NASA WFIRST-AFTA mission study includes a coronagraph instrument to find and characterize exoplanets. Various types of masks could be employed to suppress the host starlight to about 10−⁹ level contrast over a broad spectrum to enable the coronagraph mission objectives. Such masks for high-contrast internal coronagraphic imaging require various fabrication technologies to meet a wide range of specifications, including precise shapes, micron scale island features, ultralow reflectivity regions, uniformity, wave front quality, and achromaticity. We present the approaches employed at JPL to produce pupil plane and image plane coronagraph masks by combining electron beam, deep reactive ion etching, and black silicon technologies with illustrative examples of each, highlighting milestone accomplishments from the High Contrast Imaging Testbed at JPL and from the High Contrast Imaging Lab at Princeton University.

We present and discuss the design details of an extensible, modular, open-source software framework called EXOSIMS (Exoplanet Open-Source Imaging Mission Simulator), which creates end-to-end simulations of space-based exoplanet imaging missions. We motivate the development and baseline implementation of the component parts of this software with models of the wide-field infrared survey telescope-astrophysics focused telescope assets (WFIRST-AFTA) coronagraph and present initial results of mission simulations for various iterations of the WFIRST-AFTA coronagraph design. We present and discuss two sets of simulations. The first compares the science yield of completely different instruments in the form of early competing coronagraph designs for WFIRST-AFTA. The second set of simulations evaluates the effects of different operating assumptions, specifically the assumed postprocessing capabilities and telescope vibration levels. We discuss how these results can guide further instrument development and the expected evolution of science yields.

The Wide Field InfraRed Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission is a 2.4-m class space telescope that will be used across a swath of astrophysical research domains. JPL will provide a high-contrast imaging coronagraph instrument—one of two major astronomical instruments. In order to achieve the low noise performance required to detect planets under extremely low flux conditions, the electron multiplying charge-coupled device (EMCCD) has been baselined for both of the coronagraph’s sensors—the imaging camera and integral field spectrograph. JPL has established an EMCCD test laboratory in order to advance EMCCD maturity to technology readiness level-6. This plan incorporates full sensor characterization, including read noise, dark current, and clock-induced charge. In addition, by considering the unique challenges of the WFIRST space environment, degradation to the sensor’s charge transfer efficiency will be assessed, as a result of damage from high-energy particles such as protons, electrons, and cosmic rays. Science-grade CCD201-20 EMCCDs have been irradiated to a proton fluence that reflects the projected WFIRST orbit. Performance degradation due to radiation displacement damage is reported, which is the first such study for a CCD201-20 that replicates the WFIRST conditions. In addition, techniques intended to identify and mitigate radiation-induced electron trapping, such as trap pumping, custom clocking, and thermal cycling, are discussed.

The new frontier in the quest for the highest contrast levels in the focal plane of a coronagraph is now the correction of the large diffraction artifacts introduced at the science camera by apertures of increasing complexity. Indeed, the future generation of space- and ground-based coronagraphic instruments will be mounted on on-axis and/or segmented telescopes; the design of coronagraphic instruments for such observatories is currently a domain undergoing rapid progress. One approach consists of using two sequential deformable mirrors (DMs) to correct for aberrations introduced by secondary mirror structures and segmentation of the primary mirror. The coronagraph for the WFIRST-AFTA mission will be the first of such instruments in space with a two-DM wavefront control system. Regardless of the control algorithm for these multiple DMs, they will have to rely on quick and accurate simulation of the propagation effects introduced by the out-of-pupil surface. In the first part of this paper, we present the analytical description of the different approximations to simulate these propagation effects. In Appendix A, we prove analytically that in the special case of surfaces inducing a converging beam, the Fresnel method yields high fidelity for simulations of these effects. We provide numerical simulations showing this effect. In the second part, we use these tools in the framework of the active compensation of aperture discontinuities (ACAD) technique applied to pupil geometries similar to WFIRST-AFTA. We present these simulations in the context of the optical layout of the high-contrast imager for complex aperture telescopes, which will test ACAD on a optical bench. The results of this analysis show that using the ACAD method, an apodized pupil Lyot coronagraph, and the performance of our current DMs, we are able to obtain, in numerical simulations, a dark hole with a WFIRST-AFTA-like. Our numerical simulation shows that we can obtain contrast better than 2×10−⁹ in monochromatic light and better than 3×10−⁸ with 10% bandwidth between 5 and 14 λ/D.

