Coronagraphic Extreme Adaptive Optics System Research Articles

SCExAO/CHARIS Multiwavelength High-contrast Imaging of the BD+45°598 Debris Disk

We present a multiwavelength (1.16–2.37 μm) view of the debris disk around BD+45°598, using the Subaru Coronagraphic Extreme Adaptive Optics system paired with the Coronagraphic High Angular Resolution Imaging Spectrograph. With an assumed age of 23 Myr, this source allows us to study the early evolution of debris disks and search for forming planets. We fit a scattered light model to our disk using a differential evolution algorithm, and constrain its geometry. We find the disk to have a peak density radius of R 0 = 109.6 au, an inclination of i = 88.1°, and position angle PA = 111.0°. While we do not detect a substellar companion in the disk, our calculated contrast limits indicate sensitivity to planets as small as ∼10M Jup at a projected separation of 12 au of the star, and as small as ∼4M Jup beyond 38 au. When measuring intensity as a function of wavelength, the disk color constrains the minimum dust grain size within a range of ∼0.13–1.01 μm.

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.

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.

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 10−2Hz 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.

SCExAO/CHARIS Near-IR High-contrast Imaging and Integral Field Spectroscopy of the HIP 79977 Debris Disk

Abstract We present new, near-infrared (1.1–2.4 μm) high-contrast imaging of the bright debris disk surrounding HIP 79977 with the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) coupled with the CHARIS integral field spectrograph. SCExAO/CHARIS resolves the disk down to smaller angular separations of (0.″11; r ∼ 14 au) and at a higher significance than previously achieved at the same wavelengths. The disk exhibits a marginally significant east–west brightness asymmetry in H band that requires confirmation. Geometrical modeling suggests a nearly edge-on disk viewed at a position angle of ∼114.°6 east of north. The disk is best-fit by scattered-light models assuming strongly forward-scattering grains (g ∼ 0.5–0.65) confined to a torus with a peak density at r 0 ∼ 53–75 au. We find that a shallow outer density power law of α out = −1 to −3 and flare index of β = 1 are preferred. Other disk parameters (e.g., inner density power law and vertical scale height) are more poorly constrained. The disk has a slightly blue intrinsic color and its profile is broadly consistent with predictions from birth ring models applied to other debris disks. While HIP 79977's disk appears to be more strongly forward-scattering than most resolved disks surrounding 5–30 Myr old stars, this difference may be due to observational biases favoring forward-scattering models for inclined disks versus lower inclination, ostensibly neutral-scattering disks like HR 4796A’s. Deeper, higher signal-to-noise SCExAO/CHARIS data can better constrain the disk’s dust composition.

On-Sky Demonstration of Low-Order Wavefront Sensing and Control with Focal Plane Phase Mask Coronagraphs

The ability to characterize exoplanets by spectroscopy of their atmospheres requires direct imaging techniques to isolate planet signal from the bright stellar glare. One of the limitations with the direct detection of exoplanets, either with ground- or space-based coronagraphs, is pointing errors and other low-order wavefront aberrations. The coronagraphic detection sensitivity at the diffraction limit therefore depends on how well low-order aberrations upstream of the focal plane mask are corrected. To prevent starlight leakage at the inner working angle of a phase mask coronagraph, we have introduced a Lyot-based low-order wavefront sensor (LLOWFS), which senses aberrations using the rejected starlight diffracted at the Lyot plane. In this article, we present the implementation, testing, and results of LLOWFS on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) at the Subaru Telescope. We have controlled 35 Zernike modes of a H-band vector vortex coronagraph in the laboratory and 10 Zernike modes on-sky with an integrator control law. We demonstrated a closed-loop pointing residual of 0.02 mas in the laboratory and 0.15 mas on-sky for data sampled using the minimal 2-s exposure time of the science camera. We have also integrated the LLOWFS in the visible high-order control loop of SCExAO, which in closed-loop operation has validated the correction of the noncommon path pointing errors between the infrared science channel and the visible wavefront sensing channel with pointing residual of 0.23 mas on-sky.