We describe a NASA Strategic Astrophysics Technology initiative. Our objective is the implementation and closed-loop demonstration of a new optical wavefront control element for the active correction of low-order wavefront errors associated with telescope line-of-sight jitter, thermal gradients, and alignment drift. In concert with a Lyot coronagraph and Zernike wavefront sensor in a laboratory vacuum environment, this hardware demonstrates the separation of active low-order and stable high-order wavefront control at high levels of contrast and provides experimental evidence that significant sources of error have been identified and effectively controlled or mitigated.
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 Roman Space Telescope will have the first advanced coronagraph in space, with deformable mirrors (DMs) for wavefront control (WFC), low-order wavefront sensing and maintenance, and a photon-counting detector. It is expected to be able to detect and characterize mature, giant exoplanets in reflected visible light. Over the past decade, the performance of the coronagraph in its flight environment has been simulated with increasingly detailed diffraction and structural/thermal finite-element modeling. With the instrument now being integrated in preparation for launch within the next few years, the present state of the end-to-end modeling, including the measured flight components such as DMs, is described. The coronagraphic modes, including characteristics most readily derived from modeling, are thoroughly described. The methods for diffraction propagation, WFC, and structural and thermal finite-element modeling are detailed. The techniques and procedures developed for the instrument will serve as a foundation for future coronagraphic missions, such as the Habitable Worlds Observatory.
NASA is about to embark on an ambitious program to develop a Habitable Worlds Observatory (HWO) flagship mission to directly image approximately 25 potentially Earth-like planets and spectroscopically characterize them for signs of life, as recommended by the Astro2020 decadal survey. In addition, Astro2020 recommended a new approach for flagship formulation, which involves increasing the scope and depth of early, pre-phase A trades and technology maturation, as part of the new Great Observatories Maturation Program (GOMAP). The critical capability of the HWO mission is starlight suppression. To inform future architecture trades, it is necessary to survey a wide range of 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 an interim update on a new effort, initiated by NASA’s Exoplanet Exploration Program (ExEP), to survey coronagraph design options for HWO. We present a preliminary summary of the survey, including: (1) a current list of coronagraph design options; (2) proposed evaluation criteria, such as expected mission yields and feasibility of maturing to TRL5 by 2029; and (3) tools and methods which we are using to quantify evaluations of different designs. While not charged to down-select or prioritize the different coronagraph designs, this survey is expected to be valuable in informing future mission teams of coronagraph design options. All interested coronagraph researchers are welcome to participate in this survey by contacting the first two authors of this paper.
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.
The Wide Field Infrared Survey Telescope (WFIRST), which is planned for launch in 2025, is NASA’s next large space observatory after the James Webb Space Telescope. It contains two primary science instruments: A Wide Field Instrument (WFI) to carry out surveys of galaxies in the near infrared; explore the properties of dark energy and dark matter; and carry out a microlensing survey to complete the census of exoplanets, and a Coronagraph Instrument (CGI) to demonstrate high-contrast technology for exoplanet imaging and spectroscopy. Understanding how to implement the technology for CGI is a critical step toward future, larger missions targeted at direct imaging of Earthlike planets in the habitable zone of nearby stars. This paper presents an overview of the current instrument design and requirements, highlighting the critical technologies being demonstrated. These include two types of coronagraphs, closed-loop wavefront control with two deformable mirrors, a visible-light, electron multiplying detector (EMCCD), and a lenslet based integral field spectrograph. Additionally, critical algorithms will be developed and tested for low- and high-order wavefront control, spectral extraction, and post-processing for planet detection. The paper will also describe the operational plans for the instrument. A participating scientist program (PSP) will enable members of the community to engage in the technology demonstration and, if warranted by instrument performance, will incorporate science focusing on imaging and spectra of large Jupiter size planets and protoplanetary and debris disks and a blind search program for undiscovered Jupiters and possibly mini-Neptunes.
The Habex study, commissioned by NASA in preparation for the 2020 Decadal Survey, is evaluating a 4 meter space telescope for high contrast imaging and spectral characterization of extrasolar terrestrial planets. Its off-axis configuration, active structural metrology, and low-disturbance pointing control provide an optimal system for coronagraphs. We present predictions of the Habex performance using a charge 6 vortex coronagraph that have been obtained using numerical modeling techniques developed for the WFIRST coronagraph. The models include realistic optical surface and polarization-induced aberrations, pointing jitter, and thermally-induced wavefront variations. Wavefront control using dual deformable mirrors is simulated to create a dark, high-contrast hole around the star. The results show that current technologies can closely approach the Habex performance goals, and with some additional development in key areas (e.g., deformable mirror surface quality, low-polarization coatings, etc.) over the next few years they should reliably meet them.
