NASA is embarking on an ambitious program to develop the Habitable Worlds Observatory (HWO) flagship to perform transformational astrophysics, as well as directly image ∼ 25 potentially Earth-like planets and spectroscopically characterize them for signs of life. This mission was recommended by Astro2020, which additionally recommended a new approach for flagship formulation based on increasing the scope and depth of early, pre-phase A trades and technology maturation. A critical capability of the HWO mission is the suppression of starlight. To inform future architecture trades, it is necessary to survey a wide range of candidate technologies, from the relatively mature ones such as the ones described in the LUVOIR and HabEx reports to the relatively new and emerging ones, which may lead to breakthrough performance. In this paper, we present a summary of an effort, funded by NASA’s Exoplanet Exoplaration Program (ExEP), to survey potential coronagraph options for HWO. In particular, our results consist of: (1) a database of different coronagraph designs sourced from the world-wide coronagraph community that are potentially compatible with HWO; (2) evaluation criteria, such as expected mission yields and feasibility of maturing to TRL 5 before phase A; (3) a unified modeling pipeline that processes the designs from (1) and outputs values for any machine-calculable criteria from (2); (4) assessments of maturity of designs, and other criteria that are not machine-calculable; (5) a table presenting an executive summary of designs and our results. While not charged to down-select or prioritize the different coronagraph designs, the products of this survey were designed to facilitate future HWO trade studies.
The requirements for a coronagraph instrument to image and obtain spectra of rocky planets around bright stars from space are tight. Indeed, the goal of imaging an Earth-like planet requires a starlight suppression system that cancels light to a level of 10 − 10 with sufficient stability and robustness to errors. Furthermore, the key science questions necessitate an adequate sample size; consequently, the throughput of the coronagraph drives the achievable yield of a given mission. The trade among achievable raw contrast, sensitivity to wavefront errors, and throughput poses a challenging problem in coronagraph design. The complexity of this problem drives us toward the simultaneous solving of all optical elements. We present a set of methods to optimize the design of a coronagraph. We implement these for the case of the hybrid Lyot coronagraph in the context of the Nancy Grace Roman Space Telescope Coronagraph Instrument. We discuss our findings in terms of coronagraph instrument design, and optical subsystems, and performance interplay.
The Nancy Grace Roman Space Telescope, planned to launch in the mid-2020s, will be the first space-based observatory to demonstrate active wavefront correction at high contrast with its Coronagraph Instrument. As a technology demonstrator, the instrument’s main purpose is to mature the various technologies needed by future flagship mission concepts that aim to image and characterize Earth-like exoplanets. These technologies include two high-actuator-count deformable mirrors (DMs), photon-counting detectors, two complementary wavefront sensing and control loops, and two different coronagraph types. Here we describe the complete set of flight mask designs for the Roman Coronagraph. Multiple mask configurations are required to overcome the challenging pupil obscurations and enable the desired types of imaging, spectroscopy, and polarimetry. In designing each mask configuration, we considered many performance metrics, including spectral bandwidth, field of view, contrast, core throughput, encircled energy, deformable mirror surface height, and low-order aberration sensitivity
KEYWORDS: Coronagraphy, Exoplanets, Space telescopes, James Webb Space Telescope, Infrared telescopes, Visible radiation, Aerospace engineering, Space operations, Telescopes, Near infrared
Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1 ♦ 10-10 at small angular separations of 100 milliarcseconds or less in visible light with more than 50 nm bandwidth. To our knowledge, the technology needed to achieve the contrast and stability has not been demonstrated as of January 2019. The science requirements for near future NASA missions such as James Webb Space Telescope’s (JWST) Near Infrared Camera (NIRCam) coronagraph and Wide Field Infrared Space Telescope (WFIRST) Coronagraph Instrument (CGI) are at least 10 times short. To investigate and guide the technology to reach this capability, we built a high contrast coronagraph testbed at NASA’s Jet Propulsion Laboratory (JPL). Titled the Decadal Survey Testbed (DST), state-of-art testbed is based on the accumulated experience of JPL’s High Contrast Imaging Testbed (HCIT) team. Currently, the DST hosts a Hybrid Lyot Coronagraph (HLC) with an unobscured, circular pupil. The DST also has two deformable mirrors and is equipped with the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. In this paper, we present up-to-date progress of the testbed demonstration. As of January 2019, we repeatedly obtain convergence below 4 × 10-10 mean contrast with 10% broadband light centered at 550 nm in a 360 degrees dark hole with a working angle between 3 λ/D and 9 λ/D. We show the key elements used in the testbed and the performance results with associated analysis.
Direct imaging of an Earth-like exoplanet requires starlight suppression with a contrast ratio on the order of 1×10-10 at small angular separations of 100 milliarcseconds or less in visible light. To aid the technology development to reach this capability and enable future exoplanet missions, we built a high contrast coronagraph testbed, titled the Decadal Survey Testbed (DST). As of early 2019, the testbed has repeatedly demonstrated a monochromatic contrast floor about 1×10-10, and broadband performance at 550 nm with 10% color band- width <4×10-10 . The testbed has also demonstrated open-loop contrast drift rates of around 10-10/hour, temperature drift stabilities of <10 milliKelvins/day, passive pointing stability of around 0.1 λ/D per day on the occulting mask, and rms pointing jitter around 0.005 λ/D. This paper focusses primarily on the testbed hardware description, and a companion paper by Seo et al. details the experimental results.
