Coronagraphic Telescope Research Articles

Simulation of High-contrast Polarimetric Observations of Debris Disks with the Roman Coronagraph Instrument

The Nancy Grace Roman Space Telescope Coronagraph Instrument will enable the polarimetric imaging of debris disks and inner dust belts in the optical and near-infrared wavelengths, in addition to the high-contrast polarimetric imaging and spectroscopy of exoplanets. The Coronagraph uses two Wollaston prisms to produce four orthogonally polarized images and is expected to measure the polarization fraction with measurement errors <3% per spatial resolution element. To simulate the polarization observations through the Hybrid Lyot Coronagraph (HLC) and Shaped Pupil Coronagraph (SPC), we model disk scattering, the coronagraphic point-response function, detector noise, speckles, jitter, and instrumental polarization and calculate the Stokes parameters. To illustrate the potential for discovery and a better understanding of known systems with both the HLC and SPC modes, we model the debris disks around Epsilon Eridani and HR 4796A, respectively. For Epsilon Eridani, using astrosilicates with 0.37 ± 0.01 as the peak input polarization fraction in one resolution element, we recover the peak disk polarization fraction of 0.33 ± 0.01. Similarly, for HR 4796A, for a peak input polarization fraction of 0.92 ± 0.01, we obtain the peak output polarization fraction as 0.80 ± 0.03. The Coronagraph design meets the required precision, and forward modeling is needed to accurately estimate the polarization fraction.

Flight photon counting electron multiplying charge coupled device development for the Roman Space Telescope coronagraph instrument

We describe the development of flight electron multiplying charge coupled devices (EMCCDs) for the photon-counting camera system of a coronagraph instrument (CGI) to be flown on the 2.4-m Nancy Grace Roman Space Telescope. Roman is a NASA flagship mission that will study dark energy and dark matter, and search for exoplanets with a planned launch in the mid-2020s. The CGI is intended to demonstrate technologies required for high-contrast imaging and spectroscopy of exoplanets, such as high-speed wavefront sensing and pointing control, adaptive optics with deformable mirrors, and ultralow noise signal detection with photon counting, visible-sensitive (350 to 950 nm) detectors. The camera system is at the heart of these demonstrations and is required to sense both faint and bright targets (10 − 4 − 107 counts-s − 1) adaptively at up to 1000 frames-s − 1 to provide the necessary feedback to the instrument control loops. The system includes two identical cameras, one to demonstrate faint light scientific capability, and the other to provide high-speed real-time sensing of instrument pointing disturbances. Our program at the Jet Propulsion Laboratory (Pasadena, California, United States) has evaluated the low-signal performance of radiation-damaged commercial EMCCD sensors and used those measurements as a basis for targeted radiation hardening modifications developed in partnership with the Open University (Milton Keynes, United Kingdom) and Teledyne-e2v (Chelmsford, United Kingdom). A pair of EMCCDs with test features was then developed and their low signal performance is reported here. The program has resulted in the development of a flight version of the EMCCD with low signal performance improved by more than a factor of three over the commercial one after exposure to 2.6 × 109 protons-cm − 2 (10 MeV equivalent). The flight EMCCD sensors are contributed by ESA through a contract with Teledyne-e2v (Chelmsford, United Kingdom). We will describe the program requirements, sensor design, test results and metrics used to evaluate photon counting performance.

Dark hole maintenance with modal pairwise probing in numerical simulations of Roman coronagraph instrument

