Kent Wallace

Bio: Kent Wallace is an academic researcher from Jet Propulsion Laboratory. The author has contributed to research in topics: Gemini Planet Imager & Adaptive optics. The author has an hindex of 9, co-authored 17 publications receiving 234 citations.

Low order wavefront sensing and control for WFIRST coronagraph

Abstract: To maintain the required WFIRST Coronagraph starlight suppression performance in a realistic space environment, a low order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from coronagraph to sense and suppress the telescope pointing drift and jitter as well as the low order wavefront errors due to changes in thermal loading on the telescope and the rest of the observatory. In this paper we will present an overview of the low order wavefront sensing and control subsystem for the WFIRST Coronagraph. We will describe LOWFS/C’s Zernike wavefront sensor concept and control design, and present an overview of sensing performance analysis and modeling, predicted line-of-sight jitter suppression loop performance, as well as the low order wavefront error correction with the coronagraph’s deformable mirror. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show promising performance of the Zernike wavefront sensor and FSM feedback loop.

Extreme adaptive optics imaging with a clear and well-corrected off-axis telescope sub-aperture

Abstract: Rather than using an adaptive optics (AO) system to correct a telescope s entire pupil, it can instead be used to more finely correct a smaller sub-aperture. Indeed, existing AO systems can be used to correct a sub-aperture 1/3 to 1/2 the size of a 5-10 m telescope to extreme adaptive optics (ExAO) levels. We discuss the potential performance of a clear off-axis well-corrected sub-aperture (WCS), and describe our initial imaging results with a 1.5 m diameter WCS on the Palomar Observatory s Hale telescope. These include measured Strehl ratios of 0.92-0.94 in the infrared (2.17 microns), and 0.12 in the B band, the latter allowing a binary of separation 0.34 arc sec to be easily resolved in the blue. Such performance levels enable a variety of novel observational modes, such as infrared ExAO, visible-wavelength AO, and high-contrast coronagraphy. One specific application suggested by the high Strehl ratio stability obtained (1%) is the measurement of planetary transits and eclipses. Also described is a simple dark-hole experiment carried out on a binary star, in which a comatic phase term was applied directly to the deformable mirror, in order to shift the diffraction rings to one side of the point spread function.

Ground-Based Direct Detection of Exoplanets with the Gemini Planet Imager (GPI)

Abstract: The Gemini Planet (GPI) imager is an "extreme" adaptive optics system being designed and built for the Gemini Observatory GPI combines precise and accurate wavefront control, diffraction suppression, and a speckle-suppressing science camera with integral field and polarimetry capabilities GPI's primary science goal is the direct detection and characterization of young, Jovian-mass exoplanets For systems younger than 2 Gyr exoplanets more massive than 6 MJ and semimajor axes beyond 10 AU are detected with completeness greater than 50% GPI will also discover faint debris disks, explore icy moons and minor planets in the solar system, reveal high dynamic range main-sequence binaries, and study mass loss from evolved stars This white paper explains the role of GPI in exoplanet discovery and characterization and summarizes our recommendations to the NSF-NASA-DOE Astronomy and Astrophysics Advisory Committee ExoPlanet Task Force

W. M. Keck Observatory's next-generation adaptive optics facility

Abstract: We report on the preliminary design of W.M. Keck Observatory's (WMKO's) next-generation adaptive optics (NGAO) facility. This facility is designed to address key science questions including understanding the formation and evolution of today's galaxies, measuring dark matter in our galaxy and beyond, testing the theory of general relativity in the Galactic Center, understanding the formation of planetary systems around nearby stars, and exploring the origins of our own solar system. The requirements derived from these science questions have resulted in NGAO being designed to have near diffraction-limited performance in the near-IR (K-Strehl ~ 80%) over narrow fields (< 30" diameter) with modest correction down to ~ 700 nm, high sky coverage, improved sensitivity and contrast and improved photometric and astrometric accuracy. The resultant key design features include multi-laser tomography to measure the wavefront and correct for the cone effect, open loop AO-corrected near-IR tip-tilt sensors with MEMS deformable mirrors (DMs) for high sky coverage, a high order MEMS DM for the correction of atmospheric and telescope static errors to support high Strehls and high contrast companion sensitivity, point spread function (PSF) calibration to benefit quantitative astronomy, a cooled science path to reduce thermal background, and a high-efficiency science instrument providing imaging and integral field spectroscopy.

On-sky vibration environment for the Gemini Planet Imager and mitigation effort

Abstract: The Gemini Planet Imager (GPI) entered on-sky commissioning and had its rst-light at the Gemini South (GS)telescope in November 2013. GPI is an extreme adaptive optics (XAO), high-contrast imager and integral- eldspectrograph dedicated to the direct detection of hot exo-planets down to a Jupiter mass. The performanceof the apodized pupil Lyot coronagraph depends critically upon the residual wavefront error (design goal of60nmRMS with < 5mas RMS tip/tilt), and therefore is most sensitive to vibration (internal or external) ofGemini's instrument suite. Excess vibration can be mitigated by a variety of methods such as passive or activedampening at the instrument or telescope structure or Kalman ltering of speci c frequencies with the AOcontrol loop. Understanding the sources, magnitudes and impact of vibration is key to mitigation. This papergives an overview of related investigations based on instrument data (GPI AO module) as well as external datafrom accelerometer sensors placed at di erent locations on the GS telescope structure. We report the status ofrelated mitigation e orts, and present corresponding results.Keywords: High-contrast imaging, vibration absorbers, LQG, Kalman lter, telescope vibrations