High contrast imaging and characterization of faint exoplanets require a coronagraph instrument to efficiently suppress the host star light to 10-9 level contrast over a broad spectral bandwidth. The NASA WFIRST mission plan includes a coronagraph instrument to demonstrate the technology needed to image and characterize exoplanets. Lyot coronagraph masks designed to serve at the focal plane followed by a Lyot stop will be key elements in the WFIRST coronagraph and in future advanced missions such as LUVOIR (Bolcar (2019) and HabEx (Morgan 2019, Martin 2019)). Shaped pupil masks designed to work in reflective geometry are also employed in the WFIRST Coronagraph. High-contrast performance reaching much better than 10-9 contrast requires very tight design, fabrication tolerances, and material properties to meet a wide range of specifications, including precise shapes, micron-scale island features, ultra-low reflectivity regions, uniformity, wavefront quality, etc. In this paper, we present all the critical analytical and measured properties of materials and designs in relation to the results from our coronagraph testbeds.
Occulter mask fabrication for Hybrid Lyot Coronagraph (HLC) at JPL is a relatively mature technology as past successful testbed demonstrations can attest. Nevertheless, as NASA’s WFIRST mission moved into Phase B, new mask design space and fabrication process were explored for new requirements and for better performances for the CoronaGraph Instrument (CGI). To minimize the risks associated with the new explorations, CGI modeling team is tasked with assessing the viability of new designs. In this paper, we describe our HLC modeling effort and results, which identified the potential risks with early exploratory designs and modified fabrication processes. As a result, the traditional (proven) style design is kept for risk aversion. Along the way a standard procedure has been developed for systematic mask evaluation, mask baselining, and general flight performance prediction. In the second part, we describe our model validation effort for the chosen baseline mask’s testbed performance. The focus of the testbed demonstration is to address a major concern related to the CGI’s limited time for wavefront control (WFC) in flight. It includes two stages of WFC: ground seed generation WFC, and (simulated) in-orbit commissioning phase WFC. Good agreements have been achieved in both stages of WFC which affirms that the CGI is capable of digging a dark hole that meets raw contrast requirement within the required time allocation. It also represents a significant improvement in our HLC WFC modeling for an as-built real system.
In order to validate required operation of the proposed Wide-Field InfraRed Survey Telescope (WFIRST) coronagraph instrument, we have built a testbed in Jet Propulsion Laboratory (JPL), which is analogous to the baseline WFIRST coronagraph instrument architecture. Since its birth in 2016, this testbed, named as Occulting Mask Coronagraph (OMC) testbed, has demonstrated several crucial technological milestones: Broadband high contrast demonstration in both Hybrid Lyot Coronagraph (HLC) and Shape Pupil Coronagraph (SPC) modes while the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem senses and corrects the dynamic flight-like wavefront disturbances. In this paper, we present up-to-date progress of HLC mode demonstration in the OMC testbed. While injecting the flight-like low photon flux starlight with expected Line of Sight (LoS) and Wavefront Error (WFE) perturbation to the OMC testbed, we demonstrate generating high contrast dark hole images. We first study the expected photon flux in actual flight environment, and estimate detection noise and estimation accuracy of the complex electric field if the wavefront sensing algorithm is used based on the pair-wise difference imaging. Then, we introduce our improved scheme to mitigate this photon-starved flight-like low flux environment. As a result, we generate a dark hole that meets the WFIRST raw contrast requirements using the 2nd magnitude star light. We establish the key ideas, describe test setups, and demonstrate test results with data analysis.
As it has for the past few years, numerical modeling is being used to predict the on-orbit, high-contrast imaging performance of the WFIRST coronagraph, which was recently defined to be a technology demonstrator with science capabilities. A consequence has been a realignment of modeling priorities and revised applications of modeling uncertainty factors and margins, which apply to multiple factors such as pointing and wavefront jitter, thermally-induced deformations, polarization, and aberration sensitivities. At the same time, the models have increased in fidelity as additional parameters have been added, such as time-dependent pupil shear and mid-spatial-frequency deformations of the primary and secondary mirrors, detector effects, and reaction-wheel-speed-dependent pointing and wavefront jitter.
We have developed the Fast Linearized Coronagraph Optimizer (FALCO), a new software toolbox for high-contrast, coronagraphic wavefront sensing and control. FALCO rapidly calculates the linearized deformable mirror (DM) response matrices, also called control Jacobians, and can be used for the design, simulation, or testbed operation of several types of coronagraphs. In this paper, we demonstrate that the optical propagation used in FALCO is accurate and matches PROPER. In addition, we demonstrate the drastic reduction in runtime when using FALCO for DM Jacobian calculations instead of the conventional method used, for example with a model of the Wide-Field Infrared Survey Telescope (WFIRST) Coronagraph Instrument (CGI). We then compare the relative accuracy between optical models in FALCO and PROPER.
The Shaped Pupil Coronagraph (SPC) is one of the two operating modes of the WFIRST coronagraph instrument. The SPC provides starlight suppression in a pair of wedge-shaped regions over an 18% bandpass, and is well suited for spectroscopy of known exoplanets. To demonstrate this starlight suppression in the presence of expected onorbit input wavefront disturbances, we have recently built a dynamic testbed at JPL analogous to the WFIRST flight instrument architecture, with both Hybrid Lyot Coronagraph (HLC) and SPC architectures and a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to apply, sense, and correct dynamic wavefront disturbances. We present our best up-to-date results of the SPC mode demonstration from the testbed, in both static and dynamic conditions, along with model comparisons. HLC results will be reported separately.
