Coronagraph technology is advancing and promises to enable space telescopes capable of directly detecting low surface brightness circumstellar debris disks as well as giant planets as close as in the habitable zones of their host stars. One mission capable of doing this is called EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer), which in 2011 was selected by NASA's Explorer program for technology development (Category III). EXCEDE is a 0.7m space telescope concept designed to achieve raw contrasts of 10-6 at an inner working angle of 1.2 λ/D and 10-7 at 2 λ/D and beyond. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and scientific precursor for an exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the Phase Induced Amplitude Apodization (PIAA) coronagraph to provide high throughput and high contrast close to the diffraction limit, enabling aggressive performance. We report on our continuing progress of developing the SSS for EXCEDE, including (a) high contrast performance demonstrations at 1.2 λD, which includes a lab demonstration of 2x10-7 median contrast between 1.2 and 2.0 λ/D simultaneously with 6.5x10-8 median contrast between 2 and 4 λ/D in monochromatic light at 655nm, meeting a major milestone in our technology development program; (b) the installation of a new Low Order Wavefront Sensor (LOWFS) which enabled achieving deep contrasts at aggressive inner working angles; (c) implementation of more efficient model-based wavefront control algorithms; and (d) a preliminary broadband contrast result of 6x10-6 contrast at 1.2 λ/D in a 10% band.
Coronagraph technology is advancing and promises to enable space telescopes capable of seeing debris disks as well as
seeing and spectrally characterizing exo-Earths. Recently, NASA's explorer program has selected the EXCEDE
(EXoplanetary Circumstellar Environments and Disk Explorer) mission concept for technology development. EXCEDE
is a 0.7m space telescope concept designed to achieve raw contrasts of 1e-6 at an inner working angle of 1.2 λ/D and 1e-
7 at 2 λ/D. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and
scientific precursor for an exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the
Phase Induced Amplitude Apodization (PIAA) coronagraph to provide high throughput and high contrast close to the
diffraction limit, enabling aggressive performance on small telescopes. We report on the latest progress in developing
the SSS and present coronagraphic performance results from our air testbed at NASA Ames. Our results include a lab
demonstration of 1e-5 contrast at 1.2 λ/D, 1.3e-6 contrast at 1.4 λ/D and 2e-8 at 2 λ/D in monochromatic light. In
addition, we discuss tip-tilt instabilities, which are believed to be our main limiting factor at present, and ways of
characterizing them.
We present an overview of the EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE), selected by
NASA for technology development and maturation. EXCEDE will study the formation, evolution and architectures of
exoplanetary systems, and characterize circumstellar environments into stellar habitable zones. EXCEDE provides
contrast-limited scattered-light detection sensitivities ~ 1000x greater than HST or JWST coronagraphs at a much
smaller effective inner working angle (IWA), thus enabling the exploration and characterization of exoplanetary
circumstellar disks in currently inaccessible domains. EXCEDE will utilize a laboratory demonstrated high-performance
Phase Induced Amplitude Apodized Coronagraph (PIAA-C) integrated with a 70 cm diameter unobscured aperture
visible light telescope. The EXCEDE PIAA-C will deliver star-to-disk augmented image contrasts of < 10E-8 and a 1.2
λ/D IWA or 0.14” with a wavefront control system utilizing a 2000-element MEMS DM and fast steering mirror.
EXCEDE will provide 0.12” spatial resolution at 0.4 μm with dust detection sensitivity to levels of a few tens of zodis
with two-band imaging polarimetry. EXCEDE is a science-driven technology pathfinder that will advance our
understanding of the formation and evolution of exoplanetary systems, placing our solar system in broader astrophysical
context, and will demonstrate the high contrast technologies required for larger-scale follow-on and multi-wavelength
investigations on the road to finding and characterizing exo-Earths in the years ahead.
The Pupil-mapping Exoplanet Coronagraphic Observer (PECO) mission concept is a 1.4-m space-based coronagraphic telescope optimized to image exoplanets and disks at optical wavelengths and characterize them through low resolution spectroscopy and polarimetry. Thanks to a high efficiency Phase-Induced Amplitude Apodization (PIAA) coronagraph, PECO can deliver 1e-10 contrast at 2 λ/D separation (0.15") with no loss in angular resolution or throughput due to the coronagraph. PECO acquires narrow field images simultaneously in 16 spectral bands over wavelengths from 0.4 to 0.9 μm , utilizing all available photons for maximum wavefront sensing efficiency and optimal sensitivity for imaging and spectroscopy. PECO can detect and characterize potentially habitable planets around 20 known F, G, K type stars, and map exozodiacal clouds to a fraction of our own own zodiacal dust content.