Coronagraphy is a very promising method for directly imaging exoplanets, but the performance of a coronagraph is highly sensitive to quasi-static aberrations within the telescope. The resultant speckles are suppressed in the final focal plane using a wavefront control system that estimates the field at the final focal plane to avoid any noncommon path error. This requires a set of probe images that modulate the field so that it may be estimated. With an estimate of the focal plane electric field, a control law is defined to suppress the speckle field so that the planet can be imaged. Characterizing the planet requires that the speckle field be suppressed simultaneously over the bandpass of interest. The choice of control law, bandpass, estimator, and probing methodology has implications in the control solutions and contrast performance. Here, we compare wavefront probing, estimation, and control algorithms, and describe their practical implementation.

The coronagraphic instrument (CGI) currently proposed for the Wide-Field Infrared Survey Telescope–Astrophysics Focused Telescope Assets (WFIRST-AFTA) mission will be the first example of a space-based coronagraph optimized for extremely high contrasts that are required for the direct imaging of exoplanets reflecting the light of their host star. While the design of this instrument is still in progress, this early stage of development is a particularly beneficial time to consider the operation of such an instrument. We review current or planned operations on the Hubble Space Telescope and the James Webb Space Telescope with a focus on which operational aspects will have relevance to the planned WFIRST-AFTA CGI. We identify five key aspects of operations that will require attention: (1) detector health and evolution, (2) wavefront control, (3) observing strategies/postprocessing, (4) astrometric precision/target acquisition, and (5) polarimetry. We make suggestions on a path forward for each of these items.

We propose to use an extremely unbalanced interferometer (EUI) as a wavefront correcting input to a stellar coronagraph for direct exoplanet observation. Since wavefront error causes incomplete suppression of stellar light, an EUI aims to precisely correct the wavefront incident on the coronagraph to a level better than λ/5000 in the visible wavelength range. Compared to the previous unbalanced interferometer, which incorporated a nulling function, the proposed EUI does not introduce the nulling function. EUI does not use a precise deformable mirror. It increases the accuracy of a wavefront control effectively because of the coherent summation with an amplitude imbalance. It enables obtaining the desirable 10⁻⁹ coronagraphic contrast for Earth-like exoplanet imaging.

Coronagraphs of the apodized pupil and shaped pupil varieties use the Fraunhofer diffraction properties of amplitude masks to create regions of high contrast in the vicinity of a target star. Here we present a hybrid coronagraph architecture in which a binary, hard-edged shaped pupil mask replaces the gray, smooth apodizer of the apodized pupil Lyot coronagraph (APLC). For any contrast and bandwidth goal in this configuration, as long as the prescribed region of contrast is restricted to a finite area in the image, a shaped pupil is the apodizer with the highest transmission. We relate the starlight cancellation mechanism to that of the conventional APLC. We introduce a new class of solutions in which the amplitude profile of the Lyot stop, instead of being fixed as a padded replica of the telescope aperture, is jointly optimized with the apodizer. Finally, we describe shaped pupil Lyot coronagraph (SPLC) designs for the baseline architecture of the Wide-Field Infrared Survey Telescope–Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph. These SPLCs help to enable two scientific objectives of the WFIRST-AFTA mission: (1) broadband spectroscopy to characterize exoplanet atmospheres in reflected starlight and (2) debris disk imaging.

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.

