Extreme Adaptive Optics System Research Articles

The environment around young eruptive stars. SPHERE/IRDIS polarimetric imaging of seven protostars

Eruptive stars are a class of young stellar objects that show an abrupt increase in luminosity. These burst-like episodes are thought to dominate the stellar accretion process during the class 0 to class I stage. We present an overview of a survey of seven episodically accreting protostars carried out to study their potentially complex circumstellar surroundings. The observations were performed with the instrument SPHERE, mounted at the Very Large Telescope. SPHERE is equipped with an extreme adaptive optics system that allows high-contrast imaging. We observed the eruptive stars in the $H$ band with the near-infrared imager IRDIS and used the polarimeter to extract the polarized light scattered from the stars' surroundings. We produced polarized light images for three FUor objects, Z CMa, V960 Mon, and FU Ori, and four EXor objects, XZ Tau, UZ Tau, NY Ori, and EX Lup. We calculated the intrinsic polarization fraction for all the observed stars. In all systems we registered scattered light from around the primary star. FU Ori and V960 Mon are surrounded by complex structures, including spiral-like features. In Z CMa, we detected a point source 0 to the northeast of the primary. Based on the astrometric measurements from archival Keck/NIRC2 data, we find this source to be a third member of the system. Furthermore, Z CMa displays an outflow that extends for thousands of au. Unlike the other EXor objects in our sample, XZ Tau shows bright, extended scattered light structures that are also associated with an outflow on a scale of hundreds of au. The other EXors show relatively faint disk-like structures in the immediate vicinity of the coronagraph. Each object shows a unique environment, but we classified the seven objects into three categories: systems with illuminated outflows, asymmetric arms, and faint disks. Asymmetric arms were only found around FUor objects, while faint disks seem to predominantly occur around EXors. Importantly, for Z CMa the detection of the faint extended structure calls into question previous interpretations of the system's dynamic state. The streamer that was associated with a fly-by object turned out to be part of a huge outflow extending 6000 au.

Nonlinear Wave Front Reconstruction from a Pyramid Sensor using Neural Networks

The pyramid wave front sensor (PyWFS) has become increasingly popular to use in adaptive optics (AO) systems due to its high sensitivity. The main drawback of the PyWFS is that it is inherently nonlinear, which means that classic linear wave front reconstruction techniques face a significant reduction in performance at high wave front errors, particularly when the pyramid is unmodulated. In this paper, we consider the potential use of neural networks (NNs) to replace the widely used matrix vector multiplication (MVM) control. We aim to test the hypothesis that the NN's ability to model nonlinearities will give it a distinct advantage over MVM control. We compare the performance of a MVM linear reconstructor against a dense NN, using daytime data acquired on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) instrument. In a first set of experiments, we produce wavefronts generated from 14 Zernike modes and the PyWFS responses at different modulation radii (25, 50, 75, and 100 mas). We find that the NN allows for a far more precise wave front reconstruction at all modulations, with differences in performance increasing in the regime where the PyWFS nonlinearity becomes significant. In a second set of experiments, we generate a data set of atmosphere-like wavefronts, and confirm that the NN outperforms the linear reconstructor. The SCExAO real-time computer software is used as baseline for the latter. These results suggest that NNs are well positioned to improve upon linear reconstructors and stand to bring about a leap forward in AO performance in the near future.

MagAO-X and HST High-contrast Imaging of the AS209 Disk at Hα

The detection of emission lines associated with accretion processes is a direct method for studying how and where gas giant planets form, how young planets interact with their natal protoplanetary disk, and how volatile delivery to their atmosphere takes place. Hα (λ = 0.656 μm) is expected to be the strongest accretion line observable from the ground with adaptive optics systems, and is therefore the target of specific high-contrast imaging campaigns. We present MagAO-X and Hubble Space Telescope (HST) data obtained to search for Hα emission from the previously detected protoplanet candidate orbiting AS209, identified through Atacama Large Millimeter/submillimeter Array observations. No signal was detected at the location of the candidate, and we provide limits on its accretion. Our data would have detected an Hα emission with F Hα > 2.5 ± 0.3 × 10−16 erg s−1 cm−2, a factor 6.5 lower than the HST flux measured for PDS70 b. The flux limit indicates that if the protoplanet is currently accreting it is likely that local extinction from circumstellar and circumplanetary material strongly attenuates its emission at optical wavelengths. In addition, the data reveal the first image of the jet north of the star as expected from previous detections of forbidden lines. Finally, this work demonstrates that current ground-based observations with extreme adaptive optics systems can be more sensitive than space-based observations, paving the way to the hunt for small planets in reflected light with extremely large telescopes.