Hybrid Lyot Coronagraph (HLC) is one of the two operating modes of the Wide-Field InfraRed Survey Telescope (WFIRST) coronagraph instrument. Since being selected by National Aeronautics and Space Administration (NASA) in December 2013, the coronagraph technology is being matured to Technology Readiness Level (TRL) 6 by 2018. To demonstrate starlight suppression in presence of expecting on-orbit input wavefront disturbances, we have built a dynamic testbed in Jet Propulsion Laboratory (JPL) in 2016. This testbed, named as Occulting Mask Coronagraph (OMC) testbed, is designed analogous to the WFIRST flight instrument architecture: It has both HLC and Shape Pupil Coronagraph (SPC) architectures, and also has the Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to sense and correct the dynamic wavefront disturbances. We present upto-date progress of HLC mode demonstration in the OMC testbed. SPC results will be reported separately. We inject the flight-like Line of Sight (LoS) and Wavefront Error (WFE) perturbation to the OMC testbed and demonstrate wavefront control using two deformable mirrors while the LOWFS/C is correcting those perturbation in our vacuum testbed. As a result, we obtain repeatable convergence below 5 × 10−9 mean contrast with 10% broadband light centered at 550 nm in the 360 degrees dark hole with working angle between 3 λ/D and 9 λ/D. We present the key hardware and software used in the testbed, the performance results and their comparison to model expectations.
To maintain the required performance of WFIRST Coronagraph in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The LOWFS/C uses a Zernike wavefront sensor (ZWFS) with the phase shifting disk combined with the starlight rejecting occulting mask. For wavefront error corrections, WFIRST LOWFS/C uses a fast steering mirror (FSM) for line-of-sight (LoS) correction, a focusing mirror for focus drift correction, and one of the two deformable mirrors (DM) for other low order wavefront error (WFE) correction. As a part of technology development and demonstration for WFIRST Coronagraph, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned. With its configuration similar to the WFIRST flight coronagraph instrument the OMC testbed consists of two coronagraph modes, Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC), a low order wavefront sensor (LOWFS), and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront error that would be induced by the WFIRST telescope’s vibration and thermal changes. In this paper, we will introduce the concept of WFIRST LOWFS/C, describe the OMC testbed, and present the testbed results of LOWFS sensor performance. We will also present our recent results from the dynamic coronagraph tests in which we have demonstrated of using LOWFS/C to maintain the coronagraph contrast with the presence of WFIRST-like line-of-sight and low order wavefront disturbances.
Hybrid Lyot coronagraph (HLC) is one of the two operating modes of the WFIRST-AFTA coronagraph instrument. It produces starlight suppression over the full 360-deg annular region and thus is particularly suitable to improve the discovery space around WFIRST-AFTA targets. Since being selected by the National Aeronautics and Space Administration in December 2013, the coronagraph technology is being matured to technology readiness level 5 by September 2016. We present the progress of HLC key component fabrication and testbed demonstrations with the WFIRST-AFTA pupil. For the first time, a circular HLC occulter mask consisting of metal and dielectric layers is fabricated and characterized. Wavefront control using two deformable mirrors is successfully demonstrated in a vacuum testbed with narrowband light (<1-nm bandwidth at 516 nm) to obtain repeatable convergence below 8×10−9 mean contrast in the 360-deg dark hole with a working angle between 3λ/D and 9λ/D with arbitrary polarization. We detail the hardware and software used in the testbed, the results, and the associated analysis.
The prospect of extreme high-contrast astronomical imaging from space has inspired developments of new coronagraph methods for exoplanet imaging and spectroscopy. However, the requisite imaging contrast, at levels of 1 billion to one or better for the direct imaging of cool mature exoplanets in reflected visible starlight, leads to challenging new requirements on the stability and control of the optical wavefront, at levels currently beyond the reach of ground-based telescopes. We review the design, performance, and science prospects for the hybrid Lyot coronagraph (HLC) on the WFIRST-AFTA telescope. Together with a pair of deformable mirrors for active wavefront control, the HLC creates a full 360-deg high-contrast dark field of view at 10−9 contrast levels or better, extending to within angular separations of 3 λ0/D from the central star, over spectral bandwidths of 10% or more.
We review the design, performance, and future prospects for the Hybrid Lyot Coronagraph (HLC) on the WFIRSTAFTA telescope. Together with a pair of deformable mirrors for active wavefront control, the HLC creates high contrast dark fields of view at 10-9 contrast levels, extending to within angular separations of 3 λ0/D from the central star, over spectral bandwidths of 10% or more.
We have been developing focal-plane phase-mask coronagraphs ultimately aiming at direct detection and characterization of Earth-like extrasolar planets by future space coronagraph missions. By utilizing photonic-crystal technology, we manufactured various coronagraphic phase masks such as eight-octant phase masks (8OPMs), 2nd-order vector vortex masks, and a 4th-order discrete (32-sector) vector vortex mask. Our laboratory experiments show that the 4th-order vortex mask reaches to higher contrast than the 2nd-order one at inner region on a focal plane. These results demonstrate that the higher-order vortex mask is tolerant of low-order phase aberrations such as tip-tilt errors. We also carried out laboratory demonstration of the 2nd-order vector vortex masks in the High-Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL), and obtained 10-8-level contrast owing to an adaptive optics system for creating dark holes. In addition, we manufactured a polarization-filtered 8OPM, which theoretically realizes achromatic performance. We tested the manufactured polarization-filtered 8OPM in the Infrared Coronagraphic Testbed (IRCT) at the JPL. Polychromatic light sources are used for evaluating the achromatic performance. The results suggest that 10-5- level peak-to-peak contrasts would be obtained over a wavelength range of 800-900 nm. For installing the focal-plane phase-mask coronagraph into a conventional centrally-obscured telescope with a secondary mirror, pupil-remapping plates have been manufactured for removing the central obscuration to enhance the coronagraphic performance. A result of preliminary laboratory demonstration of the pupil-remapping plates is also reported. In this paper, we present our recent activities of the photonic-crystal phase coronagraphic masks and related techniques for the high-contrast imaging.
