Search

Over the past two decades, thousands of confirmed exoplanets have been detected; the next major challenge is to characterize these other worlds and their stellar systems. Much information on the composition and formation of exoplanets and circumstellar debris disks can only be achieved via direct imaging. Direct imaging is challenging because of the small angular separations ($, 23 pages, 13 figures, 7 tables, conference

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.

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.

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.

To maintain the required performance of WFIRST Coronagraph in a realistic space environment, a Low Order Wavefront Sensing and Control (LOWFS/C) subsystem is necessary. The LOWFS/C uses a Zernike wavefront sensor (ZWFS) with the phase shifting disk combined with the starlight rejecting occulting mask. For wavefront error corrections, WFIRST LOWFS/C uses a fast steering mirror (FSM) for line-of-sight (LoS) correction, a focusing mirror for focus drift correction, and one of the two deformable mirrors (DM) for other low order wavefront error (WFE) correction. As a part of technology development and demonstration for WFIRST Coronagraph, a dedicated Occulting Mask Coronagraph (OMC) testbed has been built and commissioned. With its configuration similar to the WFIRST flight coronagraph instrument the OMC testbed consists of two coronagraph modes, Shaped Pupil Coronagraph (SPC) and Hybrid Lyot Coronagraph (HLC), a low order wavefront sensor (LOWFS), and an optical telescope assembly (OTA) simulator which can generate realistic LoS drift and jitter as well as low order wavefront error that would be induced by the WFIRST telescope’s vibration and thermal changes. In this paper, we will introduce the concept of WFIRST LOWFS/C, describe the OMC testbed, and present the testbed results of LOWFS sensor performance. We will also present our recent results from the dynamic coronagraph tests in which we have demonstrated of using LOWFS/C to maintain the coronagraph contrast with the presence of WFIRST-like line-of-sight and low order wavefront disturbances.

Hybrid Lyot Coronagraph (HLC) is one of the two operating modes of the 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.

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.

The Shaped Pupil Coronagraph (SPC) is one of the two operating modes of the WFIRST (Wide-Field InfraRed Survey Telescope) coronagraph instrument. The SPC provides starlight suppression in a pair of wedge-shaped regions over an 18 percent bandpass, and is well suited for spectroscopy of known exoplanets. To demonstrate this starlight suppression in the presence of expected on-orbit input wavefront disturbances, we have recently built a dynamic testbed at JPL (Jet Propulsion Lab.) analogous to the WFIRST fight 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 static SPC mode demonstration from the testbed, along with model comparisons and performance under realistic dynamical conditions. HLC results will be reported separately.

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.

SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets and circumstellar disks in the visible (450 - 900 nm) at a spectral resolution of about 40 using both spectroscopy and polarimetry. By 2020/22, present and near-term instruments will have found several tens of planets that SPICES will be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES can preferentially access exoplanets located at several AUs (0.5-10 AU) from nearby stars ($

Early release science observations of the cluster NGC3603 with the WFC3 on the refurbished HST allow us to study its recent star formation history. Our analysis focuses on stars with Halpha excess emission, a robust indicator of their pre-main sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Halpha excess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate., 10 pages, 8 figures, accepted for publication in The Astrophysical Journal

The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.

We present narrow band, continuum subtracted Hα, [S ii], Hβ, [O iii] and [O ii] data taken with the Wide Field Camera 3 on the Hubble Space Telescope in the nearby dwarf starburst galaxy NGC 4214. From these images, we identify seventeen new planetary nebula candidates, and seven supernova remnant candidates. We use the observed emission line luminosity function of the planetary nebulae to establish a new velocity-independent distance to NGC 4214. We conclude that the PNLF technique gives a reddening independent distance to NGC 4214 of 3.19±0.36 Mpc, and that our current best-estimate of the distance to this galaxy ids 2.98±0.13 Mpc.