Analytical modelling of adaptive optics systems: Role of the influence function

Context. Adaptive optics (AO) is now a tool commonly deployed in astronomy. The real time correction of the atmospheric turbulence that AO enables allows telescopes to perform close to the diffraction limit at the core of their point spread function (PSF). Among other factors, AO-corrected PSFs depend on the ability of the wavefront corrector (WFC), generally a deformable mirror, to fit the incident wavefront corrugations. Aims. In this work, we focus on this error introduced by the WFC, the so-called fitting error. To date, analytical models only depend on the WFC cut-off frequency, and Monte Carlo simulations are the only solution for studying the impact of the WFC influence function shape on the AO-corrected PSF. We aim to develop an analytical model accounting for the influence function shape. Methods. We first obtain a general analytical model of the fitting error structure function. With additional hypotheses, we then derive an analytical model of the AO-corrected power spectral density. These two analytical solutions are compared with Monte Carlo simulations on different ideal profiles (piston, pyramid, Gaussian) as well as with real hardware (DM192 from ALPAO). Results. Our analytical predictions show a very good agreement with the Monte Carlo simulations. We show that in the image plane, the depth of the correction as well as the transition profile between the AO-corrected area and the remaining turbulent halo depend on the influence functions of the WFC. We also show that the generally assumed hypothesis of stationarity of the AO correction is actually not met. Conclusions. As the fitting error is the intrinsic optimal limit of an AO system, our analytical model allows for the assessment of the theoretical limits of extreme AO systems limited by the WFC in high-contrast imaging through a context where other errors become comparable.

Three-sided pyramid wavefront sensor, part II: preliminary demonstration on the new comprehensive adaptive optics and coronagraph test instrument testbed

The next generation of giant ground and space telescopes will have the light-collecting power to detect and characterize potentially habitable terrestrial exoplanets using high-contrast imaging for the first time. This will only be achievable if the performance of the Giant Segment Mirror Telescopes (GSMTs) extreme adaptive optics (ExAO) systems are optimized to their full potential. A key component of an ExAO system is the wavefront sensor (WFS), which measures aberrations from atmospheric turbulence. A common choice in current and next-generation instruments is the pyramid wavefront sensor (PWFS). ExAO systems require high spatial and temporal sampling of wavefronts to optimize performance and, as a result, require large detectors for the WFS. We present a closed-loop testbed demonstration of a three-sided pyramid wavefront sensor (3PWFS) as an alternative to the conventional four-sided pyramid wavefront (4PWFS) sensor for GSMT-ExAO applications on the innovative comprehensive adaptive optics and coronagraph test instrument (CACTI). The 3PWFS is less sensitive to read noise than the 4PWFS because it uses fewer detector pixels. The 3PWFS has further benefits: a high-quality three-sided pyramid optic is easier to manufacture than a four-sided pyramid. We describe the design of the two components of the CACTI system, the adaptive optics simulator and the PWFS testbed that includes both a 3PWFS and 4PWFS. We detail the error budget of the CACTI system, review its operation and calibration procedures, and discuss its current status. A preliminary experiment was performed on CACTI to study the performance of the 3PWFS to the 4PWFS in varying strengths of turbulence using both the raw intensity and slopes map signal processing methods. This experiment was repeated for a modulation radius of 1.6 and 3.25 λ / D. We found that the performance of the two wavefront sensors is comparable if modal loop gains are tuned.

