Theoretical ideal coronagraph performance is achieved when the light from an exoplanet can be coherently decomposed into a linear combination of spatial modes indistinguishable from that containing starlight, and an orthogonal mode. The intensity in the exoplanet mode orthogonal from the stellar modes as a function of separation from the star represents theoretical ideal coronagraph performance. Here we introduce a photonic coronagraph architecture capable of achieving this near-ideal exoplanet throughput at small inner working angles. We will review progress at the NASA Jet Propulsion Lab on prototype hardware implementing this photonic coronagraph concept and discuss our progress at device calibration and closed-loop control required for a photonic coronagraph in a changing wavefront environment.
NASA’s Habitable Worlds Observatory addresses the challenging goal of characterizing numerous Earth-like exoplanets orbiting nearby stars. While the baseline approach is to carry out the observations with a coronagraph, current planning calls for the observatory to be “starshade ready” so that it can take advantage of the superior throughput, working angle, contrast, and bandwidth when the telescope is paired with a starshade. We describe two starshade designs that together enable imaging in the UV, visible, and NIR bands, as well as a multi-starshade configuration that could efficiently discover and characterize exoplanets. Additionally, we estimate the stellar light leakage and solar light scatter from micrometeoroid impacts and show that after 10 years on orbit, the stellar leakage will have contrast below 10^-11 while solar scatter will be fainter than V=31 mag.
The Habitable Worlds Observatory aims to detect and characterize Earth-like exoplanets orbiting around Sunlike stars. Current coronagraph technology is not yet capable of reaching the required 1E-10 contrasts; however, advancements in photonic technologies may be able to fill this gap. A significant challenge in astrophotonics is the efficient coupling of light from the telescope into the photonic device. To address this, we have manufactured a photonic device incorporating a spatial array of photonic lanterns, designed to couple light in the focal plane into the device, even in the presence of aberrations. Additionally, we have constructed a testbed for the free-space coupling of light into photonic devices. This testbed is equipped with a segmented deformable mirror (DM) for inducing controlled phase aberrations and a vectorized Zernike wavefront sensor (vZWFS) for direct electric field measurement in the pupil plane. Our device comprises seven mode-sorting photonic lanterns arranged in a hexagonal layout, each coupling light into three modes: LP01, LP11a, and LP11b. This lantern array, paired with a dynamic photonic integrated circuit (PIC), forms the architecture of a near-ideal photonic coronagraph. We describe the development of the testbed, the preliminary characterization of the photonic lantern array, and present preliminary images through the device.
The optical edges of a starshade define its outermost perimeter. During astronomical measurements, the edges are exposed to sunlight resulting in glint seen by the telescope. Clean, sharply etched edges are capable of meeting the stringent solar glint scatter requirement. We report on the increased glint that arises from particulate contamination clinging to the edges. We measure the relationship between surface contamination and edge contamination and compare to a simple edge contaminant population model where the edge acts like a line drawn across the continuous surface. We correlate the level of edge contamination to the degree of increased scatter and derive an on-orbit contamination requirement that is compatible with the detection of exo-Earths.
The Habitable Worlds Observatory (HWO) concept is a 6-meter diameter deep space telescope operating from the ultraviolet through the near infrared. One of its primary goals is to measure the spectra of 25 Earth-like exoplanets. While the Decadal Survey described a highly stabilized off-axis telescope with a coronagraph to perform these measurements, a starshade offers an alternative approach that will be especially effective in the ultraviolet because it offers high throughput, large bandwidth, small working angle, and does not require a picometer-stabilized telescope. We show the feasibility of obtaining UV spectra and measuring the presence of Ozone in the Hartley band using a 35 m diameter starshade with a bandpass of 250 nm to 500 nm. Spectra with high SNR can be obtained in just a few days of integration on Earth-like exoplanets orbiting solar type stars.
Starshades are designed to enable the direct observation of an exoplanet by blocking the light of the planet’s star from reaching the telescope. As discussed in our companion paper [S. Shaklan et al., “Solar glint from uncoated starshade optical edges,” J. Astron. Telesc. Instrum. Syst.7(2), 021204 (2021)], diffraction and reflection of sunlight incident on the starshade’s razor-sharp uncoated edges will appear as glint that may be brighter than the feeble light of the exoplanet. We report on the measurement and modeling of thin, conformal, multilayer antireflection coatings that reduce solar glint by more than an order of magnitude when applied to uncoated edges. We used the Lumerical finite-difference time-domain simulation software suite to determine the performance of coatings designed to work on a flat surface when applied to a sharp, curved edge. Laboratory measurements of coated edges, including a 50-cm long segment, confirm the glint reduction predicted by these models. We consider two coating approaches and compare their performance: a line-of-sight coating and a coating that uniformly covers the entire terminal edge. Starting with a wide range of coating designs emphasizing different angles of incidence and bandpass characteristics, we use Lumerical to account for edge diffraction and reflection, and we optimize the designs for the Starshade Rendezvous Mission and the HabEx mission concept.