We propose an efficient approach for focal-plane wavefront estimation for high-contrast imaging. It is designed for very low photon flux regimes with the Roman Space Telescope Coronagraph Instrument in mind. During days-long exposures of Roman, the starlight wavefront will drift due to the thermal instabilities of the optical tube assembly (OTA) and deformable mirror (DM) actuators. To correct the drift in high-order spatial wavefront modes, Roman will periodically be pointed to a bright reference star to sense the speckles in the dark hole (the high-contrast region of the image) and correct them. However, this maneuver decreases observation time by several hours a day and may lead to wavefront instability by causing structural and thermal stresses on the OTA. Instead, we propose maintaining the dark hole using the DM to modulate (probe) the speckles while observing the target star. Our proposed algorithm can then be used to directly estimate the DM commands that would create such speckles, rather than first estimating the electric field of the speckles. Applying the opposite commands on the DM reduces the intensity of the speckles without slewing to the reference star. Further, we analyze this dark-hole maintenance approach using data from Roman Observing Scenario 9 (OS9) simulations while assuming that the high-order wavefront control on Roman will be carried out from the Earth (ground-in-the-loop). In our simulations of Roman’s Hybrid Lyot Coronagraph, the high-order wavefront errors were sensed and corrected while observing a dim target, 47 UMa. Despite the DM probes, the photon flux in the dark hole remained close to that in OS9 and well below detector noise. In terms of planet-detectability, dark hole maintenance kept the post-processing contrast within 10% of OS9’s, even after pointing the telescope at the target star for 6 days. The OTA stability that dark hole maintenance provides compared to slewing the telescope potentially outweighs the small negative impact it has on the contrast.

Coronagraph design with the electric field conjugation algorithm

The requirements for a coronagraph instrument to image and obtain spectra of rocky planets around bright stars from space are tight. Indeed, the goal of imaging an Earth-like planet requires a starlight suppression system that cancels light to a level of 10 − 10 with sufficient stability and robustness to errors. Furthermore, the key science questions necessitate an adequate sample size; consequently, the throughput of the coronagraph drives the achievable yield of a given mission. The trade among achievable raw contrast, sensitivity to wavefront errors, and throughput poses a challenging problem in coronagraph design. The complexity of this problem drives us toward the simultaneous solving of all optical elements. We present a set of methods to optimize the design of a coronagraph. We implement these for the case of the hybrid Lyot coronagraph in the context of the Nancy Grace Roman Space Telescope Coronagraph Instrument. We discuss our findings in terms of coronagraph instrument design, and optical subsystems, and performance interplay.

Sensitivity of the Roman Coronagraph Instrument to Exozodiacal Dust

Exozodiacal dust, warm debris from comets and asteroids in and near the habitable zone of stellar systems, reveals the physical processes that shape planetary systems. Scattered light from this dust is also a source of background flux which must be overcome by future missions to image Earthlike planets. This study quantifies the sensitivity of the Nancy Grace Roman Space Telescope Coronagraph to light scattered by exozodi, the zodiacal dust around other stars. Using a sample of 149 nearby stars, previously selected for optimum detection of habitable exoplanets by space observatories, we find the maximum number of exozodiacal disks with observable inner habitable zone boundaries is six and the number of observable outer habitable boundaries is 74. One zodi was defined as the visible-light surface brightness of 22 m V arcsec−2 around a solar-mass star, approximating the scattered light brightness in visible light at the Earth-equivalent insolation. In the speckle limited case, where the signal-to-noise ratio is limited by speckle temporal stability rather than shot noise, the median 5σ sensitivity to habitable zone exozodi is 12 zodi per resolution element. This estimate is calculated at the inner-working angle of the coronagraph, for the current best estimate performance, neglecting margins on the uncertainty in instrument performance and including a post-processing speckle suppression factor. For an log-norm distribution of exozodi levels with a median exozodi of 3× the solar zodi, we find that the Roman Coronagraph would be able to make 5σ detections of exozodiacal disks in scattered light from 13 systems with a 95% confidence interval spanning 7–20 systems. This sensitivity allows Roman Coronagraph to complement ground-based measurements of exozodiacal thermal emission and constrain dust albedos. Optimized post-processing and detection of extended sources in multiple resolution elements is expected to further improve this unprecedented sensitivity to light scattered by exozodiacal dust.