Probing Photon Statistics in Adaptive Optics Images with SCExAO/MEC* * Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

We present an experimental study of photon statistics for high-contrast imaging with the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera (MEC) located behind the Subaru Coronagraphic Extreme Adaptive Optics System at the Subaru Telescope. We show that MEC measures the expected distributions for both on-axis companion intensity and off-axis intensity, which manifests as quasi-static speckles in the image plane and currently limits high-contrast imaging performance. These statistics can be probed by any MEC observation due to the photon-counting capabilities of MKID detectors. Photon arrival time statistics can also be used to directly distinguish companions from speckles using a post-processing technique called Stochastic Speckle Discrimination (SSD). Here, we give an overview of the SSD technique and highlight the first demonstration of SSD on an extended source—the protoplanetary disk AB Aurigae. We then present simulations that provide an in-depth exploration as to the current limitations of an extension of the SSD technique called Photon-Counting SSD (PCSSD) to provide a path forward for transitioning PCSSD from simulations to on-sky results. We end with a discussion of how to further improve the efficacy of such arrival-time-based post-processing techniques applicable to both MKIDs, as well as other high-speed astronomical cameras.

Low-wind-effect impact on Shack-Hartmann-based adaptive optics

Context. The low wind effect (LWE) occurs at the aperture of 8-meter class telescopes when the spiders holding the secondary mirror get significantly cooler than the air. The effect creates phase discontinuities in the incoming wavefront at the location of the spiders. Under the LWE, the wavefront residuals after correction of the adaptive optics (AO) are dominated by low-order aberrations, pistons, and tip-tilts, contained in the pupil quadrants separated by the spiders. Those aberrations, called petal modes, degrade the AO performances during the best atmospheric turbulence conditions. Ultimately, the LWE is an obstacle for high-contrast exoplanet observations at a small angular separation from the host star. Aims. We aim to understand why extreme AO with a Shack-Hartmann (SH) wavefront sensor fails to correct for the petal tip and tilt modes, while these modes imprint a measurable signal in the SH slopes. We explore if the petal tip and tilt content of the LWE can be controlled and mitigated without an additional wavefront sensor. Methods. We simulated the sensitivity of a single subaperture of a SH wavefront sensor in the presence of a phase discontinuity across this subaperture. We explored the effect of the most important parameters: the amplitude of the discontinuity, the spider thickness, and the field of view. We then performed end-to-end simulations to reproduce and explain the behavior of extreme AO systems based on a SH in the presence of the LWE. We then evaluated the efficiency of a new mitigation strategy by running simulations, including atmosphere and realistic LWE phase perturbations. Results. For realistic parameters (i.e. a spider thickness at 25% of a SH subaperture, and a field of view of 3.5λ/d), we find that the sensitivity of the SH to a phase discontinuity is dramatically reduced, or even reversed. Under the LWE, a nonzero curl path is created in the measured slopes, which transforms into vortex-structures in the residuals when the loop is closed. While these vortexes are easily seen in the residual wavefront and slopes, they cannot be controlled by the system. We used this understanding to propose a strategy for controlling the petal tip and tilt modes of the LWE by using the measurements from the SH, but excluding the faulty subapertures. Conclusions. The proposed mitigation strategy may be of use in all extreme AO systems based on SH for which the LWE is an issue, such as SPHERE and GRAVITY+.

Toward on-sky adaptive optics control using reinforcement learning

Context. 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 control laws of XAO systems leave strong residuals. Aims. Current AO control strategies such as static matrix-based wavefront reconstruction and integrator control suffer from a temporal delay error and are sensitive to mis-registration, that is, 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 (PSF). Methods. We extend previous work in reinforcement learning for AO. The improved method, called the Policy Optimization for Adaptive Optics (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 sensor (PWFS) for the 8-m and 40-m telescope aperture cases. We further implemented PO4AO and carried out experiments in a laboratory environment using Magellan Adaptive Optics eXtreme system (MagAO-X) at the Steward laboratory. Results. PO4AO provides the desired performance by improving the coronagraphic contrast in numerical simulations by factors of 3–5 within the control region of deformable mirror and PWFS, both in simulation and in the laboratory. The presented method is also quick to train, that is, on timescales of typically 5–10 s, and the inference time is sufficiently small (<ms) to be used in real-time control for XAO with currently available hardware even for extremely large telescopes.

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.

