Alexander T. Rodack

Modeling coronagraphic extreme wavefront control systems for high contrast imaging in ground and space telescope missions

The challenges of high contrast imaging (HCI) for detecting exoplanets for both ground and space applications can be met with extreme adaptive optics (ExAO), a high-order adaptive optics system that performs wavefront sensing (WFS) and correction at high speed. We describe 2 ExAO optical system designs, one each for ground- based telescopes and space-based missions, and examine them using the angular spectrum Fresnel propagation module within the Physical Optics Propagation in Python (POPPY) package. We present an end-to-end (E2E) simulation of the MagAO-X instrument, an ExAO system capable of delivering 6x10-5 visible-light raw contrast for static, noncommon path aberrations without atmosphere. We present an E2E simulation of a laser guidestar (LGS) companion spacecraft testbed demonstration, which uses a remote beacon to increase the signal available for WFS and control of the primary aperture segments of a future large space telescope, providing of order 10 factor improvement for relaxing observatory stability requirements.

MagAO-X: project status and first laboratory results

MagAO-X is an entirely new extreme adaptive optics system for the Magellan Clay 6.5 m telescope, funded by the NSF MRI program starting in Sep 2016. The key science goal of MagAO-X is high-contrast imaging of accreting protoplanets at Hα. With 2040 actuators operating at up to 3630 Hz, MagAO-X will deliver high Strehls (> 70%), high resolution (19 mas), and high contrast (< 1 × 10-4 ) at Hα (656 nm). We present an overview of the MagAO-X system, review the system design, and discuss the current project status.

Focal plane wavefront sensing and control strategies for high-contrast imaging on the MagAO-X instrument

The Magellan extreme adaptive optics (MagAO-X) instrument is a new extreme adaptive optics (ExAO) system designed for operation in the visible to near-IR which will deliver high contrast-imaging capabilities. The main AO system will be driven by a pyramid wavefront sensor (PyWFS); however, to mitigate the impact of quasi-static and non-common path (NCP) aberrations, focal plane wavefront sensing (FPWFS) in the form of low-order wavefront sensing (LOWFS) and spatial linear dark field control (LDFC) will be employed behind a vector apodizing phase plate (vAPP) coronagraph using rejected starlight at an intermediate focal plane. These techniques will allow for continuous high-contrast imaging performance at the raw contrast level delivered by the vAPP coronagraph ( 6 x 10-5). We present simulation results for LOWFS and spatial LDFC with a vAPP coronagraph, as well as laboratory results for both algorithms implemented with a vAPP coronagraph at the University of Arizona Extreme Wavefront Control Lab.

Characterization of deformable mirrors for the MagAO-X project

The MagAO-X instrument is an upgrade of the Magellan AO system that will introduce extreme adaptive optics capabilities for high-contrast imaging at visible and near-infrared wavelengths. A central component of this system is a 2040-actuator microelectromechanical (MEMS) deformable mirror (DM) from Boston Micromachines Corp. (BMC) that will operate at 3.63 kHz for high-order wavefront control. Two additional DMs from ALPAO will perform low-order and non-common-path science-arm wavefront correction. The accuracy of the wavefront correction is limited by our ability to command these DMs to a desired shape, which requires a careful characterization of each DM surface. We have developed a characterization pipeline that uses a Zygo Verifire Interferometer to measure the surface response and a Karhunen-Loeve transform to remove noise from our measurements. We present our progress in the characterization process and the results of our pipeline applied to an ALPAO DM97 and a BMC Kilo-DM, demonstrating the ability to drive the DMs to a flat of ≤2nm and ≤4nm RMS in our beam footprint on the University of Arizona Wavefront Control (UAWFC) testbed.

Real-time estimation and correction of quasi-static aberrations in ground-based high contrast imaging systems with high frame-rates

The success of ground-based, high contrast imaging for the detection of exoplanets in part depends on the ability to differentiate between quasi-static speckles caused by aberrations not corrected by adaptive optics (AO) systems, known as non-common path aberrations (NCPAs), and the planet intensity signal. Frazin (ApJ, 2013) introduced a post-processing algorithm demonstrating that simultaneous millisecond exposures in the science camera and wavefront sensor (WFS) can be used with a statistical inference procedure to determine both the series expanded NCPA coefficients and the planetary signal. We demonstrate, via simulation, that using this algorithm in a closed-loop AO system, real-time estimation and correction of the quasi-static NCPA is possible without separate deformable mirror (DM) probes. Thus the use of this technique allows for the removal of the quasi-static speckles that can be mistaken for planetary signals without the need for new optical hardware, improving the efficiency of ground-based exoplanet detection. In our simulations, we explore the behavior of the Frazin Algorithm (FA) and the dependence of its convergence to an accurate estimate on factors such as Strehl ratio, NCPA strength, and number of algorithm search basis functions. We then apply this knowledge to simulate running the algorithm in real-time in a nearly ideal setting. We then discuss adaptations that can be made to the algorithm to improve its real-time performance, and show their efficacy in simulation. A final simulation tests the technique’s resilience against imperfect knowledge of the AO residual phase, motivating an analysis of the feasibility of using this technique in a real closed-loop Extreme AO system such as SCExAO or MagAO-X, in terms of computational complexity and the accuracy of the estimated quasi-static NCPA correction.