NASA’s WFIRST-AFTA mission concept includes the first high-contrast stellar coronagraph in space. This coronagraph will be capable of directly imaging and spectrally characterizing giant exoplanets similar to Neptune and Jupiter, and possibly even super-Earths, around nearby stars. In this paper we present the plan for maturing coronagraph technology to TRL5 in 2014-2016, and the results achieved in the first 6 months of the technology development work. The specific areas that are discussed include coronagraph testbed demonstrations in static and simulated dynamic environment, design and fabrication of occulting masks and apodizers used for starlight suppression, low-order wavefront sensing and control subsystem, deformable mirrors, ultra-low-noise spectrograph detector, and data post-processing.
We present and compare experimental results in high contrast imaging representing the state of the art in coronagraph and starshade technology. These experiments have been undertaken with the goal of demonstrating the capability of detecting Earth-like planets around nearby Sun-like stars. The contrast of an Earth seen in reflected light around a Sun-like star would be about 1.2 × 10−10. Several of the current candidate technologies now yield raw contrasts of 1.0 × 10−9 or better, and so should enable the detection of Earths, assuming a gain in sensitivity in post-processing of a factor of 10. We present results of coronagraph and starshade experiments conducted at visible and infrared wavelengths. Cross-sections of dark fields are directly compared as a function of field angle and bandwidth. The strength and differences of the techniques are compared.
We present a novel optical integral field spectrograph (IFS) called the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES), which will be a facility class instrument within the NASA Exoplanet Exploration Program's High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory. Integral field spectroscopy is ideal for imaging faint exoplanets: it enables spectral characterization of exoplanet atmospheres and can improve contrast by providing chromatic measurements of the target star's point-spread function (PSF). PISCES at the HCIT will be the first IFS to demonstrate imaging spectroscopy in the 10-9 contrast regime required for characterizing exoplanets imaged in scattered light. It is directly relevant as a prototype for IFS science instruments that could fly with the AFTA Coronagraph, the Exoplanet Probe missions currently under study, and/or the ATLAST mission concept. We present the instrument requirements, a baseline design for PISCES, a simulation of its performance, a solution to mitigate spectral crosstalk, experimental verification of our simulator, and the final vacuum compatible opto-mechanical design. PISCES will be assembled and tested at the Goddard Space Flight Center (GSFC), and subsequently delivered and integrated into the HCIT facility. Testing at HCIT will verify the performance of PISCES and its ability to meet the requirements of a space mission, will enable investigations into broadband wavefront control using the IFS as an image plane sensor, and will allow tests of contrast enhancement via multiwavelength differential imaging post-processing. Together with wavefront control and starlight suppression, PISCES is thus a key element for maturing the overall integrated system for a future coronagraphic space mission. PISCES is scheduled to receive first light in the HCIT in 2015.
The vortex coronagraph has already enabled high-contrast observations very close to bright stars on large ground-based telescopes, and it also has great potential for use on coronagraphic space missions aimed at exoplanet detection and characterization. As such, demonstrations of vortex coronagraph performance have recently been carried out in JPL’s High Contrast Imaging Testbed. Some of our recent results are presented here, including the suppression of a monochromatic, single-polarization point-source to below the 10-9 level over a dark hole covering both the 2-7 λ/D and 3-8 λ/D regions, as well as the suppression of a 10% band of white-light to approximately the 10-8 level over a 3-8 λ/D dark hole.
We update the design, performance, and future prospects for the complex apodized Lyot coronagraph. We extend previous design work for off axis telescope with unobscured circular pupils, now to designs for high-contrast exoplanet imaging and spectroscopy with complicated pupil obscurations such as the WFIRST/AFTA telescope. Together with a pair of deformable mirrors for active wavefront control, the complex apodized Lyot coronagraph creates high contrast dark fields of view extending to within angular separations of 3 λ/D from the central star, over spectral bandwidths of 10% or more, and with throughput efficiencies greater than 35%.
As part of the NASA ROSES Technology Demonstrations for Exoplanet Missions (TDEM) program, we conducted a numerical modeling study of three internal coronagraphs (PIAA, vector vortex, hybrid bandlimited) to understand their behaviors in realistically-aberrated systems with wavefront control (deformable mirrors). This investigation consisted of two milestones: (1) develop wavefront propagation codes appropriate for each coronagraph that are accurate to 1% or better (compared to a reference algorithm) but are also time and memory efficient, and (2) use these codes to determine the wavefront control limits of each architecture. We discuss here how the milestones were met and identify some of the behaviors particular to each coronagraph. The codes developed in this study are being made available for community use. We discuss here results for the HBLC and VVC systems, with PIAA having been discussed in a previous proceeding.