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Coronagraphic imaging of exoplanets and circumstellar environments using ground-based instruments on large telescopes is intrinsically limited by speckles induced by uncorrected aberrations. These aberrations originate from the imperfect correction of the atmosphere by an extreme adaptive optics system; from static optical defects; or from small opto-mechanical variations due to changes in temperature, pressure, or gravity vector. More than the speckles themselves, the performance of high-contrast imagers is ultimately limited by their temporal stability, since most post-processing techniques rely on difference of images acquired at different points in time. Identifying the origin of the aberrations and the timescales involved is therefore crucial to understanding the fundamental limits of dedicated high-contrast instruments. In previous works we demonstrated the use of a Zernike wavefront sensor called ZELDA for sensing non-common path aberrations (NCPA) in the VLT/SPHERE instrument. We now use ZELDA to investigate the stability of the instrumental aberrations using five long sequences of measurements obtained at high cadence on the internal calibration source. Our study reveals two regimes of decorrelation of the NCPA. The first, with a characteristic timescale of a few seconds and an amplitude of a few nanometers, is induced by a fast internal turbulence within the enclosure. The second is a slow quasi-linear decorrelation on the order of a few 10−3 nmrms s−1 that acts on timescales from minutes to hours. We use coronagraphic image reconstruction to demonstrate that these two NCPA contributions have a measurable impact on differences of images, and that the fast internal turbulence is a dominating term over to the slow linear decorrelation. We also use dedicated sequences where the derotator and atmospheric dispersion compensators emulate a real observation to demonstrate the importance of performing observations symmetric around the meridian, which minimizes speckle decorrelation, and therefore maximizes the sensitivity to point sources in difference of images.

Cascade adaptive optics: contrast performance analysis of a two-stage controller by numerical simulations

The contrast performance of current extreme adaptive optics (XAO) systems can be improved by adding a second AO correction stage featuring its own wavefront sensor (WFS), deformable mirror (DM), and real-time controller. We develop a dynamical model for such a cascade adaptive optics (CAO) system with two stages each controlled by a standard integrator and study its control properties. We study how such a configuration can improve an existing system without modifying the first stage. We analyze the CAO architecture in general and show how part of the disturbance is transferred from low to high temporal frequencies with a nefarious effect of the second stage integrator overshoot and suggest possible ways to mitigate this. We also carry out numerical simulations of the particular case of a first stage AO using a Shack–Hartmann WFS and a second stage AO with a smaller DM running at a higher framerate to reduce temporal error. In this case, we demonstrate that the second stage improves imaging contrast by one order of magnitude and shortens the decorrelation time of atmospheric turbulence speckles by even a greater factor. The results show that CAO presents a promising and relatively simple way to upgrade some existing XAO systems and achieve improved imaging contrasts fostering a large number of science case including the direct imaging of exoplanets.

First observation of a quadruple asteroid

Context. Extreme adaptive optics systems, such as the Spectro-Polarimetric High-contrast Exoplanet REsearch facility (SPHERE), push forward the limits in high contrast and high resolution in direct imaging. The main objectives of these instruments are exoplanet detection and characterisation. Aims. We aim to increase the contrast limits to detect new satellites orbiting known asteroids. We use cutting-edge data reduction techniques and data processing algorithms that are essential to best analyse the raw data provided by the instruments and increase their performances. Doing so, the unequalled performances of SPHERE also make it a unique tool to resolve and study asteroids in the solar system, expanding the domain of its main science targets. Methods. We applied a newly developed data reduction pipeline for integral field spectrographs on archival SPHERE data of a resolved asteroid, (130) Elektra. It was coupled with a dedicated point spread function reconstruction algorithm to model the asteroid halo. Following the halo removal, the moon signal could be extracted more accurately. The moon positions were fitted at three epochs and were used to derive the orbital parameters via a genetic-based algorithm. Results. We announce the discovery of S/2014 (130) 2, a third moon orbiting (130) Elektra, making it the first quadruple asteroid ever found. It is identified in three different epochs, 9, 30, and 31 Dec. 2014, at a respective angular separation of 258 mas (333 km), 229 mas (327 km), and 319 mas (457 km). We estimate that this moon has a period of 0.679 ± 0.001 day and a semi-major axis of 344 ± 5 km, with an eccentricity of 0.33 ± 0.05 and an inclination of 38° ±19° compared to the primary rotation axis. With a relative magnitude to the primary of 10.5 ± 0.5, its size is estimated to be 1.6 ± 0.4 km. Conclusions. The orbital parameters of S/2014 (130) 2 are poorly constrained due to the unfavourable configurations of the available fragmentary data. Additional observations are needed to better estimate its orbit and to suggest a formation model. This new detection nonetheless shows that dedicated data reduction and processing algorithms modelling the physics of the instruments can push their contrast limits further.