We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology, and photometry in continuum-subtracted Hα, [S II], Hβ, [O III], and [O II] filters, we have identified 60 supernova remnant (SNR) candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible, their Hα fluxes are given. Radiative ages and pre-shock densities are derived from those SNRs that have good photometry. The ages lie in the range 2.62 < log (τrad/yr) < 5.0, and the pre-shock densities at the blast wave range over 0.56 < n 0/cm-3 < 1680. Two populations of SNRs have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium (ISM) is pressurized by the hot X-ray plasma. We also find that the ISM in the nuclear region of M83 is characterized by a very high porosity and pressure, and infer an SNR rate of 1 per 70-150 yr for the nuclear (R < 300 pc) region. On the basis of the number of SNRs detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is M min = 16+7 –5 M ☉. Finally, we give evidence for the likely detection of the remnant of the historical supernova, SN1968L.

We discuss the results of a multi-wavelength differential imaging lab experiment with the High Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory. The HCIT combines a Lyot coronagraph with a Xinetics deformable mirror in a vacuum environment to simulate a space telescope in order to test technologies and algorithms for a future exoplanet coronagraph mission. At present, ground based telescopes have achieved significant attenuation of speckle noise using the technique of spectral differential imaging (SDI). We test whether ground-based SDI can be generalized to a non-simultaneous spectral differential imaging technique (NSDI) for a space mission. In our lab experiment, a series of 5 filter images centered around the O2(A) absorption feature at 0.762 um were acquired at nominal contrast values of 10^-6, 10^-7, 10^-8, and 10^-9. Outside the dark hole, single differences of images improve contrast by a factor of ~6. Inside the dark hole, we found significant speckle chromatism as a function of wavelength offset from the nulling wavelength, leading to a contrast degradation by a factor of 7.2 across the entire ~80 nm bandwidth. This effect likely stems from the chromatic behavior of the current occulter. New, less chromatic occulters are currently in development; we expect that these new occulters will resolve the speckle chromatism issue., Comment: 24 pages, 8 figures, 3 tables, accepted by PASP

Band-limited image plane masks, in conjunction with appropriate Lyot stops, function effectively to suppress host star light while transmitting planet light to the final image for detecting and characterizing faint exo-solar terrestrial planets. In this article, we review various types of image plane masks, their design, fabrication, and characterization. Measured characteristics and performance of sample masks in simulations and in laboratory experiments are also discussed. Star light suppression contrast better than 10−9 in the 4 to 10 λ / D region has been demonstrated with such a mask in our laboratory testbed. To cite this article: K. Balasubramanian et al., C. R. Physique 8 (2007).

"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.

Occulting focal plane masks for the Terrestrial Planet Finder Coronagraph (TPF-C) could be designed with continuous gray scale profile of the occulting pattern such as 1-sinc2 on a suitable material or with micron-scale binary transparent and opaque structures of metallic pattern on glass. We have designed, fabricated and tested both kinds of masks. The fundamental characteristics of such masks and initial test results from the High Contrast Imaging Test bed (HCIT) at JPL are presented.

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.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Io's sodium clouds result mostly from a combination of two atmospheric escape processes at Io. Neutralization of Na + and/or NaX + pickup ions produces the “stream” and the “jet” and results in a rectangular-shaped sodium nebula around Jupiter. Atmospheric sputtering of Na by plasma torus ions produces the “banana cloud” near Io and a diamond-shaped sodium nebula. Charge exchange of thermal Na + with Na in Io's atmosphere does not appear to be a major atmospheric ejection process. The total ejection rate of sodium from Io varied from 3×10 26 to 25×10 26 atoms/s over seven years of observations. Our results provide further evidence that Io's atmospheric escape is driven from collisionally thick regions of the atmosphere rather than from the exosphere.

The near-space environment of the stratosphere is far superior to terrestrial sites for optical and infrared observations. New balloon technologies will enable flights and safe recovery of 2-ton payloads at altitudes of 35 km for 100 days and longer. The combination of long flights and superb observing conditions make it possible to undertake science programs that otherwise could only be done from orbit. We propose to fly an "Ultra-Hubble" Stratospheric Telescope (UHST) equipped with a coronagraphic camera and active optics at 35 km to search for planets around 200 of the nearest stars. This ULDB mission will establish the frequency of solar-type planetary systems, and provide targets to search for earth-like planets.