This work describes the fabrication, characterization, and modeling of a second-generation occulting mask for a phase-induced amplitude apodization complex mask coronagraph, designed for use on the WFIRST-AFTA mission. The mask has many small features (∼micron lateral scales) and was fabricated at the Jet Propulsion Laboratory Microdevices Laboratory, then characterized using a scanning electron microscope, atomic force microscope, and optical interferometric microscope. The measured fabrication errors were then fed to a wavefront control model which predicts the contrast performance of a full coronagraph. The expected coronagraphic performance using this mask is consistent with observing ∼15 planetary targets with WFIRST-AFTA in a reasonable time (<1  day/target).

The contrast and angular resolution required to directly image and characterize mature exoplanetary systems place stringent requirements on the space-based telescopes and starlight suppression systems needed to study spatial distributions of debris disks, exozodiacal dust, and individual planets at multiple epochs in their orbits. A nulling interferometer (nuller) is a coronagraphic suppression system that can be used with all telescope types, including those with obscured and segmented apertures envisioned for upcoming and future observatories. One of the challenges for detection and characterization of exoplanetary signals is achieving high contrast with broad spectral coverage. This work presents design concepts for broadband nulling over four parallel ∼20% bandpasses spanning the visible spectrum. Contrast-limiting effects of stellar angular extent, residual chromaticity of broadband phase shifters, and aperture diffraction are considered to reach simultaneous ≲2×10⁻⁸ contrast over separations spanning 0.2 to 0.9 arc sec for a 2.4-m telescope observing a Sun-like star at 10 pc. With added dark hole wavefront control and postprocessing point spread function subtraction techniques to further reduce scattered starlight, such a system could be capable of detecting the very the nearest Earth-like exoplanets and spectral characterization of several nearby extrasolar gas giants.

A framework for evaluating the science yield of a coronagraph in the presence of a variety of line-of-sight jitter environments is described and the use of a tip-tilt threshold for improving science yield is proposed. The current expectations of the WFIRST-AFTA mission are used for specific distributions of line-of-sight jitter, including the current expectations for tip-tilt correction using a low-order wavefront sensor/control. The effect of the residual tip-tilt on the phase-induced amplitude apodization complex mask coronagraph (PIAACMC) architecture is considered, because the performance of the PIAACMC architecture is expected to be dominated by tip-tilt sensitivity, implying that this treatment has a large impact on the final science yield. The most important outcomes of this study are that the rms residual tip-tilt expected after correction is 0.6  mas rms/axis and that by eliminating some science frames during analysis through a tip-tilt threshold, the number of planets observable increases by ∼25% for the 550-nm imaging channel. The number of known radial velocity planets expected to be observed ranges from 29 to 78 at 550 nm and from 9 to 12 at 890 nm.

For imaging faint exoplanets and disks, a coronagraph-equipped observatory needs focal plane wavefront correction to recover high contrast. The most efficient correction methods iteratively estimate the stellar electric field and suppress it with active optics. The estimation requires several images from the science camera per iteration. To maximize the science yield, it is desirable both to have fast wavefront correction and to utilize all the correction images for science target detection. Exoplanets and disks are incoherent with their stars, so a nonlinear estimator is required to estimate both the incoherent intensity and the stellar electric field. Such techniques assume a high level of stability found only on space-based observatories and possibly ground-based telescopes with extreme adaptive optics. In this paper, we implement a nonlinear estimator, the iterated extended Kalman filter (IEKF), to enable fast wavefront correction and a recursive, nearly-optimal estimate of the incoherent light. In Princeton’s High Contrast Imaging Laboratory, we demonstrate that the IEKF allows wavefront correction at least as fast as with a Kalman filter and provides the most accurate detection of a faint companion. The nonlinear IEKF formalism allows us to pursue other strategies such as parameter estimation to improve wavefront correction.