Method of Moments Based Coronagraph Pupil Design for Exoplanet Exploration

Telescope coronagraph is an instrument to observe faint exoplanets located at very close proximities to their star by shielding the direct starlight. In this paper, the coronagraph pupil-mask system is described through a series of Fourier transforms, which leads to an eigenvalue system described by an integral equation. The method of moments (MoM) is applied to solve such an eigenvalue problem to obtain the eigen modes that represent an optimal set of refractive index distributions on the pupil. Due to the application of the MoM, the shape of coronagraph pupil can be arbitrary. A numerical example considering a circular pupil is given to demonstrate capability of the proposed method. The accuracy of the MoM solution is validate by comparing the numerical solutions with the analytical solutions given by the generalized prolate spheroidal wave functions.

Catalogue of exoplanets accessible in reflected starlight to theNancy Grace RomanSpace Telescope

Context.Reflected starlight measurements will open a new path in the characterization of directly imaged exoplanets. However, we still lack a population study of known targets to which this technique can be applied.Aims.We investigate which of the about 4300 exoplanets confirmed to date are accessible for theRomanSpace Telescope coronagraph in reflected starlight at reference wavelengthsλ= 575, 730, and 825 nm. We carry out a population study and also address the prospects for phase-curve measurements.Methods.We used the NASA Exoplanet Archive as a reference for planet and star properties and explored the effect of their uncertainties on the exoplanet detectability by applying statistical arguments. We defined a planet asRoman-accessible on the basis of the inner and outer working angles of the instrument and its minimum planet-to-star contrast (IWA, OWA, andCmin). We adopted three plausible configurations for these technical specifications, labeled pessimistic, intermediate, and optimistic. Our key outputs for each exoplanet are its probability of beingRoman-accessible (Paccess), the range of observable phase angles, the evolution of its equilibrium temperature, the number of days per orbit for which it is accessible, and its transit probability.Results.In the optimistic scenario, we find 26Roman-accessible exoplanets withPaccess&gt; 25% and host stars brighter thanV= 7 mag. This population is biased towards planets more massive than Jupiter, but also includes the super-Earths tau Cet e and f, which orbit near the habitable zone of their star. Thirteen planets are part of multi-planetary systems. Three of these planets have known transiting companions, which offers opportunities for a contemporaneous atmospheric characterization. The intermediate and pessimistic scenarios yield ten and threeRoman-accessible exoplanets, respectively. We find that inclination estimates (e.g. with astrometry) are required to refine the detectability prospects.Conclusions.A science phase of the coronagraph instrument has a remarkable potential for characterizing the atmospheres of exoplanets that cannot be studied with other techniques.

The Pale Green Dot: A Method to Characterize Proxima Centauri b Using Exo-Aurorae

Abstract We examine the feasibility of detecting auroral emission from the potentially habitable exoplanet Proxima Centauri b. Detection of aurorae would yield an independent confirmation of the planet’s existence, constrain the presence and composition of its atmosphere, and determine the planet’s eccentricity and inclination, thereby breaking the mass-inclination degeneracy. If Proxima Centauri b is a terrestrial world with an Earth-like atmosphere and magnetic field, we estimate that the power at the 5577 Å O i auroral line is on the order of 0.1 TW under steady-state stellar wind, or ∼100× stronger than that on Earth. This corresponds to a planet–star contrast ratio of in a narrow band about the 5577 Å line, though higher contrast ( ) may be possible during periods of strong magnetospheric disturbance (auroral power 1–10 TW). We searched the Proxima Centauri b HARPS data for the 5577 Å line and for other prominent oxygen and nitrogen lines, but find no signal, indicating that the O i auroral line contrast must be lower than (with power ≲3000 TW), consistent with our predictions. We find that observations of 0.1 TW auroral emission lines are likely infeasible with current and planned telescopes. However, future observations with a space-based coronagraphic telescope or a ground-based extremely large telescope (ELT) with a coronagraph could push sensitivity down to terawatt oxygen aurorae (contrast ) with exposure times of ∼1 day. If a coronagraph design contrast of 10−7 can be achieved with negligible instrumental noise, a future concept ELT could observe steady-state auroral emission in a few nights.

