Jeremy Kasdin

Closed Loop, DM Diversity-based, Wavefront Correction Algorithm for High Contrast Imaging Systems

High contrast imaging from space relies on coronagraphs to limit diffraction and a wavefront control systems to compensate for imperfections in both the telescope optics and the coronagraph. The extreme contrast required (up to 10(-10) for terrestrial planets) puts severe requirements on the wavefront control system, as the achievable contrast is limited by the quality of the wavefront. This paper presents a general closed loop correction algorithm for high contrast imaging coronagraphs by minimizing the energy in a predefined region in the image where terrestrial planets could be found. The estimation part of the algorithm reconstructs the complex field in the image plane using phase diversity caused by the deformable mirror. This method has been shown to achieve faster and better correction than classical speckle nulling.

ACCESS – A Concept Study for the Direct Imaging and Spectroscopy of Exoplanetary Systems

ACCESS is one of four medium-class mission concepts selected for study in 2008-9 by NASAs Astrophysics Strategic Mission Concepts Study program. ACCESS evaluates a space observatory designed for extreme high-contrast imaging and spectroscopy of exoplanetary systems. An actively-corrected coronagraph is used to suppress the glare of diffracted and scattered starlight to contrast levels required for exoplanet imaging. The ACCESS study considered the relative merits and readiness of four major coronagraph types, and modeled their performance with a NASA medium-class space telescope. The ACCESS study asks: What is the most capable medium-class coronagraphic mission that is possible with telescope, instrument, and spacecraft technologies available today? Using demonstrated high-TRL technologies, the ACCESS science program surveys the nearest 120+ AFGK stars for exoplanet systems, and surveys the majority of those for exozodiacal dust to the level of 1 zodi at 3 AU. Coronagraph technology developments in the coming year are expected to further enhance the science reach of the ACCESS mission concept.

Pupil mapping exoplanet coronagraphic observer (PECO)

The Pupil mapping Exoplanet Coronagraphic Observer (PECO) mission concept is a 1.4-m telescope aimed at imaging and characterizing extra-solar planetary systems at optical wavelengths. The coronagraphic method employed, Phase-Induced Amplitude Apodization or PIAA (a.k.a. pupil mapping) can deliver 1e-10 contrast at 2 lambda/D and uses almost all the starlight that passes through the aperture to maintain higher throughput and higher angular resolution than any other coronagraph or nuller, making PECO the theoretically most efficient existing approach for imaging extra-solar planetary systems. PECOs instrument also incorporates deformable mirrors for high accuracy wavefront control. Our studies show that a probe-scale PECO mission with 1.4 m aperture is extremely powerful, with the capability of imaging at spectral resolution R≈∠15 the habitable zones of already known F, G, K stars with sensitivity sufficient to detect planets down to Earth size, and to map dust clouds down to a fraction of our zodiacal cloud dust brightness. PECO will acquire narrow field images simultaneously in 10 to 20 spectral bands covering wavelengths from 0.4 to 1.0 μm and will utilize all available photons for maximum wavefront sensing and imaging/spectroscopy sensitivity. This approach is well suited for low-resolution spectral characterization of both planets and dust clouds with a moderately sized telescope. We also report on recent results obtained with the laboratory prototype of a coronagraphic low order wavefront sensor (CLOWFS) for PIAA coronagraph. The CLOWFS is a key part of PECOs design and will enable high contrast at the very small PECO inner working angle.

Toward 10 10 contrast for terrestrial exoplanet detection: demonstration of wavefront correction in a shaped-pupil coronagraph

The Shaped Pupil Coronagraph (SPC) is a high-contrast imaging system pioneered at Princeton for detection of extra-solar earthlike planets. It is designed to achieve 10-10 contrast at an inner working angle of 4λ/D. However, a critical requirement in attaining this contrast level in practice is the ability to control wavefront phase and amplitude aberrations to at least λ/104 in rms phase and 1/1000 rms amplitude, respectively. Furthermore, this has to be maintained over a large spectral band. The High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Lab (JPL) is a state-of-the-art facility for studying high contrast imaging systems and fine wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. In this paper, we present the results of testing Princetons SPC in JPLs HCIT. In particular, we present the achievement of 4x10-8 contrast using a speckle nulling algorithm, and demonstrate that this contrast is maintained across wavelengths of 785, 836nm, and for broadband light having 10% bandwidth around 800nm.