The phase-induced amplitude apodization complex mask coronagraph (PIAACMC) provides an efficient way to control diffraction propagation effects caused by the central obstruction/segmented mirrors of the telescope. PIAACMC can be optimized in a way that takes into account both chromatic diffraction effects caused by the telescope obstructed aperture and the tip-tilt sensitivity of the coronagraph. As a result, unlike classic phase-induced amplitude apodization (PIAA), the PIAACMC mirror shapes are often slightly asymmetric even for an on-axis configuration and require more care in calculating off-axis shapes when an off-axis configuration is preferred. A method to design off-axis PIAA mirror shapes given an on-axis mirror design is presented. The algorithm is based on geometrical ray tracing and is able to calculate off-axis PIAA mirror shapes for an arbitrary geometry of the input and output beams. The method is demonstrated using the third generation PIAACMC design for WFIRST-AFTA telescope. Geometrical optics design issues related to the off-axis diffraction propagation effects are also discussed.

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 coronagraph instrument (CGI) on the Wide-Field Infrared Survey Telescope will directly image and spectrally characterize planets and circumstellar disks around nearby stars. Here we estimate the expected science yield of the CGI for known radial-velocity (RV) planets and potential circumstellar disks. The science return is estimated for three types of coronagraphs: the hybrid Lyot and shaped pupil are the currently planned designs, and the phase-induced amplitude apodizing complex mask coronagraph is the backup design. We compare the potential performance of each type for imaging as well as spectroscopy. We find that the RV targets can be imaged in sufficient numbers to produce substantial advances in the science of nearby exoplanets. To illustrate the potential for circumstellar disk detections, we estimate the brightness of zodiacal-type disks, which could be detected simultaneously during RV planet observations.

To maintain the required Wide-Field Infrared Survey Telescope (WFIRST) coronagraph performance in a realistic space environment, a low-order wavefront sensing and control (LOWFS/C) subsystem is necessary. The LOWFS/C uses the rejected stellar light from the coronagraph to sense and suppress the telescope pointing errors as well as low-order wavefront errors (WFEs) due to changes in thermal loading of the telescope and the rest of the observatory. We will present a conceptual design of a LOWFS/C subsystem for the WFIRST-AFTA coronagraph. This LOWFS/C uses a Zernike phase contrast wavefront sensor (ZWFS) with a phase shifting disk combined with the stellar light rejecting occulting masks, a key concept to minimize the noncommon path error. We will present our analysis of the sensor performance and evaluate the performance of the line-of-sight jitter suppression loop, as well as the low-order WFE correction loop with a deformable mirror on the coronagraph. We will also report the LOWFS/C testbed design and the preliminary in-air test results, which show a very promising performance of the ZWFS.

The capabilities of a high (∼10−⁹ resel−¹) contrast narrow-field coronagraphic instrument (CGI) on a space-based WFIRST-C or probe-class EXO-C/S mission are particularly and importantly germane to symbiotic studies of the systems of circumstellar material from which planets have emerged and interact with throughout their lifetimes. The small particle populations in “disks” of co-orbiting materials can trace the presence of planets through dynamical interactions that perturb the spatial distribution of light-scattering debris, which is detectable at visible wavelengths and resolvable with a WFIRST-C or EXO-S/C CGI. Herein, we (1) present the scientific case to study the formation, evolution, architectures, diversity, and properties of the material in the planet-hosting regions of nearby stars; (2) discuss how a CGI under current conception can uniquely inform and contribute to those investigations; (3) consider the applicability of CGI-anticipated performance for circumstellar debris system studies; (4) investigate, through WFIRST CGI image simulations, the anticipated interpretive fidelity and metrical results from specific representative zodiacal debris disk observations; (5) comment on specific observational modes and methods germane to and augmenting circumstellar debris system observations; and (6) present a case for augmenting future CGI instrumentation with the capability to obtain full linear-Stokes imaging polarimetery, which greatly benefits characterization of the material properties of circumstellar dust and exoplanet atmospheres (discussed in other studies).