Coronagraphic methods for detecting faint astronomical objects near bright ones: Application to the 2.5-m telescope of Sternberg Astronomical Institute

Possible methods for reducing the instrumental background of astronomical telescopes are discussed: the classical Lyot coronagraphic method, the multi-cascade coronagraphic method, the use of special masks in the plane of the entrance aperture, and the use of super-smooth and medium-smooth primary optics. For the first time, analytical expressions describing the intensity distribution at the exit pupil are presented for the case when a mask is placed in the primary focal plane in order to block the light from a bright object. An algorithm for computing the point-spread function (PSF) of a coronagraphic telescope employing super-smooth primary optics and the coronagraphic method is proposed. Computed PSFs for the 2.5-m telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute are presented, without allowance for diffusive and turbulent scattering in the Earth’s atmosphere, and for operation both without coronagraphic elements and in the coronagraphic mode of the telescope.

Recent white-light coronagraphs at the Naval Research Laboratory.

A white-light externally occulted coronagraph is a telescope designed to view the very faint emission of the solar corona in the region close to the sun. It uses the Lyot principle to mitigate the effects of diffraction. The first such telescope flew on a spacecraft in 1971 and showed the dynamic nature of the corona. Since that mission, six other such coronagraphic telescopes have been flown, whose designs evolved to meet the requirements of the mission. This article describes the latest two coronagraphs and compares their capabilities and their designs.

THE CENTER OF LIGHT: SPECTROASTROMETRIC DETECTION OF EXOMOONS

Direct imaging of extrasolar planets with future space-based coronagraphic telescopes may provide a means of detecting companion moons at wavelengths where the moon outshines the planet. We propose a detection strategy based on the positional variation of the center of light with wavelength, "spectroastrometry." This new application of this technique could be used to detect an exomoon, to determine the exomoon's orbit and the mass of the host exoplanet, and to disentangle of the spectra of the planet and moon. We consider two model systems, for which we discuss the requirements for detection of exomoons around nearby stars. We simulate the characterization of an Earth-Moon analog system with spectroastrometry, showing that the orbit, the planet mass, and the spectra of both bodies can be recovered. To enable the detection and characterization of exomoons we recommend that coronagraphic telescopes should extend in wavelength coverage to 3 micron, and should be designed with spectroastrometric requirements in mind.

Experimental demonstration of binary shaped pupil mask coronagraphs for telescopes with obscured pupils

Abstract We present the fabrication and experimental demonstration of three free-standing binary shaped pupil mask coronagraphs, which are applicable for telescopes with partially obscured pupils. Three masks, designed to be complementary (labeled Mask-A, Mask-B, and Mask-C), were formed in 5 μm thick nickel. The design of Mask-A is based on a one-dimensional barcode mask. The design principle of Mask-B is similar, but has a smaller inner working angle and a lower contrast than Mask-A. Mask-C is based on a concentric ring mask and provides the widest dark region and a symmetric point spread function. Mask-A and Mask-C were both designed to produce a flexibly tailored dark region (i.e., non-uniform contrast). The contrast was evaluated using a light source comprising a broadband super-luminescent light-emitting diode with a center wavelength of 650 nm, and the measurements were carried out in a large vacuum chamber. Active wavefront control was not applied in this work. The coronagraphic images obtained by experiment were mostly consistent with the designs. The contrast of Mask-A within the ranges 3.3–8 λ/D and 8–12 λ/D was ∼ 10−4–10−7 and ∼ 10−7, respectively, where λ is the wavelength and D is the pupil diameter. The contrast of Mask-B was ∼ 10−4 and that of Mask-C over an extended field of view (5–25 λ/D) was ∼ 10−5–10−6. The effect of tilting the masks was investigated, and found to be irrelevant at the ∼ 10−7 contrast level. Therefore the masks can be tilted to avoid ghosting. These high-contrast free-standing masks have the potential to enable coronagraphic imaging over a wide wavelength range using both ground-based and space-borne general-purpose telescopes with pupil structures not specifically designed for coronagraphy.