"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. Keywords: Exoplanets, high contrast imaging, optical astronomy, space mission concepts

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.

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.

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.

SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) was proposed in 2010 for 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 (⊕), 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.

The inner part of the northeast middle radio lobe of the radio galaxy Centaurus A is the site of complex interactions. This area contains a large H I cloud as well as filaments of ionized gas and associated blue knots, several of which exist along the northeastern edge of the radio-emitting zones. We observed the filaments and blue knots with the Hubble Space Telescope using WFPC2, and the ionized gas from the ground. Our sensitive, high angular resolution WFPC2 images reveal the presence of young stars, many concentrated in what appear to be OB associations, superimposed on a background sheet of older stars that is typical of the Cen A halo. The ages of the OB associations are estimated to be less than 15 Myr from a comparison of color-magnitude diagrams with those for the Large Magellanic Cloud star cluster NGC 2004, and younger stellar groups may be nearer regions of Hα emission. We discuss our data in the context of models for star formation stimulated by interactions between the radio jet and gas cloud.

Jupiter's low and mid-latitudes are examined using ultraviolet (UV) images obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 over the period of 1994–1997. Effective atmospheric pressures probed by the F160W, F218W, and F255W are estimated at 100, 165, and 370 mbar, respectively, at low latitudes. Far-ultraviolet images of Jupiter taken with the F160W filter reveal bright and dark latitudinal bands at low and mid-latitudes. These bands were found to be correlated with the zonal winds; the bright bands are at latitudes of subsidences, and the dark bands at upwellings. The sense of this correlation is opposite to that seen in visible images of Jupiter where the bright zones are located over upwellings, and dark belts over subsidences. The locations of the dark bands seen in the F160W images are consistent with enhanced concentrations of NH_3 and/or aerosols in the upwellings near the tropopause. Anticyclonic ovals, like the Great Red Spot, are regions of upwellings, and also appear dark in the F160W images. Temporal variations were also observed in the contrast of the low-latitude bands. An increase in the contrast of the low-latitude bands appeared to coincide with a visible wavelength broadening of the North Equatorial Belt that occurred in early to mid-1996. Changes at mid-southern latitudes were observed in the aftermath of the SL-9 impacts in July 1994.

Jupiter's polar regions are examined using ultraviolet (UV, 120–320 nm) images obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 over the period of 1994–1997. These images probe the stratospheric aerosols at pressures of a few tens of millibars. We discovered a UV-darkened segment in the polar shading just equatorward of the north polar hood that remained aligned with the southern-most portion of the north auroral oval. The darkened segment generally extended over 100°–260° System III longitude, and 39°–53°N planetocentric latitude in the F160W, F218W, and F255W images. No single type of feature in the F218W and F255W images appeared consistently aligned within the outline of the north auroral oval, but several transient features were observed. It is not certain whether these transient features were associated with auroral processes, or just coincidentally aligned. A “south dark patch” was consistently observed within the outline of the south auroral oval and was temporally variable in size and shape. By tracking selected features, we obtained the first direct measurements of the zonal motions in Jupiter's high-latitude stratosphere. Primarily retrograde motions were observed from 41°N to 60°N. Both prograde and retrograde motions appeared from 48°S to 71°S. We propose that the asymmetric appearance of the polar hoods (R. A. West 1979, Icarus 38, 12–33; R. Wagener and J. Caldwell 1988, Icarus 74, 141–152) is influenced, in part, by meridional mixing in the stratosphere. Enhanced meridional mixing would tend to be more effective at dispersing the polar aerosols to lower latitudes in the north than in the south.