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.
The Wide Field Infrared Survey Telescope (WFIRST) Coronagraph Instrument (CGI) will be the first high-performance stellar coronagraph using active wavefront control for deep starlight suppression in space, providing unprecedented levels of contrast and spatial resolution for astronomical observations in the optical. One science case enabled by the CGI will be taking visible images and (R~50) spectra of faint interplanetary dust structures present in the habitable zone of nearby sunlike stars (~10 pc) and within the snow-line of more distant ones (~20 pc), down to dust brightness levels commensurate with that of the solar system zodiacal cloud. Reaching contrast levels below 10-7 at sub-arcsecond angular scales for the first time, CGI will cross an important threshold in debris disks physics, accessing disks with low enough optical depths that their structure is dominated by transport mechanisms rather than collisions. Hence, CGI will help us understand how exozodiacal dust grains are produced and transported in low-density disks around mature stars. Additionally, CGI will be able to measure the brightness level and constrain the degree of asymmetry of exozodiacal clouds around individual nearby sunlike stars in the optical, at the ~3x solar zodiacal emission level. This information will be extremely valuable for optimizing the observational strategy of possible future exo-Earth direct imaging missions, especially those planning to operate at optical wavelengths as well, such as the Habitable Exoplanet Observatory (HabEx) and the Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR).
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.
The Wide Field Infrared Survey Telescope (WFIRST), which is entering Phase B for a launch in 2026, is NASA’s next large space observatory after the James Webb Space Telescope. In addition to the primary science carried out by The Wide Field Instrument (WFI), which is designed to carry out surveys of galaxies in the near infrared, explore the properties of dark energy and dark matter, and carry out a microlensing survey to complete the census of exoplanets, there will be a technology demonstration of a Coronagraph Instrument (CGI) for very high-contrast imaging and spectroscopy of nearby exoplanets. The CGI will incorporate two coronagraph types and demonstrate low- and high-order wavefront correction for the first time on a space telescope. Operating in the visible, it will consist of a direct imaging camera and a lenslet based integral field spectrograph, both using electron-multiplying CCDs in the focal plane, as well as polarizers allowing direct imaging in separate polarization states. Written by the lead science and engineering team, supported by two science investigation teams (SITs – https://wfirst.gsfc.nasa.gov/science/fswg/scienceteam.html), this paper presents an overview of the technology requirements on the instrument, the instrument design, and the operational plans to demonstrate exoplanet imaging and spectroscopic capability. Also described is how CGI will advance algorithms for extracting planet images from the background and retrieving spectra from a space IFS. Once the core performance is successfully demonstrated, CGI will also be used in the latter part of the mission for a dedicated science and Guest Observer (GO) program. This paper thus also describes the potentially revolutionary science that will be enabled through direct imaging and spectroscopy of known radial velocity planets and debris disks as seen in reflected light.
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.
The WFIRST/AFTA 2.4 m space telescope currently under study includes a stellar coronagraph for the imaging and the spectral characterization of extrasolar planets. The coronagraph employs sequential deformable mirrors to compensate for phase and amplitude errors. Using the optical model of an Occulting Mask Coronagraph (OMC) testbed at the Jet Propulsion Laboratory (JPL), we have investigated the sensitivity of a Hybrid Lyot Coronagraph (HLC) broadband contrast performance to DM actuator errors and actuator limits. Considered case include drifts in actuator gains or actuator response curves, paired actuators, as well as the limits imposed by a neighboring-actuator rule. Actual data about the actuator drifts and the knowledge about the paired-actuators obtained in several DM characterization experiments conducted at JPL, as well as the neighboring-actuator rule implemented on the OMC testbed were used in simulations. We obtained good agreement between the model prediction and the testbed measurement in terms of static HLC contrast floor and contrast chromaticity.
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.
We present a concept study to develop a new instrument to sequentially and over a long time measure methane abundance
on Mars and find out its global seasonal variations, if any. The Planetary Homodyne Interferometer (PHI) can offer
integrated spectra over a wide field-of-view (FOV) in high spectral resolution (R~105) in a compact design using no (or a
small < 1m) primary mirror. PHI is best suited to studies of sources where temporally tracing specific spectral features
sensitivity, and spectral resolution is of higher significance than spatial fidelity.