Phase-induced amplitude apodization complex mask coronagraphy (PIAACMC) for large segmented apertures (Conference Presentation)

High contrast imaging of exoplanets around nearby stars with future large segmented apertures requires starlight suppression systems optimized for such geometries, with the ability to control diffraction created by gaps between segments. The PIAACMC approach is well-suited for high high efficiency coronagraphic imaging of exoplanets at small angular separations, offering an inner working angle (IWA) as small as 1 lambda/D. We show that PIAACMC can be designed for segmented apertures and present a few representative designs. The design process can mitigate leaks due to stellar angular size and chromatic diffraction by segment gaps by co-optimizing a multi-zone diffractive focal plane mask and a Lyot stop. The resulting performance is ultimately limited by stellar angular size, and the IWA must be carefully traded against contrast and throughput at small angular separations. We show that PIAACMCs small IWA enables space-based near-IR imaging and spectroscopy of exoplanets around Sun-stars, and ground-based imaging and characterization of habitable planets around nearby M-type stars. We review the current status of PIAACMC laboratory development and near-term prospects for ground-based use.

Design of the MagAO-X pyramid wavefront sensor

Adaptive optics systems correct atmospheric turbulence in real time. Most adaptive optics systems used routinely correct in the near infrared, at wavelengths greater than 1 μm. MagAO- X is a new extreme adaptive optics (ExAO) instrument that will offer corrections at visible-to- near-IR wavelengths. MagAO-X will achieve Strehl ratios of ≥70% at Hα when running the 2040 actuator deformable mirror at 3.6 kHz. A visible pyramid wavefront sensor (PWFS) optimized for sensing at 600-1000 nm wavelengths will provide the high-order wavefront sensing on MagAO-X. We present the optical design and predicted performance of the MagAO-X pyramid wavefront sensor.

Results of the astrometry and direct imaging testbed for exoplanet detection

Measuring masses of long-period planets around F, G, and K stars is necessary to characterize exoplanets and assess their habitability. Imaging stellar astrometry offers a unique opportunity to solve radial velocity system inclination ambiguity and determine exoplanet masses. The main limiting factor in sparse-field astrometry, besides photon noise, is the non-systematic dynamic distortions that arise from perturbations in the optical train. Even space optics suffer from dynamic distortions in the optical system at the sub-μas level. To overcome this limitation we propose a diffractive pupil that uses an array of dots on the primary mirror creating polychromatic diffraction spikes in the focal plane, which are used to calibrate the distortions in the optical system. By combining this technology with a high-performance coronagraph, measurements of planetary systems orbits and masses can be obtained faster and more accurately than by applying traditional techniques separately. In this paper, we present the results of the combined astrometry and and highcontrast imaging experiments performed at NASA Ames Research Center as part of a Technology Development for Exoplanet Missions program. We demonstrated 2.38x10-5 λ/D astrometric accuracy per axis and 1.72x10-7 raw contrast from 1.6 to 4.5 λ/D. In addition, using a simple average subtraction post-processing we demonstrated no contamination of the coronagraph field down to 4.79x10-9 raw contrast.

Adaptive optics self-calibration using differential OTF (dOTF)

We demonstrate self-calibration of an adaptive optical system using differential OTF [Codona, JL; Opt. Eng. 0001; 52(9):097105-097105. doi:10.1117/1.OE.52.9.097105]. We use a deformable mirror (DM) along with science camera focal plane images to implement a closed-loop servo that both flattens the DM and corrects for non-common-path aberrations within the telescope. The pupil field modification required for dOTF measurement is introduced by displacing actuators near the edge of the illuminated pupil. Simulations were used to develop methods to retrieve the phase from the complex amplitude dOTF measurements for both segmented and continuous sheet MEMS DMs and tests were performed using a Boston Micromachines continuous sheet DM for verification. We compute the actuator correction updates directly from the phase of the dOTF measurements, reading out displacements and/or slopes at segment and actuator positions. Through simulation, we also explore the effectiveness of these techniques for a variety of photons collected in each dOTF exposure pair.

Deconvolution of differential OTF (dOTF) to measure high-resolution wavefront structure

Differential OTF uses two images taken with a telescope pupil modification between them to measure the complex field over most of the pupil. If the pupil modification involves a non-negligible region of the pupil, the dOTF field is blurred by convolution with the complex conjugate of the pupil field change. In some cases, the convolution kernel, or difference field, can cause significant blurring. We explore using deconvolution to recover a highresolution measurement of the complex pupil field. In particular, by assuming we know something about the area and nature of the difference field, we can construct a Wiener filter that increases the resolution of the complex pupil field estimate in the presence of noise. By introducing a controllable pupil modification, such as actuating a telescope primary mirror segment in piston-tip-tilt to make the measurement, we explain added features to the difference field which can be used to increase the signal-to-noise ratio for information in arbitrary ranges of spatial frequency. We will present theory and numerical simulations to discuss key features of the difference field which lead to its utility for deconvolution of dOTF measurements.