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construct complete ``population boxes'' for the LMC based entirely on color-magnitude diagrams; remarkably, these qualitatively reproduce the age-metallicity relation observed in LMC clusters. We discuss some of the uncertainties in deriving star formation histories. We find, independently of the models, that the LMC bar field has a larger relative component of older stars than the outer fields. The main implications suggested by this study are: 1) the star formation history of field stars appears to differ from the age distribution of clusters, 2) there is no obvious evidence for bursty star formation, but our ability to measure bursts shorter in duration than $\sim$ 25% of any given age is limited by the statistics of the observed number of stars, 3) there may be some correlation of the star formation rate with the last close passage of the LMC/SMC/Milky Way, but there is no dramatic effect, and 4) the derived star formation history is probably consistent with observed abundances, based on recent chemical evolution models.

From images taken with the Hubble Space Telescope's WFPC2, we have obtained photometry of a field in the Sculptor dwarf spheroidal galaxy to 3 mag below the main‐sequence turnoff. We determine an age equal to that of the earliest globular clusters for the bulk of the stars in our field of view. We attempt to constrain the star formation history of the Sculptor dwarf by examining the main‐sequence luminosity function. The presence of a half‐dozen blue straggler candidates blueward of the turnoff points to a possibly complex star formation history. However, the contribution of any intermediate‐age population is difficult to measure conclusively, because of the uncertain origin of blue stragglers and the sparseness of the photometric sample.

We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40 kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function down to I ~25 provides evidence for a roughly constant SFR of approximately 0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc) during the past 250-350 Myr. Structure in the blue loop luminosity function implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy element abundance of these young stars is 1/10th solar. The best explanation for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under excellent seeing conditions. The AGB-star luminosity function is consistent with a period of continuous star formation over at least the age range 2-10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 to similar data for three other Local Group dwarf galaxies, and find that it most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216)., Comment: To appear in the September 1999 Astronomical Journal. LaTeX, uses AASTeX v4.0, emulateapj style file, 19 pages, 12 postscript figures, 2 tables. 5 of the figures available separately via the WWW

We report Hubble Space Telescope observations of variability within the reflection nebulosity of HH 30, a compact bipolar nebula which is a nearly edge-on accretion disk system.

Surface brightness fluctuations (SBF) have been detected for three elliptical galaxies-NGC 3379 in the Leo group, NGC 4406 in the Virgo cluster, and NGC 4373 in the Hydra-Centaurus supercluster-using marginally-sampled, deep images taken with the Planetary Camera of the WFPC-2 instrument on the Hubble Space Telescope (HST). The power spectrum of the fluctuations image is well-fit by an empirical model of the point-spread function (PSF) constructed using point sources identified in the field. The previous ground-based SBF measurements for NGC 3379 and NGC 4406 are recovered, thereby demonstrating the capability of the Planetary Camera of WFPC-2 to measure distances using the SBF technique despite the marginal sampling of the images. The residual variance due to unresolved sources in all three galaxies is only 2-5% of the detected fluctuations signal, which confirms the advantage of HST imaging in minimizing the uncertainty of this SBF correction. Extensive consistency checks, including an independent SBF analysis using an alternate software package, suggest that our internal uncertainties are < 0.02 mag. The fluctuations magnitude for NGC 4373 is I814bar = 31.31 +/- 0.05 mag, corresponding to a distance modulus of DM = 32.99 +/- 0.11. This implies a peculiar velocity for this galaxy of 415 +/- 330 km/s, which is smaller than derived from the D_n-sigma relation. These results demonstrate the power of the post-repair HST to measure distances to elliptical galaxies at significant redshifts using the SBF technique., to appear in The Astrophysical Journal; 25 pages, including 7 Postscript figures and 2 tables; uses AAS LaTeX style files

We report observations of dramatic morphological changes in the extremely young Herbig-Haro object ejected by the pre-main-sequence binary XZ Tauri. Hubble Space Telescope images taken in 1995 showed a filled bubble of emission nebulosity extending 4'' north-northeast of the system in the direction of a previously known jet. New images obtained in 1998 show that the bubble has undergone a remarkable transition into a limb-brightened structure as it continues its motion away from the binary. The new images suggest that we may be witnessing the initial formation of the postshock cooling zone in a newly emerging Herbig-Haro bow shock system, probably accompanied by significant temporal evolution in the nebula's emission line spectrum.