Photonic crystal, an artificial periodic nanostructure of refractive indices, is one of the attractive technologies for
coronagraph focal-plane masks aiming at direct imaging and characterization of terrestrial extrasolar planets. We
manufactured the eight-octant phase mask (8OPM) and the vector vortex coronagraph (VVC) mask very precisely using
the photonic crystal technology. Fully achromatic phase-mask coronagraphs can be realized by applying appropriate
polarization filters to the masks. We carried out laboratory experiments of the polarization-filtered 8OPM coronagraph
using the High-Contrast Imaging Testbed (HCIT), a state-of-the-art coronagraph simulator at the Jet Propulsion
Laboratory (JPL). We report the experimental results of 10-8-level contrast across several wavelengths over 10%
bandwidth around 800nm. In addition, we present future prospects and observational strategy for the photonic-crystal
mask coronagraphs combined with differential imaging techniques to reach higher contrast. We proposed to apply the
polarization-differential imaging (PDI) technique to the VVC, in which we built a two-channel coronagraph using
polarizing beam splitters to avoid a loss of intensity due to the polarization filters. We also proposed to apply the
angular-differential imaging (ADI) technique to the 8OPM coronagraph. The 8OPM/ADI mode mitigates an intensity
loss due to a phase transition of the mask and provides a full field of view around central stars. We present results of
preliminary laboratory demonstrations of the PDI and ADI observational modes with the phase-mask coronagraphs.
We review the design, fabrication, performance, and future prospects for a complex apodized Lyot coronagraph for highcontrast
exoplanet imaging and spectroscopy. We present a newly designed circular focal plane mask with an inner
working angle of 2.5 λ/D. Thickness-profiled metallic and dielectric films superimposed on a glass substrate provide
control over both the real and imaginary parts of the coronagraph wavefront. Together with a deformable mirror for
control of wavefront phase, the complex Lyot coronagraph potentially exceeds billion-to-one contrast over dark fields
extending to within angular separations of 2.5 λ/D from the central star, over spectral bandwidths of 20% or more, and
with throughput efficiencies better than 50%.
Our approach is demonstrated with a linear occulting mask, for which we report our best laboratory imaging contrast
achieved to date. Raw image contrasts of 3×10-10 over 2% bandwidths, 6×10-10 over 10% bandwidths, and 2×10-9 over
20% bandwidths are consistently achieved across high contrast fields extending from an inner working angle of 3 λ/D to
a radius of 15 λ/D. Occulter performance is analyzed in light of experiments and optical models, and prospects for
further progress are summarized.
The science capability of the hybrid Lyot coronagraph is compared with requirements for ACCESS, a representative
space coronagraph concept for the direct imaging and spectroscopy of exoplanet systems. This work has been supported
by NASA’s Strategic Astrophysics Technology / Technology Demonstrations for Exoplanet Missions (SAT/TDEM)
program.
Debris disks around nearby stars are tracers of the planet formation process, and they are a key element of our understanding of the formation and evolution of extrasolar planetary systems. With multi-color images of a significant number of disks, we can probe important questions: can we learn about planetary system evolution; what materials are the disks made of; and can they reveal the presence of planets? Most disks are known to exist only through their infrared flux excesses as measured by the Spitzer Space Telescope, and through images measured by Herschel. The brightest, most extended disks have been imaged with HST, and a few, such as Fomalhaut, can be observed using ground-based telescopes. But the number of good images is still very small, and there are none of disks with densities as low as the disk associated with the asteroid belt and Edgeworth Kuiper belt in our own Solar System.
Direct imaging of disks is a major observational challenge, demanding high angular resolution and extremely high dynamic range close to the parent star. The ultimate experiment requires a space-based platform, but demonstrating much of the needed technology, mitigating the technical risks of a space-based coronagraph, and performing valuable measurements of circumstellar debris disks, can be done from a high-altitude balloon platform. In this paper we present a balloon-borne telescope concept based on the Zodiac II design that could undertake compelling studies of a sample of debris disks.
The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and
space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and
in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength
ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of
technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of
Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris-
Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid
crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete
perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive
optics, extremely large ground-based telescopes, and space-based internal coronagraphs.
As part of the NASA ROSES Technology Development for Exoplanet Missions (TDEM) program, we are conducting a
study of three internal coronagraphs (PIAA, vector vortex, hybrid bandlimited) to understand their behaviors in
realistically-aberrated systems with wavefront control (deformable mirrors). This study consists of two milestones: (1)
develop wavefront propagation codes appropriate for each coronagraph that are accurate to 1% or better (compared to a
reference algorithm) but are also time and memory efficient, and (2) use these codes to determine the wavefront control
limits of each architecture. We discuss the results from the study so far, with emphasis on representing the PIAA
coronagraph and its wavefront control behavior.
The Vector Vortex Coronagraph (VVC) is an attractive internal coronagraph solution to image and characterize
exoplanets. It provides four key pillars on which efficient high contrast imaging instruments can be built for
ground- and space-based telescopes: small inner working angle, high throughput, clear off-axis discovery space,
and simple layout. We present the status of the VVC technology development supported by NASA. We will
review recent results of the optical tests of the second-generation topological charge 4 VVC on the actively
corrected High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL). New VVC contrast
records have been established.
We report our best laboratory contrast demonstrations achieved to date. We review the design, fabrication, performance,
and future prospects of a hybrid focal plane occulter for exoplanet coronagraphy. Composed of thickness-profiled
metallic and dielectric thin films vacuum deposited on a fused silica substrate, the hybrid occulter uses two
superimposed thin films for control over both the real and imaginary parts of the complex attenuation pattern. Together
with a deformable mirror for adjustment of wavefront phase, the hybrid Lyot coronagraph potentially exceeds billion-toone
contrast over dark fields extending to within angular separations of 3 λ/D from the central star, over spectral
bandwidths of 20% or more, and with throughput efficiencies up to 60%.
We report laboratory contrasts of 3×10-10 over 2% bandwidths, 6×10-10 over 10% bandwidths, and 2×10-9 over 20%
bandwidths, achieved across high contrast fields extending from an inner working angle of 3 λ/D to a radius of 15 λ/D.
Occulter performance is analyzed in light of recent experiments and optical models, and prospects for further
improvements are summarized.