The Scorpion Planet Survey: Wide-orbit Giant Planets Around Young A-type Stars

Abstract The first directly imaged exoplanets indicated that wide-orbit giant planets could be more common around A-type stars. However, the relatively small number of nearby A-stars has limited the precision of exoplanet demographics studies to ≳10%. We aim to constrain the frequency of wide-orbit giant planets around A-stars using the VLT/SPHERE extreme adaptive optics system, which enables targeting ≳100 A-stars between 100 and 200 pc. We present the results of a survey of 84 A-stars within the nearby ∼5–17 Myr old Sco OB2 association. The survey detected three companions—one of which is a new discovery (HIP 75056Ab), whereas the other two (HD 95086b and HIP 65426b) are now-known planets that were included without a priori knowledge of their existence. We assessed the image sensitivity and observational biases with injection and recovery tests combined with Monte Carlo simulations to place constraints on the underlying demographics. We measure a decreasing frequency of giant planets with increasing separation, with measured values falling between 10% and 2% for separations of 30–100 au, and 95% confidence-level upper limits of ≲45%–8% for planets on 30–100 au orbits, and ≲5% between 200 and 500 au. These values are in excellent agreement with recent surveys of A-stars in the solar neighborhood−supporting findings that giant planets out to separations of ≲100 au are more frequent around A-stars than around solar-type hosts. Finally, the relatively low occurrence rate of super-Jupiters on wide orbits, the positive correlation with stellar mass, and the inverse correlation with orbital separation are consistent with core accretion being their dominant formation mechanism.

Multiband Imaging of the HD 36546 Debris Disk: A Refined View from SCExAO/CHARIS*

Abstract We present the first multiwavelength (near-infrared; 1.1–2.4 μm) imaging of HD 36546's debris disk, using the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system coupled with the Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS). As a 3–10 Myr old star, HD 36546 presents a rare opportunity to study a debris disk at very early stages. SCExAO/CHARIS imagery resolves the disk over angular separations of ρ ∼ 0.″25–1.″0 (projected separations of rproj ∼ 25–101 au) and enables the first spectrophotometric analysis of the disk. The disk’s brightness appears symmetric between its eastern and western extents, and it exhibits slightly blue near-infrared colors on average (e.g., J−K = −0.4 ± 0.1)—suggesting copious submicron-sized or highly porous grains. Through detailed modeling adopting a Hong scattering phase function (SPF), instead of the more common Henyey–Greenstein function, and using the differential evolution optimization algorithm, we provide an updated schematic of HD 36546's disk. The disk has a shallow radial dust density profile (α in ≈ 1.0 and α out ≈ −1.5), a fiducial radius of r 0 ≈ 82.7 au, an inclination of i ≈ 79.°1, and a position angle of PA ≈ 80.°1. Through spine tracing, we find a spine that is consistent with our modeling, but also with a “swept-back wing” geometry. Finally, we provide constraints on companions, including limiting a companion responsible for a marginal Hipparcos–Gaia acceleration to a projected separation of ≲0.″2 and to a minimum mass of ≲11 M Jup.

First on-sky demonstration of spatial Linear Dark Field Control with the vector-Apodizing Phase Plate at Subaru/SCExAO

