arXiv subjects

Kyohoon Ahn

Publications and source records attributed to Kyohoon Ahn.

36 records · Page 2Linked to original sources

arXiv2023

Nonlinear wavefront reconstruction from a pyramid sensor using neural networks

The pyramid wavefront 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 wavefront reconstruction techniques face a significant reduction in performance at high wavefront 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 neural network (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 wavefront 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 dataset 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.

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arXiv2023

Experimental demonstration of spectral linear dark field control at NASA's high contrast imaging testbeds

Due to the low flux of exoEarths, long exposure times are required to spectrally characterize them. During these long exposures, the contrast in the dark hole will degrade as the the optical system drifts from its initial DH state. To prevent such contrast drift, a wavefront sensing and control (WFSC) algorithm running in parallel to the science acquisition can stabilize the contrast. However, pairwise probing (PWP) cannot be reused to efficiently stabilize the contrast since it relies on strong temporal modulation of the intensity in the image plane, which would interrupt the science acquisition. The use of small amplitude probes has been demonstrated but requires multiple measurements from each science sub-band to converge. Conversely, spectral linear dark field control (LDFC) takes advantage of the linear relationship between the change in intensity of the post-coronagraph out-of-band image and small changes in wavefront in the science band to preserve the DH region during science exposures. In this paper, we show experimental results that demonstrate spectral LDFC stabilizes the contrast to levels of a few $10^{-9}$ on a Lyot coronagraph testbed which is housed in a vacuum chamber. Promising results show that spectral LDFC is able to correct for disturbances that degrade the contrast by more than 100$\times$. To our knowledge, this is the first experimental demonstration of spectral LDFC and the first demonstration of spatial or spectral LDFC on a vacuum coronagraph testbed and at contrast levels less than $10^{-8}$.

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arXiv2023

Photonic spectro-interferometry with SCExAO/FIRST at the Subaru Telescope: towards H-alpha imaging of protoplanets

FIRST is a post Extreme Adaptive-Optics (ExAO) spectro-interferometer operating in the Visible (600-800 nm, R~400). Its exquisite angular resolution (a sensitivity analysis of on-sky data shows that bright companions can be detected down to 0.25lambda/D) combined with its sensitivity to pupil phase discontinuities (from a few nm up to dozens of microns) makes FIRST an ideal self-calibrated solution for enabling exoplanet detection and characterization in the future. We present the latest on-sky results along with recent upgrades, including the integration and on-sky test of a new spectrograph (R~3,600) optimized for the detection of H-alpha emission from young exoplanets accreting matter.

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arXiv2023

Single-aperture spectro-interferometry in the visible at the Subaru telescope with FIRST: First on-sky demonstration on Keho'oea (α Lyrae) and Hokulei (α Aurigae)

FIRST is a spectro-interferometer combining, in the visible, the techniques of aperture masking and spatial filtering thanks to single-mode fibers. This instrument aims to deliver high contrast capabilities at spatial resolutions that are inaccessible to classical coronagraphic instruments. The technique implemented is called pupil remapping: the telescope is divided into subpupils by a segmented deformable mirror conjugated to a micro-lens array injecting light into single-mode fibers. The fiber outputs are rearranged in a nonredundant configuration, allowing simultaneous measurement of all baseline fringe patterns. The fringes are also spectrally dispersed, increasing the coherence length and providing precious spectral information. The optical setup of the instrument has been adapted to fit onto the SCExAO platform at the Subaru Telescope. We present the first on-sky demonstration of the FIRST instrument at the Subaru telescope. We used eight subapertures, each with a diameter of about 1 m. Closure phase measurements were extracted from the interference pattern to provide spatial information on the target. We tested the instrument on two types of targets : a point source (Keho'oea) and a binary system (Hokulei). An average accuracy of 0.6 degree is achieved on the closure phase measurements of Keho'oea, with a statistical error of about 0.15 degree at best. We estimate that the instrument can be sensitive to structures down to a quarter of the telescope spatial resolution. We measured the relative positions of Hokulei Aa and Ab with an accuracy about 1 mas. FIRST opens new observing capabilities in the visible wavelength range at the Subaru Telescope. With SCExAO being a testing platform for high contrast imaging instrumentation for future 30-meter class telescopes, FIRST is an important stepping stone for future interferometric instrumentation on extremely large telescopes.

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arXiv2022

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

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 $64\times64$ 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 $64\times64$ 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.