Comets as solar probes

By rapidly evaporating as they fly through the Sun’s hot outer atmosphere, sungrazing comets reveal an otherwise invisible magnetic field that shapes the very beginnings of the solar wind.

SPICES a small space coronagraph to characterize giant and telluric planets in reflected light

SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) was proposed in 2010 for a a five-year M-class mission in the context of ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets located at several AUs (0.5-10 AU) from nearby stars (<25 pc) with masses ranging from a few Jupiter masses down to Super-Earths (∼2 Earth radii, ∼10M⊕), possibly habitable. In addition, circumstellar disks as faint as a few times the zodiacal light in the Solar System can be studied. SPICES is based on a 1.5-m off-axis telescope and can perform spectro- polarimetric measurements in the visible (450-900 nm) at a spectral resolution of about 40. This paper summarizes the top science program and the choices made to conceive the instrument. The performance is illustrated for a few emblematic cases. 1. SCIENCE PROGRAM SPICES is an evolution of the former SEE-COAST proposal (11) and belongs to the category of so-called small coronagraphic telescopes derived from the Terrestrial Planet Finder concept (8). It is a direct imaging mission designed to achieve very high contrasts in order to characterize exoplanetary systems previously identified by other instruments/techniques (radial velocity, astrometry, direct imaging). This challenging goal clearly requires a specific instrumentation, which is not available on any other present or future facility and must be optimized for a very specific task. Typical star/planet brightness ratios in reflected light are in the range of 10 8 to 10 10 at less than 1 arcsec. SPICES combines several techniques for high contrast imaging in a single instrument designed to maximize the astrophysical return while reducing risks. A more precise description of the proposal can be found in (3) while detailed simulations are presented in (9). We here extract a few important results from these two reference papers. The primary objective of SPICES is to characterize the atmospheres and possibly surfaces of various types of planets: gaseous and iced giants as well as Super-Earths. For that purpose, SPICES combines direct imaging with spectro-polarimetry in the visible (450-900nm) and measures the total flux F and the linearly polarized fluxes Q and U, all as functions of the wavelength , for every pixel in an image.

EXOPLANET ALBEDO SPECTRA AND COLORS AS A FUNCTION OF PLANET PHASE, SEPARATION, AND METALLICITY

First generation optical coronagraphic telescopes will obtain images of cool gas and ice giant exoplanets around nearby stars. The albedo spectra of exoplanets at planet-star separations larger than about 1 AU are dominated by reflected light to beyond 1 {\mu}m and are punctuated by molecular absorption features. We consider how exoplanet albedo spectra and colors vary as a function of planet-star separation, metallicity, mass, and observed phase for Jupiter and Neptune analogs from 0.35 to 1 {\mu}m. We model Jupiter analogs with 1x and 3x the solar abundance of heavy elements, and Neptune analogs with 10x and 30x. Our model planets orbit a solar analog parent star at separations of 0.8 AU, 2 AU, 5 AU, and 10 AU. We use a radiative-convective model to compute temperature-pressure profiles. The giant exoplanets are cloud-free at 0.8 AU, have H2O clouds at 2 AU, and have both NH3 and H2O clouds at 5 AU and 10 AU. For each model planet we compute moderate resolution spectra as a function of phase. The presence and structure of clouds strongly influence the spectra. Since the planet images will be unresolved, their phase may not be obvious, and multiple observations will be needed to discriminate between the effects of planet-star separation, metallicity, and phase. We consider the range of these combined effects on spectra and colors. For example, we find that the spectral influence of clouds depends more on planet-star separation and hence temperature than metallicity, and it is easier to discriminate between cloudy 1x and 3x Jupiters than between 10x and 30x Neptunes. In addition to alkalis and methane, our Jupiter models show H2O absorption features near 0.94 {\mu}m. We also predict that giant exoplanets receiving greater insolation than Jupiter will exhibit higher equator to pole temperature gradients than are found on Jupiter and thus may have differing atmospheric dynamics.