The Wide-Field Infrared Survey Telescope (WFIRST) is a NASA observatory concept, now in phase A study, which is designed to perform wide-field imaging and slitless spectroscopic surveys for dark energy research and other astrophysical studies. It will also perform microlensing surveys to look for distant exoplanets in our galaxy, and direct imaging studies of some of the very nearest exoplanets. The current astrophysics focused telescope assets (AFTA) design of the mission makes use of an existing 2.4-m telescope, which yields enhanced sensitivity and imaging performance in all these science programs. AFTA also enables the addition of a coronagraph instrument (CGI) for direct imaging and spectroscopy of nearby giant exoplanets (including some that were discovered by radial velocity and other methods), and also for observing debris disks around the candidate host stars. This paper outlines the context for the other papers in this special volume on the WFIRST-AFTA CGI, covering the science, design, engineering, and technology development of the observatory and its CGI.
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.
"Exo-C" is NASAs first community study of a modest aperture space telescope mission that is optimized for high contrast observations of exoplanetary systems. The mission will be capable of taking optical spectra of nearby exoplanets in reflected light, discovering previously undetected planets, and imaging structure in a large sample of circumstellar disks. It will obtain unique science results on planets down to super-Earth sizes and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable Earth-like exoplanets. We present the mission/payload design and highlight steps to reduce mission cost/risk relative to previous mission concepts. Key elements are an unobscured telescope aperture, an internal coronagraph with deformable mirrors for precise wavefront control, and an orbit and observatory design chosen for high thermal stability. Exo-C has a similar telescope aperture, orbit, lifetime, and spacecraft bus requirements to the highly successful Kepler mission (which is our cost reference). Much of the needed technology development is being pursued under the WFIRST coronagraph study and would support a mission start in 2017, should NASA decide to proceed. This paper summarizes the study final report completed in March 2015.
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.
“Exo-C” is NASA’s first community study of a modest aperture space telescope designed for high contrast observations of exoplanetary systems. The mission will be capable of taking optical spectra of nearby exoplanets in reflected light, discover previously undetected planets, and imaging structure in a large sample of circumstellar disks. It will obtain unique science results on planets down to super-Earth sizes and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable exoplanets. We present the mission/payload design and highlight steps to reduce mission cost/risk relative to previous mission concepts. At the study conclusion in 2015, NASA will evaluate it for potential development at the end of this decade.
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 describe the algorithms and results of an estimation of the science yield for five candidate coronagraph designs for the WFIRST-AFTA space mission. The targets considered are of three types, known radial-velocity planets, expected but as yet undiscovered exoplanets, and debris disks, all around nearby stars. The results of the original estimation are given, as well as those from subsequently updated designs that take advantage of experience from the initial estimates.
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.
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%.
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 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.
The Debris Disk Explorer (DDX) is a proposed balloon-borne 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. DDX will measure the size, shape, brightness, and color of tens of disks. These measurements will enable us to place the Solar System in context. By imaging debris disks around nearby stars, DDX will reveal the presence of perturbing planets via their influence on disk structure, and explore the physics and history of debris disks by characterizing the size and composition of disk dust. The DDX instrument is a 0.75-m diameter off-axis telescope and a coronagraph carried by a stratospheric balloon. DDX will take high-resolution, multi-wavelength images of the debris disks around tens of nearby stars. Two flights are planned; an overnight test flight within the United States followed by a month-long science flight launched from New Zealand. The long flight will fully explore the set of known debris disks accessible only to DDX. It will achieve a raw contrast of 10−7, with a processed contrast of 10−8. A technology benefit of DDX is that operation in the near-space environment will raise the Technology Readiness Level of internal coronagraphs, deformable mirrors, and wavefront sensing and control, all potentially needed for a future space-based telescope for high-contrast exoplanet imaging.
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.
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.
To evaluate space-based coronagraphic techniques, end-to-end modeling is necessary to simulate realistic fields
containing speckles caused by wavefront errors. Real systems will suffer from pointing errors and thermal and motioninduced
mechanical stresses that introduce time-variable wavefront aberrations that can reduce the field contrast. A loworder
wavefront sensor (LOWFS) is needed to measure these changes at a sufficiently high rate to maintain the contrast
level during observations. We implement here a LOWFS and corresponding low-order wavefront control subsystem
(LOWFCS) in end-to-end models of a space-based coronagraph. Our goal is to be able to accurately duplicate the effect
of the LOWFS+LOWFCS without explicitly evaluating the end-to-end model at numerous time steps.