The Eclipse mission: a direct imaging survey of nearby planetary systems

Eclipse is a proposed Discovery-class mission to perform a sensitive imaging survey of nearby planetary systems, including a complete survey for Jupiter-sized planets orbiting 5 AU from all stars of spectral types A-K to distances of 15 pc. Eclipse is a coronagraphic space telescope concept designed for high-contrast visible wavelength imaging and spectrophotometry. Its optical design incorporates essential elements: a telescope with an unobscured aperture of 1.8 meters and optical surfaces optimized for smoothness at critical spatial frequencies, a coronagraphic camera for suppression of diffracted light, and precision active optical correction for suppression of light scattered by residual mirror surface irregularities. For reference, Eclipse is predicted to reduce diffracted and scattered starlight between 0.25 and 2.0 arcseconds from the star by at least three orders of magnitude compared to any HST instrument. The Eclipse mission offers precursor science explorations and critical technology validation in support of coronagraphic concepts for NASAs Terrestrial Planet Finder (TPF). A baseline three-year science mission would provide a survey of the nearby stars accessible to TPF before the end of this decade, promising fundamental new insights into the nature and evolution of possibly diverse planetary systems associated with our Suns nearest neighbors.

ACCESS - A NASA mission concept study of an actively-corrected coronagraph for exoplanet system studies

ACCESS (Actively-Corrected Coronagraph for Exoplanet System Studies) develops the science and engineering case for an investigation of exosolar giant planets, super-earths, exo-earths, and dust/debris fields that would be accessible to a medium-scale NASA mission. The study begins with the observation that coronagraph architectures of all types (other than the external occulter) call for an exceptionally stable telescope and spacecraft, as well as active wavefront correction with one or more deformable mirrors (DMs). During the study, the Lyot, shaped pupil, PIAA, and a number of other coronagraph architectures will all be evaluated on a level playing field that considers science capability (including contrast at the inner working angle (IWA), throughput efficiency, and spectral bandwidth), engineering readiness (including maturity of technology, instrument complexity, and sensitivity to wavefront errors), and mission cost so that a preferred coronagraph architecture can be selected and developed for a medium-class mission.

External occulters for direct observation of exoplanets: an overview

Perhaps the most compelling piece of science and exploration now under discussion for future space missions is the direct study of planets circling other stars. Indirect means have established planets as common in the universe but have given us a limited view of their actual characteristics. Direct observation holds the potential to map entire planetary systems, view newly forming planets, find Earth-like planets and perform photometry to search for major surface features. Direct observations will also enable spectroscopy of exoplanets and the search for evidence of simple life in the universe. Recent advances in the design of external occulters - starshades that block the light from the star while passing exoplanet light - have lowered their cost and improved their performance to the point where we can now envision a New Worlds Observer that is both buildable and affordable with todays technology. We will summarize recent studies of such missions and show they provide a very attractive alternative near term mission.

The SCExAO high contrast imager: transitioning from commissioning to science

SCExAO is the premier high-contrast imaging platform for the Subaru Telescope. It offers high Strehl ratios at near-IR wavelengths (y-K band) with stable pointing and coronagraphs with extremely small inner working angles, optimized for imaging faint companions very close to the host. In the visible, it has several interferometric imagers which offer polarimetric and spectroscopic capabilities. A recent addition is the RHEA spectrograph enabling spatially resolved high resolution spectroscopy of the surfaces of giant stars, for example. New capabilities on the horizon include post-coronagraphic spectroscopy, spectral differential imaging, nulling interferometry as well as an integral field spectrograph and an MKID array. Here we present the new modules of SCExAO, give an overview of the current commissioning status of each of the modules and present preliminary results.

Analysis of external occulters in the presence of defects

Fifty meter-class external occulters have been proposed to detect earth-like planets. The THEIA concept1, a forty-meter diameter occulter with twenty ten-meter petals has the necessary nominal performance to achieve this goal. This paper examines whether this design is robust against expected manufacturing and deployment errors. The development of a numerical algorithm that represents the mask defects as a collection of rectangular apertures mitigates the problems associated with modeling diffraction phenomena produced by an occulter with characteristic physical dimensions that span five orders of magnitude. The field from each of these rectangles, which is proportional to a two-dimensional sinc function at the telescope, is added to the diffracted field from the nominal occulter. Results for a set of representative defects are presented. A single-petal, single-defect error budget, based on a minimum contrast of 10-12 at 75 or 118 milli-arcseconds from the host star from 0.3 μ to 0.9 μ, is quoted. A Monte Carlo-type simulation that predicts the performance of the occulter in the presence of random combinations of all of the error demonstrates that the system contrast can maintained to better than 10-11 from 0.3 μ to 0.9 μ if the values in the error budget can be achieved.

Direct studies of exo-planets with the New Worlds Observer

The New World Observer has the potential to discover and study planets around other stars without expensive and risky technical heroics. We describe the starshade, a large, deployable sheet on a separate spacecraft that is flown into position along the line of sight to a nearby star. We show how a starshade can be designed and built in a practical and affordable manner to fully remove starlight and leave only planet light entering a telescope. The simulations demonstrate That NWI can detect planetary system features as faint as comets, perform spectroscopy to look for water and life signs, and perform photometry to search for oceans, continents, clouds and polar caps.