Apodized-pupil Lyot coronagraphs: multistage designs for extremely large telescopes

<i>Context. <i/>Earlier apodized-pupil Lyot coronagraphs (APLC) have been studied and developed to enable high-contrast imaging for exoplanet detection and characterization with present-day ground-based telescopes. With the current interest in the development of the next generation of telescopes, the future extremely large telescopes (ELTs), alternative APLC designs involving multistage configuration appear attractive.<i>Aims. <i/>I study the relevance of these designs for the ELTs. My analysis is designed to find out the implications of inherent error sources occurring on large coronagraphic telescopes to the performance of multistage APLC configurations.<i>Methods. <i/>Performance and sensitivity of multistage APLC to ELT specificities are analyzed and discussed, taking into account several ineluctable coronagraphic telescope error sources by means of numerical simulations. Additionally, a first laboratory experiment with a two-stages-APLC in the near-infrared (<i>H<i/>-band) is presented to further support the numerical treatment.<i>Results. <i/>Multistage configurations are found to be inappropriate to ELTs. The theoretical gain offered by a multistage design over the classical single-stage APLC is largely compromised by the presence of inherent error sources occurring in a coronagraphic telescope, and in particular in ELTs. The APLC remains an attractive solution for ELTs, but rather in its conventional single-stage configuration.

ALIEN MAPS OF AN OCEAN-BEARING WORLD

When Earth-mass extrasolar planets first become detectable, one challenge will be to determine which of these worlds harbor liquid water, a widely used criterion for habitability. Some of the first observations of these planets will consist of disc-averaged, time-resolved broadband photometry. To simulate such data, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper, we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning 1 day, taken at gibbous phases of 57° and 77°, respectively. As expected, the time-averaged spectrum of Earth is blue at short wavelengths due to Rayleigh scattering, and gray redward of 600 nm due to reflective clouds. The rotation of the planet leads to diurnal albedo variations of 15%-30%, with the largest relative changes occurring at the reddest wavelengths. To characterize these variations in an unbiased manner, we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only two dominant eigencolors. We use the time variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment, we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.

Requirements on optical density and phase dispersion of imperfect band-limited occulting masks in a broadband coronagraph

We investigate the effects of the parasitic phase of imperfect band-limited occulting masks on the broadband contrast performance of a high-contrast imaging system through modeling and simulations. We also examine the effects of the phase and the optical-density dispersions of occulting masks whose parasitic phase has been compensated at the center wavelength but is nonzero at other wavelengths. Two types of occulting masks are considered: gray-scale masks such as those made on a high-energy beam-sensitive glass, and recently proposed spatially profiled metal masks, both having 1D Sinc2(linear-Sinc2) amplitude transmission coefficient (Sinc4 intensity transmittance) profiles. We determine the conditions for obtaining 1x10(-9) and 1x10(-10) contrast values with a light centered at a 785 nm wavelength and having a 10% bandwidth in a coronagraphic telescope having ideal optical surfaces but imperfect band-limited image-plane occulting masks.

A laboratory demonstration of the capability to image an Earth-like extrasolar planet

The detection and characterization of an Earth-like planet orbiting a nearby star requires a telescope with an extraordinarily large contrast at small angular separations. At visible wavelengths, an Earth-like planet would be 1 x 10(-10) times fainter than the star at angular separations of typically 0.1 arcsecond or less. There are several proposed space telescope systems that could, in principle, achieve this. Here we report a laboratory experiment that reaches these limits. We have suppressed the diffracted and scattered light near a star-like source to a level of 6 x 10(-10) times the peak intensity in individual coronagraph images. In a series of such images, together with simple image processing, we have effectively reduced this to a residual noise level of about 0.1 x 10(-10). This demonstrates that a coronagraphic telescope in space could detect and spectroscopically characterize nearby exoplanetary systems, with the sensitivity to image an 'Earth-twin' orbiting a nearby star.

Reflectivity and optical surface height requirements in a broadband coronagraph 1Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraph's deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.