Small-angle coronagraphy is technically and scientifically appealing because it enables the use of smaller telescopes,
allows covering wider wavelength ranges, and potentially increases the yield and completeness of circumstellar
environment – exoplanets and disks – detection and characterization campaigns. However, opening up
this new parameter space is challenging. Here we will review the four posts of high contrast imaging and their
intricate interactions at very small angles (within the first 4 resolution elements from the star). The four posts
are: choice of coronagraph, optimized wavefront control, observing strategy, and post-processing methods. After
detailing each of the four foundations, we will present the lessons learned from the 10+ years of operations of
zeroth and first-generation adaptive optics systems. We will then tentatively show how informative the current
integration of second-generation adaptive optics system is, and which lessons can already be drawn from this
fresh experience. Then, we will review the current state of the art, by presenting world record contrasts obtained
in the framework of technological demonstrations for space-based exoplanet imaging and characterization mission
concepts. Finally, we will conclude by emphasizing the importance of the cross-breeding between techniques
developed for both ground-based and space-based projects, which is relevant for future high contrast imaging
instruments and facilities in space or on the ground.
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.
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.
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.
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.
The NIRCam instrument on the James Webb Space Telescope will have a Lyot coronagraph for high contrast imaging of
extrasolar planets and circumstellar disks at λ=2 - 5 μm. Half-tone patterns are used to create graded-transmission image
plane masks. These are generated using electron beam lithography and reactive ion etching of a metal layer on an antireflection
coated sapphire substrate. We report here on the manufacture and evaluation of the flight occulters.
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 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.
The NIRCam instrument on the James Webb Space Telescope will provide coronagraphic imaging from λ =1-5 μm of
high contrast sources such as extrasolar planets and circumstellar disks. A Lyot coronagraph with a variety of circular
and wedge-shaped occulting masks and matching Lyot pupil stops will be implemented. The occulters approximate
grayscale transmission profiles using halftone binary patterns comprising wavelength-sized metal dots on anti-reflection
coated sapphire substrates. The mask patterns are being created in the Micro Devices Laboratory at the Jet Propulsion
Laboratory using electron beam lithography. Samples of these occulters have been successfully evaluated in a
coronagraphic testbed. In a separate process, the complex apertures that form the Lyot stops will be deposited onto
optical wedges. The NIRCam coronagraph flight components are expected to be completed this year.
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.
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. PECO's 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 PECO's design and will enable high
contrast at the very small PECO inner working angle.
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.
KEYWORDS: Coronagraphy, Stars, Planets, James Webb Space Telescope, Point spread functions, Wavefronts, Telescopes, Space telescopes, Sensors, Diffraction
The expected stable point spread function, wide field of view, and sensitivity of the NIRCam instrument on the James
Webb Space Telescope (JWST) will allow a simple, classical Lyot coronagraph to detect warm Jovian-mass companions
orbiting young stars within 150 pc as well as cool Jupiters around the nearest low-mass stars. The coronagraph can also
be used to study protostellar and debris disks. At λ = 4.5 μm, where young planets are particularly bright relative to their
stars, and at separations beyond ~0.5 arcseconds, the low space background gives JWST significant advantages over
ground-based telescopes equipped with adaptive optics. We discuss the scientific capabilities of the NIRCam
coronagraph, describe the technical features of the instrument, and present end-to-end simulations of coronagraphic
observations of planets and circumstellar disks.
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.
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 in broadband light. 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 such high contrast imaging systems and wavefront control methods. It consists of a vacuum chamber containing a configurable coronagraph setup with a Xinetics deformable mirror. Previously, we demonstrated 4x10-8 contrast with the SPC at HCIT in 10% broadband light. The limiting factors were subsequently identified as (1) manufacturing defects due to minimal feature size constraints on our shaped pupil masks and (2) the inefficiency of the wavefront correction algorithm we used (classical speckle nulling) to correct for these defects. In this paper, we demonstrate the solutions to both of these problems. In particular, we present a method to design masks with practical minimal feature sizes and show new manufactured masks with few defects. These masks were installed at HCIT and tested using more sophisticated wavefront control algorithms based on energy minimization of light in the dark zone. We present the results of these experiments, notably a record 2.4×10-9 contrast in 10% broadband light.