Context. One of the key noise sources that currently limits high-contrast imaging observations for exoplanet detection is quasi-static speckles. Quasi-static speckles originate from slowly evolving non-common path aberrations (NCPA). These NCPA are related to the different optics encountered in the wavefront sensing path and the science path, and they also exhibit a chromatic component due to the difference in the wavelength between the science camera and the main wavefront sensor. These speckles degrade the contrast in the high-contrast region (or dark hole) generated by the coronagraph and make the calibration in post-processing more challenging. Aims. The purpose of this work is to present a proof-of-concept on-sky demonstration of spatial Linear Dark Field Control (LDFC). The ultimate goal of LDFC is to stabilize the point spread function by addressing NCPA using the science image as additional wavefront sensor. Methods. We combined spatial LDFC with the Asymmetric Pupil vector-Apodizing Phase Plate (APvAPP) on the Subaru Coronagraphic Extreme Adaptive Optics system at the Subaru Telescope. To allow for rapid prototyping and easy interfacing with the instrument, LDFC was implemented in Python. This limited the speed of the correction loop to approximately 20 Hz. With the APvAPP, we derive a high-contrast reference image to be utilized by LDFC. LDFC is then deployed on-sky to stabilize the science image and maintain the high-contrast achieved in the reference image. Results. In this paper, we report the results of the first successful proof-of-principle LDFC on-sky tests. We present results from two types of cases: (1) correction of instrumental errors and atmospheric residuals plus artificially induced static aberrations introduced on the deformable mirror and (2) correction of only atmospheric residuals and instrumental aberrations. When introducing artificial static wavefront aberrations on the DM, we find that LDFC can improve the raw contrast by a factor of 3–7 over the dark hole. In these tests, the residual wavefront error decreased by ∼50 nm RMS, from ∼90 nm to ∼40 nm RMS. In the case with only residual atmospheric wavefront errors and instrumental aberrations, we show that LDFC is able to suppress evolving aberrations that have timescales of < 0.1–0.4 Hz. We find that the power at 10−2 Hz is reduced by a factor of ∼20, 7, and 4 for spatial frequency bins at 2.5, 5.5, and 8.5λ/D, respectively. Conclusions. We have identified multiplied challenges that have to be overcome before LDFC can become an integral part of science observations. The results presented in this work show that LDFC is a promising technique for enabling the high-contrast imaging goals of the upcoming generation of extremely large telescopes.

Self-optimizing adaptive optics control with reinforcement learning for high-contrast imaging

Current and future high-contrast imaging instruments require extreme adaptive optics systems to reach contrasts necessary to directly imaged exoplanets. Telescope vibrations and the temporal error induced by the latency of the control loop limit the performance of these systems. One way to reduce these effects is to use predictive control. We describe how model-free reinforcement learning can be used to optimize a recurrent neural network controller for closed-loop predictive control. First, we verify our proposed approach for tip–tilt control in simulations and a lab setup. The results show that this algorithm can effectively learn to mitigate vibrations and reduce the residuals for power-law input turbulence as compared to an optimal gain integrator. We also show that the controller can learn to minimize random vibrations without requiring online updating of the control law. Next, we show in simulations that our algorithm can also be applied to the control of a high-order deformable mirror. We demonstrate that our controller can provide two orders of magnitude improvement in contrast at small separations under stationary turbulence. Furthermore, we show more than an order of magnitude improvement in contrast for different wind velocities and directions without requiring online updating of the control law.

Characterizing deformable mirrors for the MagAO-X instrument

The MagAO-X instrument is an extreme adaptive optics system for high-contrast imaging at visible- and near-infrared wavelengths on the Magellan Clay Telescope. A central component of this system is a 2040-actuator microelectromechanical deformable mirror (DM) from Boston Micromachines Corp. that operates at 3.63 kHz for high-order wavefront control (the tweeter). Two additional DMs from ALPAO perform the low-order (the woofer) and non-common-path science-arm wavefront correction (the NCPC DM). Prior to integration with the instrument, we characterized these devices using a Zygo Verifire Interferometer to measure each DM surface. We present the results of the characterization effort here, demonstrating the ability to drive the tweeter to a flat of 6.9 nm root-mean-square (RMS) surface (and 0.56 nm RMS surface within its control bandwidth), the woofer to 2.2-nm RMS surface, and the NCPC DM to 2.1-nm RMS surface over the MagAO-X beam footprint on each device. Using focus-diversity phase retrieval on the MagAO-X science cameras to estimate the internal instrument wavefront error, we further show that the integrated DMs correct the instrument WFE to 18.7 nm RMS, which, combined with a 11.7% pupil amplitude RMS, produces a Strehl ratio of 0.94 at Hα.