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arXiv2022

Probing Photon Statistics in Adaptive Optics Images with SCExAO/MEC

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 (SCExAO) 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 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.

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arXiv2022

Controlling petals using fringes: discontinuous wavefront sensing through sparse aperture interferometry at Subaru/SCExAO

Low wind and petaling effects, caused by the discontinuous apertures of telescopes, are poorly corrected -- if at all -- by commonly used workhorse wavefront sensors (WFSs). Wavefront petaling breaks the coherence of the point spread function core, splitting it into several side lobes, dramatically shutting off scientific throughput. We demonstrate the re-purposing of non-redundant sparse aperture masking (SAM) interferometers into low-order WFSs complementing the high-order pyramid WFS, on the SCExAO experimental platform at Subaru Telescope. The SAM far-field interferograms formed from a 7-hole mask are used for direct retrieval of petaling aberrations, which are almost invisible to the main AO loop. We implement a visible light dual-band SAM mode, using two disjoint 25 nm wide channels, that we recombine to overcome the one-lambda ambiguity of fringe-tracking techniques. This enables a control over petaling with sufficient capture range yet without conflicting with coronagraphic modes in the near-infrared. We present on-sky engineering results demonstrating that the design is able to measure petaling well beyond the range of a single-wavelength equivalent design.

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arXiv2022

Advanced wavefront sensing and control demonstration with MagAO-X

The search for exoplanets is pushing adaptive optics systems on ground-based telescopes to their limits. Currently, we are limited by two sources of noise: the temporal control error and non-common path aberrations. First, the temporal control error of the AO system leads to a strong residual halo. This halo can be reduced by applying predictive control. We will show and described the performance of predictive control with the 2K BMC DM in MagAO-X. After reducing the temporal control error, we can target non-common path wavefront aberrations. During the past year, we have developed a new model-free focal-plane wavefront control technique that can reach deep contrast (<1e-7 at 5 $λ$/D) on MagAO-X. We will describe the performance and discuss the on-sky implementation details and how this will push MagAO-X towards imaging planets in reflected light. The new data-driven predictive controller and the focal plane wavefront controller will be tested on-sky in April 2022.

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arXiv2022

High Contrast Imaging at the Photon Noise Limit with WFS-based PSF Calibration

Speckle Noise is the dominant source of error in high contrast imaging with adaptive optics system. We discuss the potential for wavefront sensing telemetry to calibrate speckle noise with sufficient precision and accuracy so that it can be removed in post-processing of science images acquired by high contrast imaging instruments. In such a self-calibrating system, exoplanet detection would be limited by photon noise and be significantly more robust and efficient than in current systems. We show initial laboratory and on-sky tests, demonstrating over short timescale that residual speckle noise is indeed calibrated to an accuracy exceeding readout and photon noise in the high contrast region. We discuss immplications for the design of space and ground high-contrast imaging systems.

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arXiv2022

High Contrast and High Angular Imaging at Subaru Telescope

Adaptive Optics projects at Subaru Telescope span a wide field of capabilities ranging from ground-layer adaptive optics (GLAO) providing partial correction over a 20 arcmin FOV to extreme adaptive optics (ExAO) for exoplanet imaging. We describe in this paper current and upcoming narrow field-of-view capabilities provided by the Subaru Extreme Adaptive Optics Adaptive Optics (SCExAO) system and its instrument modules, as well as the upcoming 3000-actuator upgrade of the Nasmyth AO system.

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arXiv2022

A Visible-light Lyot Coronagraph for SCExAO/VAMPIRES

We describe the design and initial results from a visible-light Lyot coronagraph for SCExAO/VAMPIRES. The coronagraph is comprised of four hard-edged, partially transmissive focal plane masks with inner working angles of 36 mas, 55 mas, 92 mas, and 129 mas, respectively. The Lyot stop is a reflective, undersized design with a geometric throughput of 65.7%. Our preliminary on-sky contrast is 1e-2 at 0.1" to 1e-4 at 0.75" for all mask sizes. The coronagraph was deployed in early 2022 and is available for open use.