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.
We present a status report on a study on the effects of instrumental polarization on the fine structure of the stellar point spread function (PSF). These effects are important to understand because the the aberration caused by instrumental polarization on an otherwise diffraction-limited PSF will likely have have severe consequences for extreme high contrast imaging systems such as NASA's proposed Terrestrial Planet Finder (TPF) mission and the proposed NASA Eclipse mission. The report here, describing our efforts to examine these effects, includes two parts: 1) a numerical analysis of the effect of metallic reflection, with some polarization-specific retardation, on a spherical wavefront; 2) an experimental approach for observing this effect, along with a status report on preliminary laboratory results. The numerical analysis indicates that the inclusion of polarization-specific phase effects (retardation) results in a point spread function (PSF) aberration more severe than the amplitude (reflectivity) effects previously recorded in the literature. Preliminary in-lab results are consistent with our numerical predictions.
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 Princeton's SPC in JPL's 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 Terrestrial Planet Finder Coronagraph (TPF-C) is a deep space mission designed to detect and characterize Earth-like planets around nearby stars. TPF-C will be able to search for signs of life on these planets. TPF-C will use spectroscopy to measure basic properties including the presence of water or oxygen in the atmosphere, powerful signatures in the search for habitable worlds. This capability to characterize planets is what allows TPF-C to transcend other astronomy projects and become an historical endeavor on a par with the discovery voyages of the great navigators.
To detect Earth-like planets in the visible with a coronagraphic telescope, two major noise sources have to be overcome: the photon noise of the diffracted star light, and the speckle noise due to the star light scattered by instrumental defects. Coronagraphs tackle only the photon noise contribution. In order to decrease the speckle noise below the planet level, an active control of the wave front is required. We have developed analytical methods to measure and correct the speckle noise behind a coronagraph with a deformable mirror. In this paper, we summarize these methods, present numerical simulations, and discuss preliminary experimental results obtained with the High-Contrast Imaging Testbed at NASA's Jet Propulsion Laboratory.
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.
We present here results from an experimental and theoretical study in the use of graded focal-plane occulting masks to improve high-contrast astronomical imaging at near-infrared wavelengths. The study includes investigations of both high-energy beam sensitive (HEBS) glass (a product of Canyon Materials, San Diego, CA) and binary notch-filter technologies to create precision graded occulting masks. In conjunction with this investigation, we conduct computer simulations showing expected high-contrast levels for various graded masks being considered for installation in the PHARO camera of the Palomar 200-inch (5m) Hale Telescope Adaptive Optics (AO) system. Our results demonstrate that the implementation of a graded exponential mask in the Palomar system should improve high-contrast sensitivities by about 2.4-mag in K-band (2.0-2.4 μm), for 0.75-1.5 arcsec separations. We also demonstrate that both HEBS and binary notch-filter technologies present adequate platforms for necessary occulting requirements. We conclude with a discussion of theinsights our study yields for planned space-based high-contrast observatories such as NASA's planned Terrestrial Planet Finder Coronagraph (TPF-C) and the proposed Eclipse mission.
Relative to ground-based telescopes, the James Webb Space Telescope (JWST) will have a substantial sensitivity advantage in the 2.2-5μm wavelength range where brown dwarfs and hot Jupiters are thought to have significant brightness enhancements. To facilitate high contrast imaging within this band, the Near-Infrared Camera (NIRCAM) will employ a Lyot coronagraph with an array of band-limited image-plane occulting spots. In this paper, we provide the science motivation for high contrast imaging with NIRCAM, comparing its expected performance to that of the Keck, Gemini and 30 m (TMT) telescopes equipped with Adaptive Optics systems of different capabilities. We then describe our design for the NIRCAM coronagraph that enables imaging over the entire sensitivity range of the instrument while providing significant operational flexibility. We describe the various design tradeoffs that were made in consideration of alignment and aberration sensitivities and present contrast performance in the presence of JWST's expected optical aberrations. Finally we show an example of a two-color image subtraction that can provide 10-5 companion sensitivity at sub-arcsecond separations.
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.