The science capabilities of the hybrid Lyot coronagraph are compared with requirements of the ACCESS mission, a
representative exoplanet space telescope concept study for the direct imaging and spectroscopy of exoplanet systems.
This work has been supported by NASA's Technology Demonstration for Exoplanet Missions (TDEM) program.
Zodiac II is a proposed balloon-borne science investigation of debris disks around nearby stars. Debris disks are
analogs of the Asteroid Belt (mainly rocky) and Kuiper Belt (mainly icy) in our Solar System. Zodiac II will
measure the size, shape, brightness, and color of a statistically significant sample of disks. These measurements
will enable us to probe these fundamental questions: what do debris disks tell us about the evolution of planetary
systems; how are debris disks produced; how are debris disks shaped by planets; what materials are debris disks
made of; how much dust do debris disks make as they grind down; and how long do debris disks live? In addition,
Zodiac II will observe hot, young exoplanets as targets of opportunity.
The Zodiac II instrument is a 1.1-m diameter SiC telescope and an imaging coronagraph on a gondola carried
by a stratospheric balloon. Its data product is a set of images of each targeted debris disk in four broad visiblewavelength
bands. Zodiac II will address its science questions by taking high-resolution, multi-wavelength images
of the debris disks around tens of nearby stars. Mid-latitude flights are considered: overnight test flights within
the United States followed by half-global flights in the Southern Hemisphere. These longer flights are required to
fully explore the set of known debris disks accessible only to Zodiac II. On these targets, it will be 100 times more
sensitive than the Hubble Space Telescope's Advanced Camera for Surveys (HST/ACS); no existing telescope
can match the Zodiac II contrast and resolution performance. A second objective of Zodiac II is to use the
near-space environment to raise the Technology Readiness Level (TRL) of SiC mirrors, internal coronagraphs,
deformable mirrors, and wavefront sensing and control, all potentially needed for a future space-based telescope
for high-contrast exoplanet imaging.
The optical vortex coronagraph has great potential for enabling high-contrast observations very close to bright stars, and
thus for reducing the size of space telescopes needed for exoplanet characterization missions. Here we discuss several
recent developments in optical vortex coronagraphy. In particular, we describe multi-stage vortex configurations that
allow the use of on-axis telescopes for high-contrast coronagraphy, and also enable the direct measurement of the
amplitudes and phases of focal plane speckles. We also briefly describe recent laboratory demonstrations of the optical
properties of the dual-stage vortex, and of the broadband performance of single stage vortex masks. Indeed, the
demonstrated performance of the vector vortex phase masks already in hand, ≈ 10-8, is approximately that needed for an
initial coronagraphic mission, such as an exoplanet explorer, aimed at detecting exozodiacal light and jovian exoplanets.
KEYWORDS: Stars, Space telescopes, Telescopes, Coronagraphy, Space operations, Optical isolators, Control systems, Cameras, Point spread functions, Observatories
ACCESS (Actively-Corrected Coronagraph for Exoplanet System Studies) was one of four medium-class exoplanet
concepts selected for the NASA Astrophysics Strategic Mission Concept Study (ASMCS) program in 2008/2009 [14,
15]. The ACCESS study evaluated four major coronagraph concepts under a common space observatory. This paper
describes the high precision pointing control system (PCS) baselined for this observatory.
ACCESS is one of four medium-class mission concepts selected for study in 2008-9 by NASA's 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.
The Vector Vortex Coronagraph is a phase-based coronagraph, one of the most efficient in terms of inner working
angle, throughput, discovery space, contrast, and simplicity. Using liquid-crystal polymer technology, this new
coronagraph has recently been the subject of lab demonstrations in the near-infrared, visible and was also used
on sky at the Palomar observatory in the H and K bands (1.65 and 2.2 μm, respectively) to image the brown
dwarf companion to HR 7672, and the three extra-solar planets around HR 8799. However, despite these recent
successes, the Vector Vortex Coronagraph is, as are most coronagraphs, sensitive to the central obscuration
and secondary support structures, low-order aberrations (tip-tilt, focus, etc), bandwidth (chromaticism), and
polarization when image-plane wavefront sensing is performed. Here, we consider in detail these sensitivities as
a function of the topological charge of the vortex and design features inherent to the manufacturing technology,
and show that in practice all of them can be mitigated to meet specific needs.
The NASA exoplanet exploration program is dedicated to developing technologies for detecting and characterizing
extrasolar planets. In support of that program we have evaluated three different coronagraphic techniques (bandlimited
Lyot, optical vortex, and phase-induced pupil apodization) using optical propagation simulations. These utilized a
complete hypothetical telescope+coronagraph system with phase and amplitude aberrations. Wavefront control using
dual sequential deformable mirrors was performed. We discuss the different computational techniques necessary to
accurately simulate each coronagraph.
We report the status of JPL and JDSU ongoing technological developments and contrast results of the vector
vortex coronagraph (VVC) made out of liquid crystal polymers (LCP). The first topological charge 4 VVC was
tested on the high contrast imaging testbed (HCIT) around 800 nm, under vacuum and with active wavefront
control (32x32 Xinetics deformable mirror). We measured the inner working angle or IWA (50% off-axis transmission)
at ~ 1.8λ/d. A one-sided dark hole ranging from 3λ/d to 10λ/d was created in polarized light, showing
a mean contrast of ~ 2 × 10-7 over a 10% bandwidth. This contrast was maintained very close in (3 λ/d) in
a reduced 2% bandwidth. These tests begin to demonstrate the potential of the LCP technology in the most
demanding application of a space-based telescope dedicated to extrasolar planet characterization. The main limitations
were identified as coming from incoherent sources such as multiple reflections, and residual chromaticity.
