SAs. We proposed a sensitivity analysis, and we explore how these parameters impacts the final performance. In particular, we introduce the concept of super resolution, which allows one to reduce the pupil sampling per WFS and opens an opportunity to propose potential LGSWFS designs providing the best performance for ELT scales.

Correction of Distorted Wavefront Using Dual Liquid Crystal Spatial Light Modulators

In space optical communication, owing to the influence of atmospheric turbulence, optical beams lose focus and become phase-distorted, which reduces the communication quality. Considering the polarization dependence of liquid crystal spatial light modulators and the dispersion effect of liquid crystal materials, the energy utilization rate of liquid crystal adaptive optics systems is low. In this study, a dual liquid crystal spatial light modulator adaptive optics system based on the GS algorithm is used to correct the wavefront distortion of a signal beam under different atmospheric turbulence intensities, and the Strehl ratio (SR) is used as the evaluation index. The simulation results show that the SR of the corrected system can be increased from 0.23, 0.41, and 0.72 to 0.77, 0.89, and 0.95, respectively. The corrected beam spot was more concentrated and the light intensity at the center of the beam spot was stronger. The experimental results show that, after the distortion wavefront is corrected by the dual liquid crystal spatial light modulator, the average gray value of the 10 × 10 pixels in the center of the spot increases from 159.3, 113.1, and 58.4 to 253.4, 247.7, and 198.3, respectively.

Subaru/IRCS L-band spectro-polarimetry of the HD 142527 disk scattered light

Abstract Spatially resolved L-band spectro-polarimetric observations were carried out toward a protoplanetary disk around the Herbig Fe star HD 142527 using the polarimetry mode of the InfraRed Camera and Spectrometer on the 8.2 m Subaru Telescope. The 2.9–4.1 μm polarimetric spectra of the bright central source and the scattered light at a distance of ${0{^{\prime \prime}_{.}}78}$ from the center toward the PA ∼ 290° were obtained using the AO188 adaptive optics system. The polarization spectra of the central region show no significant evidence of the polarization which is expected with the (non-aligned) hot dust emission near the central star, while the disk scattered light spectra shows linear polarization of $\sim\!\! 10\%$, which is consistent with the nature of scattered light. In addition, the polarization PA of ∼20° is also geometrically consistent with the scattered light from the central region. We also derived the flux density spectra of each region and confirmed the 3 μm water-ice absorption feature in the scattered light spectra, which is consistent with the findings in Honda et al. (2009, ApJ, 690, L110). By comparing the model predictions by Tazaki et al. (2021b, ApJ, 921, 173), we estimated the grain properties of the extracted disk region. Assuming the power-law size distribution of silicate and ice, the maximum grain size amax of 3 μm and fractional ice abundance relative to Pollack et al. (1994, ApJ, 421, 615) (fice) of ∼0.1 is inferred, which is consistent with the finding by Tazaki et al. ( 2021b, ApJ, 921, 173).

FAST: Fourier domain adaptive optics simulation tool for bidirectional ground-space optical links through atmospheric turbulence.

Free space optical links between the ground and space may be severely degraded by atmospheric turbulence. Adaptive Optics, a technique allowing partial correction of this degradation, is beginning to see use in the field with the potential to achieve more robust and higher bandwidth links. Here we present a simulation tool, FAST, which utilises an analytical Fourier domain Adaptive Optics model developed for astronomy. Using the reciprocity principle, the simulation may be applied either to downlink post-compensated or uplink pre-compensated beams. We show that FAST gives similar results to full end-to-end simulations with wave-optical propagation whilst being between 10 and 200 times faster, enabling the characterisation of optical links with complex Adaptive Optics systems in timely fashion.

Improving on Atmospheric Turbulence Profiles Derived from Dual Beacon Hartmann Turbulence Sensor Measurements

Atmospheric turbulence is an inevitable source of wavefront distortion in all fields of long range laser propagation and sensing. However, the distorting effects of turbulence can be corrected using wavefront sensors contained in adaptive optics systems. Such systems also provide deeper insight into surface layer turbulence, which is not well understood. A unique method of profile generation by a dual source Hartmann Turbulence Sensor (HTS) technique is introduced here. Measurements of optical turbulence along a horizontal path were taken to create Cn2 profiles. Two helium-neon laser beams were directed over an inhomogeneous horizontal path and captured by the HTS. The measured differential tilt variances imposed on the laser wavefronts were used in conjunction with a set of computed weighting functions to profile the turbulence over the sensing path. The weighting function matrix is inherently ill-conditioned, therefore, Tikhonov regularization was applied to produce accurate Cn2 profiles. A distribution of sonic anemometers and a co-located boundary layer scintillometer (BLS) collected independent Cn2 measurements to add confidence to the HTS profiles. The Cn2 profiles generated by this approach agree very well with the auxiliary anemometer and scintillometer measurements. This method of producing turbulence profiles may be useful in future multi-conjugate adaptive optics applications.

Predictive wavefront control on Keck II adaptive optics bench: on-sky coronagraphic results

The behavior of an adaptive optics (AO) system for ground-based high contrast imaging dictates the achievable contrast of the instrument. In conditions where the coherence time of the atmosphere is short compared with the speed of the AO system, the servo-lag error can become the dominant error term of the AO system. While the AO system measures the wavefront error and subsequently applies a correction (typically taking a total of one or a few milliseconds), the atmospheric turbulence above the telescope has changed resulting in the servo-lag error. In addition to reducing the Strehl ratio, the servo-lag error causes a build-up of speckles along the direction of the dominant wind vector in the coronagraphic image, severely limiting the contrast at small angular separations. One strategy to mitigate this problem is to predict the evolution of the turbulence over the delay time. Our predictive wavefront control algorithm minimizes, in a mean square sense, the wavefront error over the delay and has been implemented on the Keck II AO bench. We report on the latest results of our algorithm and discuss updates to the algorithm itself. We explore how to tune various filter parameters based on both daytime laboratory tests and on-sky tests. We show a reduction in residual-mean-square wavefront error for the predictor compared with the leaky integrator (the standard controller for Keck) implemented on Keck for three separate nights. Finally, we present contrast improvements for daytime and on-sky tests for the first time. Using the L-band vortex coronagraph for Keck’s NIRC2 instrument, we find a contrast gain of up to 2 at a separation of 3 λ / D and up to 3 for larger separations (3 − 7 λ / D).