The Exoplanet Handbook

Abstract: 1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

The Exoplanet Handbook: Formation and evolution

Abstract: PLANETARY SYSTEMS, the solar system amongst them, are believed to form as inevitable and common byproducts of star formation For orientation, an overview of the processes described in this chapter is as follows The present paradigm starts with star formation in molecular clouds Brown dwarfs are formed as the lowmass tail of this process, although some may be formed as a high-mass tail of planet formation Gas and dust in the collapsing molecular cloud which does not fall directly onto the protostar resides in a relatively long-lived accretion disk which provides the environment for the subsequent stages of planet formation Terrestrial-mass planets are formed within the disk through the progressive agglomeration of material denoted, as it grows in size, as dust, rocks, planetesimals and protoplanets A similar process typically occurring further out in the disk results in the cores of giant planets, which then gravitationally accumulate their mantles of ice and/or gas As the planet-forming bodies grow in mass, growth and dynamics become more dominated by gravitational interactions Towards the final phases, and before the remaining gas is lost through accretion or dispersal, the gas provides a viscous medium at least partially responsible for planetary migration Some migration also occurs during these later stages as a result of gravitational scattering between the (proto-)planets and the residual sea of planetesimals The final structural stabilisation of the planetary system may be affected by planet–planet interactions, until a configuration emerges which may be dynamically stable over billions of years The current observational data for exoplanet systems is broadly compatible with this overall picture Other constraints come from a substantial body of detailed observations of the solar system (Chapter 12) Context and present paradigm An understanding of howplanets formis essential in understanding and interpreting the considerable range of observed planetary system architectures and dynamics Today, the most widely considered solar nebula theory holds that planet formation in the solar system, and by inference in other exoplanet systems, follows on from the process of star formation and accretion disk formation, through the agglomeration of residual material as the protoplanetary disk collapses and evolves

An image of an exoplanet separated by two diffraction beamwidths from a star

Abstract: Three exoplanets around the star HR 8799 have recently been discovered by means of differential imaging with large telescopes. Bright scattered starlight limits high-contrast imaging to large angular offsets, currently of the order of ten diffraction beamwidths, 10λ/D, of the star (where λ is the wavelength and D is the aperture diameter). Imaging faint planets at smaller angles calls for reducing the starlight and associated photon and speckle noise before detection, while efficiently transmitting nearby planet light. To carry out initial demonstrations of reduced-angle high-contrast coronagraphy, we installed a vortex coronagraph capable of reaching small angles behind a small, well-corrected telescope subaperture that provides low levels of scattered starlight. Here we report the detection of all three HR 8799 planets with the resultant small-aperture (1.5 m) system, for which only 2λ/D separate the innermost planet from the star, with a final noise level within a factor of two of that given by photon statistics. Similar well-corrected small-angle coronagraphs should thus be able to detect exoplanets located even closer to their host stars with larger ground-based telescopes, and also allow a reduction in the size of potential space telescopes aimed at the imaging of very faint terrestrial planets.

Optical Vectorial Vortex Coronagraphs using Liquid Crystal Polymers: theory, manufacturing and laboratory demonstration.

Abstract: In this paper, after briefly reviewing the theory of vectorial vortices, we describe our technological approach to generating the necessary phase helix, and report results obtained with the first optical vectorial vortex coronagraph (OVVC) in the laboratory. To implement the geometrical phase ramp, we make use of Liquid Crystal Polymers (LCP), which we believe to be the most efficient technological path to quickly synthesize optical vectorial vortices of virtually any topological charge. With the first prototype device of topological charge 2, a maximum peak-to-peak attenuation of 1.4×10^(-2) and a residual light level of 3×10^(-5) at an angular separation of 3.5 λ/d (at which point our current noise floor is reached) have been obtained at a wavelength of 1.55 μm. These results demonstrate the validity of using space-variant birefringence distributions to generate a new family of coronagraphs usable in natural unpolarized light, opening a path to high performance coronagraphs that are achromatic and have low-sensitivity to low-order wavefront aberrations.

Polarimetry with the gemini planet imager: Methods, performance at first light, and the circumstellar ring around HR 4796A

Abstract: We present the first results from the polarimetry mode of the Gemini Planet Imager (GPI), which uses a new integral field polarimetry architecture to provide high contrast linear polarimetry with minimal systematic biases between the orthogonal polarizations. We describe the design, data reduction methods, and performance of polarimetry with GPI. Point-spread function (PSF) subtraction via differential polarimetry suppresses unpolarized starlight by a factor of over 100, and provides sensitivity to circumstellar dust reaching the photon noise limit for these observations. In the case of the circumstellar disk around HR 4796A, GPI's advanced adaptive optics system reveals the disk clearly even prior to PSF subtraction. In polarized light, the disk is seen all the way in to its semi-minor axis for the first time. The disk exhibits surprisingly strong asymmetry in polarized intensity, with the west side 9 times brighter than the east side despite the fact that the east side is slightly brighter in total intensity. Based on a synthesis of the total and polarized intensities, we now believe that the west side is closer to us, contrary to most prior interpretations. Forward scattering by relatively large silicate dust particles leads to the strong polarized intensity on the west side, and the ring must be slightly optically thick in order to explain the lower brightness in total intensity there. These findings suggest that the ring is geometrically narrow and dynamically cold, perhaps shepherded by larger bodies in the same manner as Saturn's F ring.