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arXiv2022

Direct Imaging Discovery and Dynamical Mass of a Substellar Companion Orbiting an Accelerating Hyades Sun-like Star with SCExAO/CHARIS

We present the direct-imaging discovery of a substellar companion in orbit around a Sun-like star member of the Hyades open cluster. So far, no other substellar companions have been unambiguously confirmed via direct imaging around main-sequence stars in Hyades. The star HIP 21152 is an accelerating star as identified by the astrometry from the Gaia and Hipparcos satellites. We have detected the companion, HIP 21152 B, in multi-epoch using the high-contrast imaging from SCExAO/CHARIS and Keck/NIRC2. We have also obtained the stellar radial-velocity data from the Okayama 188cm telescope. The CHARIS spectroscopy reveals that HIP 21152 B's spectrum is consistent with the L/T transition, best fit by an early T dwarf. Our orbit modeling determines the semi-major axis and the dynamical mass of HIP 21152 B to be 17.5$^{+7.2}_{-3.8}$ au and 27.8$^{+8.4}_{-5.4}$ $M_{\rm{Jup}}$, respectively. The mass ratio of HIP 21152 B relative to its host is $\approx$2\%, near the planet/brown dwarf boundary suggested from recent surveys. Mass estimates inferred from luminosity evolution models are slightly higher (33--42 $M_{\rm{Jup}}$). With a dynamical mass and a well-constrained age due to the system's Hyades membership, HIP 21152 B will become a critical benchmark in understanding the formation, evolution, and atmosphere of a substellar object as a function of mass and age. Our discovery is yet another key proof-of-concept for using precision astrometry to select direct imaging targets.

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arXiv2021

High contrast imaging wavefront sensor referencing from coronagraphic images

A key challenge of high contrast imaging (HCI) is to differentiate a speckle from an exoplanet signal. The sources of speckles are a combination of atmospheric residuals and aberrations in the non-common path. Those non-common path aberrations (NCPA) are particularly challenging to compensate for as they are not directly measured, and because they include static, quasi-static and dynamic components. The proposed method directly addresses the challenge of compensating the NCPA. The algorithm DrWHO - Direct Reinforcement Wavefront Heuristic Optimisation - is a quasi-real-time compensation of static and dynamic NCPA for boosting image contrast. It is an image-based lucky imaging approach, aimed at finding and continuously updating the ideal reference of the wavefront sensor (WFS) that includes the NCPA, and updating this new reference to the WFS. Doing so changes the point of convergence of the AO loop. We show here the first results of a post-coronagraphic application of DrWHO. DrWHO does not rely on any model nor requires accurate wavefront sensor calibration, and is applicable to non-linear wavefront sensing situations. We present on-sky performances using a pyramid WFS sensor with the Subaru coronagraph extreme AO (SCExAO) instrument.

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arXiv2021

On-sky validation of image-based adaptive optics wavefront sensor referencing

Differentiating between an exoplanet signal and residual speckle noise is a key challenge in high-contrast imaging. Speckles are due to a combination of fast, slow and static wavefront aberrations introduced by atmospheric turbulence and instrument optics. While wavefront control techniques developed over the last decade have shown promise in minimizing fast atmospheric residuals, slow and static aberrations such as non-common path aberrations (NCPAs) remain a key limiting factor for exoplanet detection. NCPA are not seen by the wavefront sensor (WFS) of the adaptive optics (AO) loop, hence the difficulty in correcting them. We propose to improve the identification and rejection of those aberrations. The algorithm DrWHO, performs frequent compensation of static and quasi-static aberrations to boost image contrast. By changing the WFS reference at every iteration of the algorithm, DrWHO changes the AO point of convergence to lead it towards a compensation of the static and slow aberrations. References are calculated using an iterative lucky-imaging approach, where each iteration updates the WFS reference, ultimately favoring high-quality focal plane images. We validate this concept through numerical simulations and on-sky testing on the SCExAO instrument at the 8.2-m Subaru telescope. Simulations show a rapid convergence towards the correction of 82% of the NCPAs. On-sky tests are performed over a 10-minute run in the visible (750 nm). We introduce a flux concentration (FC) metric to quantify the point spread function (PSF) quality and measure a 15.7% improvement. The DrWHO algorithm is a robust focal-plane wavefront sensing calibration method that has been successfully demonstrated on sky. It does not rely on a model nor requires wavefront sensor calibration or linearity. It is compatible with different wavefront control methods, and can be further optimized for speed and efficiency.