We report initial results from Hubble Space Telescope (HST) images of Mars obtained at a near-IR wavelength of 1042 nm between February 1995 and June 1997. The images provide the highest spatial resolution global near-IR measurements of Mars obtained to date. The data have been calibrated to radiance factor ( I / F ) to an absolute accuracy of 2 to 5% (including new refinements to several HST calibration parameters) and now exist as a set of coregistered, map-projected images spanning a range of phase angles between 10° and 41°. The relatively high spatial resolution (20–30 km/pixel) global maps, when compared with Viking-era global albedo maps, reveal that substantial variations in Mars surface albedo patterns have occurred in the past 20 years. The most obvious changes include several classical dark regions that have become substantially brighter, and a few formerly bright regions that have darkened. The normal albedo of typical surface regions in the near-IR is derived from the multiple phase angle observations and is found to be bimodal with typical bright and dark values of 0.36 and 0.18, respectively. The surface near-IR phase coefficient ranges from 0.0 to 0.013 mag/deg, and there is no systematic correlation between phase coefficient and normal albedo. This observation is consistent with the diffusely scattering nature of the martian atmosphere even during times of low dust opacity. The data presented here may facilitate the planning and optimization of measurements to be obtained by the Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA), which will begin mapping the planet in 1999. These HST data also provide a basis for comparison with future MOLA and Mars Orbiter Camera (MOC) data in searches for seasonal and interannual variations in surface and polar cap albedo.

Narrowband images of the Crab Nebula taken with the Hubble Space Telescope (HST) WFPC2 show the morphology and ionization structure of the filaments in great detail. At HST resolution, low- and high-ionization emission from filaments in the Crab differ in two complementary respects. First, low-ionization emission is found to be concentrated in very sharp structures, while high-ionization emission is predominantly found in a much more diffuse component. For example, approximately 80% of emission from [O I]λ6300 arises in features with scales of less than 0".5, while only 10% of [O III]λ5007 emission arises in such compact structures. Second, individual filaments are found to lie along a sequence of ionization structures, ranging from features in which all lines are concentrated in the same compact volume through features with low-ionization cores surrounded by high-ionization envelopes. Hester and coworkers proposed in 1996 that this sequence can be understood as the result of the nonlinear development of magnetic Rayleigh-Taylor (R-T) instabilities along the interface between the Crab synchrotron nebula and swept-up ejecta. We present photoionization models of cylindrically symmetrical filaments consisting of a quadratic core surrounded by an extended envelope. A good deal of the observed variation in filament structure in the Crab can be matched by varying the assumed density profiles in these models. This implies that variations in the development of R-T instabilities in the Crab account for much of the spectral variation within the remnant. We also present a photoionization model of a uniform, low-density medium, which reasonably matches the extended diffuse component that dominates the high-ionization emission. This envelope model produces strong [O III] but virtually no [O I]. While the He I/Hβ ratio remains fairly constant throughout a range of filament models, this ratio is a factor of 5 lower in the envelope model. We find that the apertures used in ground-based spectroscopy of the Crab generally include emission from several discrete filaments as well as a component of diffuse emission. This places a fundamental limit on what can be inferred reliably from comparison of spectra with one-dimensional photoionization models. Many filament cores coincide with dust extinction features seen in a continuum image of the Crab. We consider one such feature in detail and find that the extinction of about 1.2 mag suggests that the dust-to-gas mass ratio may be an order of magnitude higher than is typical in the interstellar medium.

First results from a Hubble Space Telescope Wide Field Planetary Camera 2 Hα imaging survey of young planetary nebulae (PNs) are reported. The PNs have been selected on the basis of their low excitation characteristics. All objects imaged so far show highly aspherical morphology, with a majority characterized by multipolar bubbles distributed roughly point-symmetrically around the central star. In some objects, bipolar ansae or collimated radial structures are seen, indicating the presence of jets, whereas in others bright structures near the minor axes indicate the presence of disks or torii. The complexity, organization, and symmetry of the above structures lead us to propose that the primary agent for shaping PNs is high-speed collimated outflows or jets that operate during the late asymptotic galactic branch (AGB) and/or early post-AGB evolutionary phase, and not a preexisting equatorial density enhancement as envisioned in the currently popular model. These outflows carve out a complex imprint within an intrinsically spherical AGB circumstellar envelope (CSE). Subsequent expansion of a hot, tenuous stellar wind from the post-AGB star inside the imprinted AGB CSE then produces the observed PN, whose shape and structure depend in detail on how the characteristics of the jets change with time.

Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC 2) images of Jupiter's aurora have been obtained close in time with Galileo ultraviolet spectrometer (UVS) spectra and in situ particles, fields, and plasma wave measurements between June 1996 and July 1997, overlapping Galileo orbits G1, G2, G7, G8, and C9. This paper presents HST images of Jupiter's aurora as a first step toward a comparative analysis of the auroral images with the in situ Galileo data. The WFPC 2 images appear similar to earlier auroral images, with the main ovals at similar locations to those observed over the preceding 2 years, and rapidly variable emissions poleward of the main ovals. Further examples have been observed of the equatorward surge of the auroral oval over 140–180° longitude as this region moves from local morning to afternoon. Comparison of the WFPC 2 reference auroral ovals north and south with the VIP4 planetary magnetic field model suggests that the main ovals map along magnetic field lines exceeding 15 RJ, and that the Io footprint locations have lead angles of 0–10° from the instantaneous magnetic projection. There was an apparent dawn auroral storm on June 23, 1996, and projections of the three dawn storms imaged with HST to date demonstrate that these appear consistently along the WFPC 2 reference oval. Auroral emissions have been consistently observed from Io's magnetic footprints on Jupiter. Possible systematic variations in brightness are explored, within factor of 6 variations in brightness with time. Images are also presented marked with expected locations of any auroral footprints associated with the satellites Europa and Ganymede, with localized emissions observed at some times but not at other times.

Wide field and planetary camera 2/Hubble Space Telescope (WFPC2/HST) images of Saturn's far ultraviolet aurora reveal emissions confined to a narrow band of latitudes near Saturn's north and south poles. The aurorae are most prominent in the morning sector with patterns that appear fixed in local time. The geographic distribution and vertical extent of the auroral emissions seen in these images provide the first new observational insights into the physical mechanisms that power Saturn's aurora since the Voyager encounters 17 years ago.

We have observed the field of FS Tauri (Haro 6-5) with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. Centered on Haro 6-5B and adjacent to the nebulous binary system of FS Tauri A there is an extended complex of reflection nebulosity that includes a diffuse, hourglass-shaped structure. H6-5B, the source of a bipolar jet, is not directly visible but appears to illuminate a compact, bipolar nebula which we assume to be a protostellar disk similar to HH 30. The bipolar jet appears twisted, which explains the unusually broad width measured in ground-based images. We present the first resolved photometry of the FS Tau A components at visual wavelengths. The fluxes of the fainter, eastern component are well matched by a 3360 K blackbody with an extinction of A_V = 8. For the western star, however, any reasonable, reddened blackbody energy distribution underestimates the K-band photometry by over 2 mag. This may indicate errors in the infrared photometry or errors in our visible measurements due to bright reflection nebulosity very close to the star. The binary was separated by 0."239 ± 0."005 at a position angle of 84° ± 1."5 on 1996 January 25. There is no nebulosity around FS Tau A at the orientation suggested for a disk based on previous, ground-based polarization measurements.