The Terrestrial Planet Finder (TPF) Coronagraph study involves exploring the technologies that enable a coronagraph-style instrument to image and characterize earth-like planets orbiting nearby stars. Test beds have been developed to demonstrate the emerging technologies needed for this effort and an architecture study has resulted in designs of a facility that will provide the environment needed for the technology to function in this role. A broad community of participants is involved in this work through studies, analyses, fabrication of components, and participation in the design effort. The scope of activities - both on the technology side and on the architecture study side - will be presented in this paper. The status and the future plans of the activities will be reviewed.
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. High density deformable mirror technology is based on a modular actuator arrays that are scalable to 1000s of actuator elements coupled to the surface of a thin mirror facesheet. Precision actuator control is done by using a low-power, vacuum compatible multiplexed driver system. Mirror surface figure, actuator influence function, and dimensional stability will be given in the context of the Eclipse point design for a coronagraphic space telescope.
KEYWORDS: Error analysis, Point spread functions, Wavefronts, Magnesium, Actuators, Wavefront sensors, Data modeling, Spatial frequencies, Coronagraphy, Near field optics
The Terrestrial Planet Finder (TPF) high contrast imaging testbed (HCIT) facilitates the investigation into the diversity of engineering challenges presented by the goal of direct exo-planet detection. For instance, HCIT offers a high-density deformable mirror to control the optical wavefront errors, a configurable coronagraph to control the diffracted light, and translatable cameras for measuring the focal and pupil planes before and after the coronagraph. One of the principle challenges for a coronagraphic space telescope is the extreme level of wavefront control required to make the very faint planet signal reasonably detectable. A key component, the extremely accurate sensing of the wavefront aberrations, was recently shown to be achievable using a sufficiently constrained image-based approach. In this paper, we summarize the experimental performance a focus-diverse phase-retrieval method that uses symmetrically defocus point-spread function measurements that are obtained about the coronagraph occulter focal plane. Using the HCIT, we demonstrate the high level of wavefront sensing repeatability achieved with our particular choices of focus diversity, data fidelity and processing methodologies. We compare these results to traceable simulations to suggest a partitioning of the error sources that may be limiting the experimental results.
The science program for the Next Generation Space Telescope (NGST) relies heavily on a high performance nearinfrared imager. A design which supports the observations outlined in the Design Reference Mission (DRM) and which also supports enhanced searches for "first light" objects and planets has been developed. Key features of the design include use of refractive optics to minimize the volume and mass required, tunable filters for spectroscopic imaging, and redundant imagers for fail-safe wavefront sensing.
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.
The Eclipse instrument concept is being optimzed to produce direct images of cool Jovian planets around several hundred candidate nearby stars, and consists of matched large telescope, wavefront sensing and control, and coronagraphic camera modules. Designed to fully comply with a Discovery Program budget and schedule, it operates in a wavelength band from 550 nm to 950 nm, and includes an unobscured telescope 1.8m in diameter. Spaceborne direct exoplanet imaging is now practical through deformable mirror (DM) technlogy that permits a hundred-fold or more quasi-static correction of a Hubble Space Telescope level primary mirror surface error at the mid-spatial frequencies that scatter starlight over its planet. Due to the stability of the spaceborne environment, wavefront error sensing can be accomplished in calibration runs preceding planet observation. Eclipse development is aided by a JPL testbed including many attributes of the flight article, and by validation of diffractive propagation algorithms that define system performance. As such, Eclipse both may serve as a pathfinder for envisioned larger instruments for terrestial planet finding, and provide the first sensitive survey of nearby planetary systems. Eclipse is in the process of definition and design, and the results shown here may be modified with further analysis and design. The emphasis of this paper is our approach to the Eclipse Space Element, and the definition of a checklist of features for coronagraphic design. The ground element, and specific devices and algorithms, and the next stages of design will be subjects of future papers. It is not sufficient to design a system capable of providing 10-9 contrast. A coronagraphic system must have sufficient stability that this level of contrast can be maintained over a reasonable observing interval.
A key element in the Eclipse coronagraph is the apodized occulting spot. For exo-planet detection in the presence of a sun-like star, the specification for intensity transmittance at the center of the spot is less than 1×10-8. It must also taper smoothly with a desired functional form to avoid diffraction artifacts in the angular region of planet detection. From a fabrication point of view, these requirements are very challenging. A candidate technology for fabricating such spots is electron-beam exposure of high-energy beam sensitive (HEBS) glass (a product of Canyon Materials, San Diego, CA). In this work, we have calibrated HEBS glass optical density as a function of electron-beam exposure and attained optical densities up to 7.66 without saturation. We then fabricated occulting spots having various functional forms including circular Gaussian, one-dimensional sinc2, and circular sinc2. Preliminary quantitative analysis of the circular sinc2 occulting spot is encouraging.