Calibration of residual aberrations in exoplanet imagers with large numbers of degrees of freedom

Imaging faint objects, such as exoplanets or disks, around nearby stars is extremely challenging because host star images are dominated by the telescope diffraction pattern. Using a coronagraph is an efficient solution for removing diffraction but requires an incoming wavefront with good quality to maximize starlight rejection. On the ground, the most advanced exoplanet imagers use extreme adaptive optics (ExAO) systems that are based on a deformable mirror (DM) with a large number of actuators to efficiently compensate for high-order aberrations and provide diffraction-limited images. While several exoplanet imagers with DMs using ∼1500 actuators are now routinely operating on large telescopes to observe gas giant planets, future systems may require a tenfold increase in the number of degrees of freedom to look for rocky planets. In this paper, we explore wavefront correction with a secondary adaptive optics system that controls a very large number of degrees of freedom that are not corrected by the primary ExAO system. Using Marseille Imaging Testbed for High Contrast (MITHiC), we implement a second stage of adaptive optics with ZELDA, a Zernike wavefront sensor, and a spatial light modulator to compensate for the phase aberrations of the bench downstream residual aberrations from adaptive optics. We demonstrate that their correction up to 137 cycles per pupil with nanometric accuracy is possible, provided there is a simple distortion calibration of the pupil and a moderate wavefront low-pass filtering. We also use ZELDA for a fast compensation of ExAO residuals, showing its promising implementation as a second-stage correction for the observation of rocky planets around nearby stars. Finally, we present images with a classical Lyot coronagraph on MITHiC and validate our ability to reach its theoretical performance with our calibration.

Diffraction-limited integral-field spectroscopy for extreme adaptive optics systems with the multicore fiber-fed integral-field unit

Direct imaging instruments have the spatial resolution to resolve exoplanets from their host star. This enables direct characterization of the exoplanets atmosphere, but most direct imaging instruments do not have spectrographs with high enough resolving power for detailed atmospheric characterization. We investigate the use of a single-mode diffraction-limited integral-field unit that is compact and easy to integrate into current and future direct imaging instruments for exoplanet characterization. This achieved by making use of recent progress in photonic manufacturing to create a single-mode fiber-fed image reformatter. The fiber link is created with three-dimensional printed lenses on top of a single-mode multicore fiber that feeds an ultrafast laser inscribed photonic chip that reformats the fiber into a pseudoslit. We then couple it to a first-order spectrograph with a triple stacked volume phase holographic grating for a high efficiency over a large bandwidth. The prototype system has had a successful first-light observing run at the 4.2-m William Herschel Telescope. The measured on-sky resolving power is between 2500 and 3000, depending on the wavelength. With our observations, we show that single-mode integral-field spectroscopy is a viable option for current and future exoplanet imaging instruments.

SCExAO/CHARIS Direct Imaging Discovery of a 20 au Separation, Low-mass Ratio Brown Dwarf Companion to an Accelerating Sun-like Star *

Abstract We present the direct imaging discovery of a substellar companion to the nearby Sun-like star, HD 33632 Aa, at a projected separation of ∼20 au, obtained with SCExAO/CHARIS integral field spectroscopy complemented by Keck/NIRC2 thermal infrared imaging. The companion, HD 33632 Ab, induces a 10.5σ astrometric acceleration on the star as detected with the Gaia and Hipparcos satellites. SCExAO/CHARIS JHK (1.1–2.4 μm) spectra and Keck/NIRC2 L p (3.78 μm) photometry are best matched by a field L/T transition object: an older, higher-gravity, and less dusty counterpart to HR 8799 cde. Combining our astrometry with Gaia/Hipparcos data and archival Lick Observatory radial velocities, we measure a dynamical mass of 46.4 ± 8 M J and an eccentricity of e < 0.46 at 95% confidence. HD 33632 Ab’s mass and mass ratio (4.0% ± 0.7%) are comparable to the low-mass brown dwarf GJ 758 B and intermediate between the more massive brown dwarf HD 19467 B and the (near-)planet-mass companions to HR 2562 and GJ 504. Using Gaia to select for direct imaging observations with the newest extreme adaptive optics systems can reveal substellar or even planet-mass companions on solar system–like scales at an increased frequency compared to blind surveys.