We present an analysis of compact star clusters in deep HST/WFPC2 images of NGC 1275. B and R band photometry of roughly 3000 clusters shows a bimodality in the B-R colors, suggesting that distinct old and young cluster populations are present. The small spread in the colors of the blue clusters is consistent with the hypothesis that they are a single age population, with an inferred age of 0.1 to 1 Gyr. The luminosity function shows increasing numbers of blue clusters to the limit of our photometry, which reaches several magnitudes past the turnover predicted if the cluster population were identical to current Galactic globulars seen at a younger age. The blue clusters have a spatial distribution which is more centrally peaked than that of the red clusters. The individual clusters are slightly resolved, with core radii

Careful examination of 28,460 selected Wide Field and Planetary Camera 2 (WFPC2) long exposures from 1994, 1995, and early 1996 has revealed trails of 96 distinct moving objects. They have been reported to the International Astronomical Union's (IAU) Minor Planet Center for their asteroid database and a few have been identified with known asteroids and used to update their orbits. Most of the objects are new, as they are too faint to show up on ground-based surveys. The trails often show a characteristic curvature due to the parallax induced by HST's orbital motion during the exposures. Using ephemerides for HST, the distance to each object can be directly determined from the parallax contribution to the trail shapes. Based on these distances, constraints on the orbits, and photometry of the trails (16

The protoEplanetary nebula, CRL 2688, has been imaged through a wideband -lter centered at 606 nm (F606W) with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. CRL 2688 is the prototypical bipolar reNection nebula in which a star is surrounded by a dense, Nattened cocoon of dust seen nearly edge-on and starlight escapes preferentially along the polar directions producing a pair of bright nebulosities, one above and one below the equatorial plane. We -nd a pair of radial "" searchlight beams II emerging from within the dusty cocoon which intersect at the position expected for the central star when extrapolated inside the cocoon. The beams are crisscrossed by a large number of roughly round arcs with their center of curvature in the vicinity of the central star. The arcs are not systematically elongated along the polar axis of the nebula, as would be expected in the current model of CRL 2688 where the nebular density decreases with latitude. Our image directly shows the last D13,000 yr history of mass ejection from the central star while it was on the tip of the asymptotic giant branch (AGB). We -nd that the average surface brightness varies as r~3.7, implying that the average mass-loss rate or the scattering opacity of the dust grains varies as r~0.7, i.e., one or both of these parameters have steadily increased with time. The temporal resolution of B25 yr in our images has provided direct evidence for episodic increases in the mass-loss rate by factors of D2 or more, occurring every 150E450 yr, and lasting over periods of 75E200 yr. These irregularities in the mass-loss rate are roughly spherically symmetric. We have resolved the edges of the searchlight beams, as well as the peculiar structure of the inner region of the nebula Our data (1A.5\radius \ 6A). require a new model for CRL 2688 in which the beams result from starlight escaping through a pair of nonuniform annular holes in the dust cocoon which are coaxial with the polar axis of the nebula. The holes have probably been generated by a young (less than 200 yr) high-velocity outNow which streams out through these holes and interacts with the surrounding dense AGB wind to produce the peculiar structure of the inner nebula. The cocoon contains dust grains of size about 0.6 km, signi-cantly larger than those in the extended nebula. Subject headings: ISM: individual (CRL 2688) E ISM: jets and outNows E ISM: structure E reNection nebulae E stars: AGB and post-AGB E stars: mass loss

High-resolution near-infrared (1.65-2.1 μm) images and 2 μm polarimetric images of the inner 195 × 193 region of the bipolar protoplanetary nebula CRL 2688, taken with the newly installed Near-Infrared Camera and Multiobject Spectrometer (NICMOS) aboard the Hubble Space Telescope (HST), are reported. The NICMOS images reveal a wealth of structure also seen in HST Wide Field Planetary Camera 2 images but not detected in previous, ground-based near-infrared imaging studies. In particular, we detect a system of concentric arcs centered on, and twin "searchlight beams" emanating from, the obscured central star. The images also show two sharply bounded spindle-shaped polar cavities with point-symmetric structure, and the 2.122 μm H2 S(1) v = 1-0 emission-line image clearly resolves the sharp interface between the high-velocity outflow that produces these polar cavities and the surrounding slow outflow that forms the extended nebula. The H2 image also resolves the bright equatorial H2 emission into distinct components and elucidates their detailed morphologies. We have discovered, within the dark lane that bifurcates the bipolar lobes of CRL 2688, a compact source of unpolarized light. Our imaging polarimetry shows that this source is not the post-asymptotic giant branch star that illuminates the nebula; we conclude that the compact source is a companion star.