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.
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 NASA's 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 Sun's nearest neighbors.
KEYWORDS: Planets, Stars, Signal to noise ratio, Space telescopes, Hubble Space Telescope, Coronagraphy, Wavefronts, Exoplanets, Telescopes, Point spread functions
Recent advances in deformable mirror technology for correcting wavefront errors and in pupil shapes and masks for coronagraphic suppression of diffracted starlight enable a powerful approach to detecting extrasolar planets in reflected (scattered) starlight at visible wavelengths. We discuss the planet-finding performance of Hubble-like telescopes using these technical advances. A telescope of aperture of at least 4 meters could accomplish the goals of the Terrestrial Planet Finder (TPF) mission. The '4mTPF' detects an Earth around a Sun at five parsecs in about one hour of integration time. It finds molecular oxygen, ozone, water vapor, the 'red edge' of chlorophyll-containing land-plant leaves, and the total atmospheric column density -- all in forty hours or less. The 4mTPF has a strong science program of discovery and characterization of extrasolar planets and planetary systems, including other worlds like Earth. With other astronomical instruments sharing the focal plane, the 4mTPF could also continue and expand the general program of astronomical research of the Hubble Space Telescope.
A set of 48 ultraviolet-visible filters in the Wide Field Camera 3 will be deployed on the Hubble Space Telescope in 2004. We summarize the specifications for the filters, derived through interaction with the Science Oversight Committee. A detailed characterization of the 48 filters is presented.
In June 1997, NASA made the decision to extend the end of the Hubble Space Telescope (HST) mission from 2005 until 2010. As a result, the age of the instruments on board the HST became a consideration. After careful study, NASA decided to ensure the imaging capabilities of the HST by replacing the Wide Field Planetary Camera 2 with a low-cost facility instrument, the Wide Field Camera 3. This paper provides an overview of the scientific goals and capabilities of the instrument.
The Hubble Space Telescope is arguably one of the most important and successful scientific endeavors undertaken in the twentieth century. Hubble, a modest-sized 2.4-m telescope, outperforms much larger terrestrial telescopes because it is diffraction limited, and because the sky seen from orbit is darker than the terrestrial night sky. If we increase the diameter of Hubble to 8.4-m, a diameter comparable to Keck and the VLT, the increase in capability will be comparable to that which was first achieved by Hubble's launch and subsequent repair. HST10X will allow a fast track solution of outstanding problems in astronomy. Perhaps foremost among these is the detection of earth-like planets orbiting nearby stars. HST10X can detect earth-like planets around stars at distances up to 10 parsecs. Furthermore, HST10X will enable spectroscopic examination of earth-like planets to search for atmospheric oxygen, a certain sign of life.
The wide field planetary camera was launched onboard the Hubble Space Telescope in April 1990 and returned to earth during the HST first servicing mission in December 1993. We report on the ongoing examination of the returned hardware. In particular, a significant decline in performance at UV wavelengths in the exterior optics has been found and studied. This appears to be the result of polymerization of molecular contamination on the external optics by UV light reflected off the Earth's atmosphere. Some conlusions from a partial disassembly of the instrument and an examination of its filter elements are presented. We also discuss the effects of radiation on the CCD detectors during their stay in orbit. Radiation damage increased the numbers of hot pixels over time but had no other discernible effects on the performance of the CCDs.
Correcting the spherical aberration of the Hubble Space Telescope (HST) requires precise optical alignment and stability. To assure that the required alignment can be achieved and maintained on-orbit, the pickoff mirror and three of the four fold mirrors of the second generation Wide Field and Planetary Camera (WFPC-2) have been made actively controllable in tip and tilt. The Pickoff Mirror Mechanism (POMM) and the Articulating Fold Mirrors (AFMs) are commanded from the ground to their required positions once the WFPC-2 is installed in the HST. The POMM is a set-and-forget device that utilizes stepper motors, while the AFMs are maintained in position by the continuous application of control voltages to electrostrictive ceramic actuators. This paper describes the assembly level testing and calibration of the AFMs, and the development of a software tool that generates the commands for adjusting the positions of the POMM and AFMs to achieve system level optical alignment. Our experience with the POMM and AFMs through system level calibration and testing of the WFPC-2 instrument is described.
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