A second generation of improved masks tackling these issues is being manufactured and will be tested on the
HCIT in the coming months.
We present the current status of our testing of a phase-induced amplitude apodization (PIAA) coronagraph at the Jet
Propulsion Lab's High Contrast Imaging Testbed (HCIT) vacuum facilities. These PIAA optics were designed to
produce a point-spread function containing a region whose intensity is below 10-9 over a 20-percent fractional bandpass,
comparable to the requirements for direct imaging of exoplanets from space. The results presented here show contrast
levels of 4×10-7 in monochromatic light, with an inner working angle of 2.4 λ/D. The instrumentation is described here,
as well as the testing procedures, wavefront control, and results.
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.
Coronagraph focal-plane occulting masks have generally been described in terms of attenuation profiles free of any
phase shift. However, phase shifts are expected and observed in physical occulting masks, with significant effect at
billion-to-one coronagraph contrast levels in spectrally broad light, as required for the direct imaging and spectroscopy
of nearby exoplanet systems.
We report progress in the design and fabrication of hybrid focal-plane masks for Lyot coronagraphy. These masks,
composed of thickness-profiled metallic and dielectric thin films superimposed on a glass substrate, are in principle
band-limited in both the real and imaginary parts of the occulter characteristics. Together with a deformable mirror for
control of wavefront phase, these masks offer Lyot coronagraph contrast performance better than 10-9 over spectral
bandwidths of 30% or more with throughput efficiencies up to 67%. We report recent laboratory coronagraph
demonstrations with vacuum-deposited nickel masks on fused silica, and preparations for the fabrication of masks with
superimposed metal and dielectric layers.
Space coronagraphy is a promising method for direct imaging of planetary systems orbiting the nearby stars. The High
Contrast Imaging Testbed is a laboratory facility at JPL that integrates the essential hardware and control algorithms
needed for suppression of diffracted and scattered light near a target star that would otherwise obscure an associated
exo-planetary system. Stable suppression of starlight by a factor of 5×10−10 has been demonstrated consistently in
narrowband light over fields of view as close as four Airy radii from the star. Recent progress includes the extension of
spectral bandwidths to 10% at contrast levels of 2×10−9, with work in progress to further improve contrast levels,
bandwidth, and instrument throughput. We summarize recent laboratory results and outline future directions. This
laboratory experience is used to refine computational models, leading to performance and tolerance predictions for
future space mission architectures.
Coronagraph focal-plane occulting masks have generally been described as attenuation profiles free of any phase shift.
However, phase shifts are expected and observed in physical occulting masks, and they can impose significant
limitations on coronagraph contrast at the billion-to-one level in spectrally broad light, as required for the direct imaging
of planetary systems orbiting the nearby stars. Here we explore design options for a physically realizable occulting mask
composed of a metallic and a dielectric thin film, each profiled in thickness and superimposed on a glass substrate. We
show that such hybrid masks, together with a deformable mirror for control of wavefront phase, offer contrast
performance better than 10-9 over spectral bandwidths up to 30% with Lyot coronagraph throughput efficiencies of 66%
or more.
Great strides have been made in recent years toward the goal of high-contrast imaging with a sensitivity adequate
to detect earth-like planets around nearby stars. It appears that the hardware − optics, coronagraph masks,
deformable mirrors, illumination systems, thermal control systems − are up to the task of obtaining the required
10-10 contrast. But in broadband light (e.g., 10% bandpass) the wavefront control algorithms have been a
limiting factor. In this paper we describe a general correction methodology that works in broadband light with
one or multiple deformable mirrors by conjugating the electric field in a predefined region in the image where
terrestrial planets would be found. We describe the linearized approach and demonstrate its effectiveness through
laboratory experiments. This paper presents results from the Jet Propulsion Laboratory High Contrast Imaging
Testbed (HCIT) for both narrow-band light (2%) and broadband light (10%) correction.
KEYWORDS: Coronagraphy, Light sources, Photomasks, Algorithm development, Wavefronts, Modeling and simulation, Near field optics, Phase measurement, Data modeling, Optical components
The performance of the high-contrast imaging testbed (HCIT) at JPL is investigated through optical modeling and
simulations. The analytical tool is an optical simulation algorithm developed by combining the HCIT's optical model
with a speckle-nulling algorithm that operates directly on coronagraphic images, an algorithm identical to the one
currently being used on the HCIT to actively suppress scattered light via a deformable mirror. It is capable of
performing full three-dimensional end-to-end near-field diffraction analysis on the HCIT's optical system. By
conducting extensive speckle-nulling optimization, we clarify the HCIT's capability and limitations in terms of its
contrast performance under various realistic conditions. Considered cases include non-ideal occulting masks, such as a
mask with parasitic phase-delay errors (i.e., a not band-limited occulting mask) and one with damped ripples in its
transmittance profiles, as well as the phase errors of all optics. Most of the information gathered on the HCIT's optical
components through measurement and characterization over the last several years at JPL has been used in this analysis to
make the predictions as accurate as possible. Our simulations predict that the contrast values obtainable on the HCIT
with narrow-band (monochromatic) illumination at 785nm wavelength are Cm=1.58x10-11 (mean) and C4=5.11x10-11(at
4λ/D), in contrast to the measured results of Cm~6×10-10 and C4~8×10-10, respectively. In this paper we report our
findings about the monochromatic light performance of the HCIT. We will describe the results of our investigation
about the HCIT's broad-band performance in an upcoming paper.