The perimeter of a sunflower-like starshade has hundreds of meters of sharp edges that are directly exposed to sunlight. The sunlight diffracts and reflects from the edge resulting in a dual-lobed glint pattern that can be brighter than an exoplanet. We present estimates of the glint brightness distribution for the Starshade Rendezvous Mission and the HabEx Starshade Mission concepts based on measurements of flight-like, environmentally tested, uncoated metallic edges using custom-built scatterometers. A companion paper addresses the performance for edges coated with a thin anti-reflection coating.
A starshade is a large flower-shaped screen designed to enable the direct imaging of exoplanets with a space telescope. The starshade perimeter is composed of sharp, precisely shaped edges to minimize the glint of sunlight into the telescope. Past work has focused on bare edges to minimize the terminal radius. This paper describes the broadband, wide-angle performance of edges coated with a thin multi-layer anti-reflection coating. This coating uses a combination of interference and absorption to reduce the surface reflectivity and to avoid the negative effects associated with a large cross-sectional area. A custom scattered light testbed has been developed to quantify the amount of light scattered from sample edges and to validate Finite-Difference Time-Domain (FDTD) models of the optical scatter. We show that optical edge samples with this coating significantly reduce the solar glint pattern compared to similar uncoated optical edges.
A sunflower-like starshade positioned between an exoplanet host star and a telescope forms a deep shadow at the telescope enabling the faint exoplanet to be viewed without being overwhelmed by veiling glare from the star. The starshade perimeter has hundreds of meters of sharp edge that are directly exposed to sunlight. The sunlight diffracts and reflects from the edge resulting in a glint pattern that can be brighter than the exoplanet. We have developed models of the edge glint to explain laboratory measurements, to guide the development of edges with minimum glint, and to determine the fundamental glint floor which is set by diffraction. The models include finite difference time domain calculations, Sommerfeld's half-plane diffraction expressions, and a micro-facet scattering model. Models successfully reproduce the features and magnitude of the measured polarization-dependent scatter and show that measured edges are performing near the theoretical limit.
A starshade enables direct imaging of Earth-like exoplanets in the habitable zone of nearby stars by suppressing light from a target star so that orbiting planets are revealed. The perimeter of a starshade, known as the optical edge, has two critical functions. First, it must meet a precise in-plane profile specification to form a deep shadow in which the telescope is placed. Second, it must minimize reflected sunlight, as scattered sunlight significantly degrades the achievable contrast. Prior work on small scales and in a laboratory environment has shown that these requirements can be met using a chemically etched amorphous metal foil. This paper describes the next step of development, a first ever demonstration of assembled optical edge segments that meet both requirements simultaneously. The segments were constructed using space-compatible components and tested to relevant thermal and mechanical environments. A thorough assessment of edge performance, including in-plane profile, sunlight scatter and mechanical survivability was performed both before and after environmental testing. Furthermore, a custom scattered light testbed has been developed to quantify the magnitude of scattered sunlight over the entire length of the optical edge. The results of this study inform the future development of optical edge technology and pave the way towards eventual flight implementation.
Starshades, combined with future space telescopes, provide the ability to detect Earth-like exoplanets in the habitable zone by producing high contrast ratios at small inner working angles. The primary function of a starshade is to suppress light from a target star such that its orbiting planets are revealed. In order to do so, the optical edges of the starshade must maintain their precise in-plane profile to produce the necessary apodization function. However, an equally important consideration is the interaction of these edges with light emanating from our own Sun as scattered and/or diffracted sunlight can significantly degrade the achievable contrast. This paper describes the technical efforts performed to obtain precision, low-scatter optical edges for future starshades. Trades between edge radius (i.e. sharpness) and surface reflectivity have been made and small-scale coupons have been produced using scalable manufacturing processes. A custom scattered light testbed has been developed to quantify the magnitude of scattered light over all sun angles. Models have also been developed to make predictions on the level of reflected and/or diffracted light for various edge architectures. The results of these studies have established a current baseline approach which implements photochemical etching techniques on thin metal foils.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.