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arXiv2021

High contrast imaging at the photon noise limit with self-calibrating WFS/C systems

High contrast imaging (HCI) systems rely on active wavefront control (WFC) to deliver deep raw contrast in the focal plane, and on calibration techniques to further enhance contrast by identifying planet light within the residual speckle halo. Both functions can be combined in an HCI system and we discuss a path toward designing HCI systems capable of calibrating residual starlight at the fundamental contrast limit imposed by photon noise. We highlight the value of deploying multiple high-efficiency wavefront sensors (WFSs) covering a wide spectral range and spanning multiple optical locations. We show how their combined information can be leveraged to simultaneously improve WFS sensitivity and residual starlight calibration, ideally making it impossible for an image plane speckle to hide from WFS telemetry. We demonstrate residual starlight calibration in the laboratory and on-sky, using both a coronagraphic setup, and a nulling spectro-interferometer. In both case, we show that bright starlight can calibrate residual starlight.

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arXiv2021

SCExAO, a testbed for developing high-contrast imaging technologies for ELTs

To directly detect exoplanets and protoplanetary disks, the development of high accuracy wavefront sensing and control (WFS&C) technologies is essential, especially for ground-based Extremely Large Telescopes (ELTs). The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a high-contrast imaging platform to discover and characterize exoplanets and protoplanetary disks. It also serves as a testbed to validate and deploy new concepts or algorithms for high-contrast imaging approaches for ELTs, using the latest hardware and software technologies on an 8-meter class telescope. SCExAO is a multi-band instrument, using light from 600 to 2500 nm, and delivering a high Strehl ratio (>80% in median seeing in H-band) downstream of a low-order correction provided by the facility AO188. Science observations are performed with coronagraphs, an integral field spectrograph, or single aperture interferometers. The SCExAO project continuously reaches out to the community for development and upgrades. Existing operating testbeds such as the SCExAO are also unique opportunities to test and deploy the new technologies for future ELTs. We present and show a live demonstration of the SCExAO capabilities (Real-time predictive AO control, Focal plane WFS&C, etc) as a host testbed for the remote collaborators to test and deploy the new WFS&C concepts or algorithms. We also present several high-contrast imaging technologies that are under development or that have already been demonstrated on-sky.

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arXiv2021

A New Type of Exoplanet Direct Imaging Search: The SCExAO/CHARIS Survey of Accelerating Stars

We present first results from a new exoplanet direct imaging survey being carried out with the Subaru Coronagraphic Extreme Adaptive Optics project (SCExAO) coupled to the CHARIS integral field spectrograph and assisted with Keck/NIRC2, targeting stars showing evidence for an astrometric acceleration from the Hipparcos and Gaia satellites. Near-infrared spectra from CHARIS and thermal infrared photometry from NIRC2 constrain newly-discovered companion spectral types, temperatures, and gravities. Relative astrometry of companions from SCExAO/CHARIS and NIRC2 and absolute astrometry of the star from Hipparcos and Gaia together yield direct dynamical mass constraints. Even in its infancy, our survey has already yielded multiple discoveries, including at least one likely jovian planet. We describe how our nascent survey is yielding a far higher detection rate than blind surveys from GPI and SPHERE, mass precisions reached for known companions, and the path forward for imaging and characterizing planets at lower masses and smaller orbital separations than previously possible.

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arXiv2021

Full characterization of the instrumental polarization effects of the spectropolarimetric mode of SCExAO-CHARIS

SCExAO at the Subaru telescope is a visible and near-infrared high-contrast imaging instrument employing extreme adaptive optics and coronagraphy. The instrument feeds the near-infrared light (JHK) to the integral-field spectrograph CHARIS. The spectropolarimetric capability of CHARIS is enabled by a Wollaston prism and is unique among high-contrast imagers. We present a detailed Mueller matrix model describing the instrumental polarization effects of the complete optical path, thus the telescope and instrument. From measurements with the internal light source, we find that the image derotator (K-mirror) produces strongly wavelength-dependent crosstalk, in the worst case converting ~95% of the incident linear polarization to circularly polarized light that cannot be measured. Observations of an unpolarized star show that the magnitude of the instrumental polarization of the telescope varies with wavelength between 0.5% and 1%, and that its angle is exactly equal to the altitude angle of the telescope. Using physical models of the fold mirror of the telescope, the half-wave plate, and the derotator, we simultaneously fit the instrumental polarization effects in the 22 wavelength bins. Over the full wavelength range, our model currently reaches a total polarimetric accuracy between 0.08% and 0.24% in the degree of linear polarization. We propose additional calibration measurements to improve the polarimetric accuracy to <0.1% and plan to integrate the complete Mueller matrix model into the existing CHARIS post-processing pipeline. Our calibrations of CHARIS' spectropolarimetric mode will enable unique quantitative polarimetric studies of circumstellar disks and planetary and brown dwarf companions.

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