The NASA Terrestrial Planet Finder Coronagraph (TPF-C) mission envisions using a space telescope with an
approximately 8 m by 3 m diameter primary mirror to image and spectroscopically characterize at visible wavelengths
Earth-like planets orbiting nearby stars. Such terrestrial planets have intensities of 10-10 relative to their stars at
separations of a fraction of an arcsecond, requiring extremely high-contrast imaging capabilities. A simple optical
system with a minimal number of surfaces will likely have the best chance to image a very faint source near a bright
star. A proposed version of a TPF-C integrated starlight suppression system and camera called SpeckleCam
encompasses such a design. It incorporates two high-density deformable mirrors to control phase and amplitude
wavefront errors, a coronagraph to suppress the stellar diffraction pattern, and simultaneous imaging in three passbands.
We use the SpeckleCam concept to examine the utility of the PROPER modeling package, a set of IDL routines that
simulate wavefront propagation in an optical system.
Knowledge of wavefront amplitude is as important as the knowledge of phase for a coronagraphic high contrast imaging system. Efforts have been made to understand various contributions to the amplitude variation in Terrestrial Planet Finder's (TPF) High Contrast Imaging Testbed (HCIT). Modeling of HCIT with as-built mirror surfaces has shown an amplitude variation of 1.3% due to the phase-amplitude mixing for the testbed's front-end optics. Experimental measurements on the testbed have shown the amplitude variation is about 2.5% with the testbed's illumination pattern having a major contribution to the low order amplitude variation.
Predictions of contrast performance for the Eclipse coronagraphic telescope are based on computational models that are tested and validated with laboratory experience. We review recent laboratory work in the key technology areas for an actively-corrected space telescope designed for extremely high contrast imaging of nearby planetary systems. These include apodized coronagraphic masks, precision deformable mirrors, and coronagraphic algorithms for wavefront sensing and correction, as integrated in the high contrast imaging testbed at JPL. Future work will focus on requirements for the Terrestrial Planet Finder coronagraph mission.
The Terrestrial Planet Finder (TPF) mission is planning to launch a visible coronagraphic space telescope in 2014. To achieve TPF science goals, the coronagraph must have extreme levels of wavefront correction (less than 1 Å rms over controllable spatial frequencies) and stability to get the necessary suppression of diffracted starlight (~1E-10 contrast at an angular separation ~4 lambda/D). TPF Coronagraph’s primary platform for experimentation is the High Contrast Imaging Testbed, which will provide laboratory validation of key technologies as well as demonstration of a flight-traceable approach to implementation. Precision wavefront control in the testbed is provided by a high actuator density deformable mirror. Diffracted light control is achieved through use of occulting or apodizing masks and stops. Contrast measurements will establish the technical feasibility of TPF requirements, while model and error budget validation will demonstrate implementation viability. This paper describes the current testbed design, development approach, and recent experimental results.
Very high contrast imagery, required for exoplanet image acquisition, imposes significantly different criteria upon telescope architecture than do the requirements imposed upon most spaceborne telescopes. For the Eclipse Mission, the fundamental figure-of-merit is a stellar contrast, or brightness reduction ratio, reaching a factor of 10-9 or better at star-planet distances as close as the 4th Airy ring. Factors necessary to achieve such contrast ratios are both irrelevant and largely ignored in contemporary telescope design. Although contemporary telescoeps now meet Hubble Space Telescope performance at substantially lower mass and cost than HST, control of mid-spatial-frequency (MSF) errors, crucial to coronagraphy, has not been emphasized. Accordingly, roughness at MSF has advanced little since HST. Fortunately, HST primary mirror smoothness would nearly satisfy Eclipse requirements, although other aspects of HST are undesirable for stellar coronagraphy. Conversely, the narrow field required for Eclipse eases other drivers of traditional telescope design. A systematic approach to telescope definition, with primary and sub-tier figures-of-merit, will be discussed in the context of the Eclipse Mission.
Coronagraphs for extra-solar planet detection remove diffracted stellar light through the combination of a coronagraphic mask and a Lyot stop. When the entrance pupil contains a nearly perfect wave front, most of the stellar light is absorbed at the mask. Light scattered around the spot due to mid- and high-spatial frequency phase errors in the pupil appears at the Lyot plane as speckles whose amplitudes are proportional to the local wave front phase residuals. The speckles scale with optical wavelength but are not radially smeared. The Eclipse deformable mirror (DM) can be used to modify the Lyot amplitude distribution, providing a simple means of estimating the residual phase content and controlling the wave front. To reduce the detrimental noise carried by uncontrollable high-spatial frequency wave front components, the Lyot plane signal is filtered at the science plane to pass only the controllable spatial frequencies that contribute to the dark hole. The Lyot stop is then reimaged onto a detector. We demonstrate through simulations that this approach significantly improves the signal-to-noise ratio of the planet measurement.
Active wavefront correction of a space telescope provides a technology path for extremely high contrast imaging astronomy at levels well beyond the capabilities of current telescope systems. A precision deformable mirror technology intended specifically for wavefront correction in a visible/near-infrared space telescope
has been developed at Xinetics and extensively tested at JPL over the past several years. Active wavefront phase correction has been demonstrated to 1 Angstrom rms over the spatial frequency range accessible to a mirror with an array of actuators on a 1 mm pitch. It is based on a modular electroceramic design that is scalable to
1000s of actuator elements coupled to the surface of a thin mirror facesheet. It is controlled by a low-power multiplexed driver system. Demonstrated surface figure control, high actuator density, and low power dissipation are described. Performance specifications are discussed in the context of the Eclipse point design for a coronagraphic space telescope.
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