PECO's key technologies are currently under active development at several testbeds, and will enable efficient exoplanet imaging missions across a wide range of telescope sizes, from a sub-meter debris disk and giant planet imager to a ~4-m life-finding mission.
The Wide Field Coronagraph Telescope (WFCT) is a 4-meter space telescope for general astrophysics and exoplanet
observations that meets the 2000 Decadal Committee requirements.
This paper presents a design for a 4-m diameter, off-axis space telescope that offers high performance in both wide field
and coronagraphic imaging modes. A 3.8 x 3.3-m unobstructed elliptical pupil is provided for direct coronagraphic
imaging of exoplanets and a 4-m diameter pupil for wide-field imaging from far-ultraviolet (UV) to near-infrared (IR).
The off-axis wide-field optics are all reflective and designed to deliver an average of 12 nm wavefront aberrations over a
6 x 24 arcminute field of view (FOV), therefore providing diffraction-limited images down to 300 nm wavelength and
15 mas images down to a wavelength limit set only by the mirror coatings. The coronagraph with phase-induced
amplitude apodization (PIAA) provides diffraction suppression around a 360-degree field with high Strehl and
sensitivity at the 1e-10 level to an inner working angle of 2 λ/D (or 50 mas at 500 nm wavelength).
This paper focuses on the optical design that allows the above imaging features to be combined in single telescope, and
gives a preliminary spacecraft design and costing, assuming a distant trailing orbit.
The Pupil-mapping Exoplanet Coronagraphic Observer (PECO) mission concept uses a coronagraphic 1.4-m
space-based telescope to both image and characterize extra-solar planetary systems at optical wavelengths.
PECO delivers 10-10 contrast at 2 λ/D separation (0.15") using a high-performance Phase-Induced Amplitude
Apodization (PIAA) coronagraph which remaps the telescope pupil and uses nearly all of the light coming into
the aperture. For exoplanet characterization, PECO acquires narrow field images simultaneously in 16 spectral
bands over wavelengths from 0.4 to 0.9 μm, utilizing all available photons for maximum wavefront sensing and
sensitivity for imaging and spectroscopy. The optical design is optimized for simultaneous low-resolution spectral
characterization of both planets and dust disks using a moderate-sized telescope. PECO will image the habitable
zones of about 20 known F, G, K stars at a spectral resolution of R≈15 with sensitivity sufficient to detect
and characterize Earth-like planets and to map dust disks to within a fraction of our own zodiacal dust cloud
brightness. The PIAA coronagraph adopted for PECO reduces the required telescope diameter by a factor of two
compared with other coronagraph approaches that were considered for Terrestrial Planet Finder Coronagraph
Flight Baseline 1, and would therefore also be highly valuable for larger telescope diameters. We report on
ongoing laboratory activities to develop and mature key PECO technologies, as well as detailed analysis aimed
at verifying PECO's wavefront and pointing stability requirement can be met without requiring development of
new technologies.
The Pupil mapping Exoplanet Coronagraphic Observer (PECO) mission concept is a 1.4-m telescope aimed at
imaging and characterizing extra-solar planetary systems at optical wavelengths. The coronagraphic method
employed, Phase-Induced Amplitude Apodization or PIAA (a.k.a. pupil mapping) can deliver 1e-10 contrast at
2 lambda/D and uses almost all the starlight that passes through the aperture to maintain higher throughput and
higher angular resolution than any other coronagraph or nuller, making PECO the theoretically most efficient
existing approach for imaging extra-solar planetary systems. PECO's instrument also incorporates deformable
mirrors for high accuracy wavefront control. Our studies show that a probe-scale PECO mission with 1.4 m
aperture is extremely powerful, with the capability of imaging at spectral resolution R≈∠15 the habitable zones
of already known F, G, K stars with sensitivity sufficient to detect planets down to Earth size, and to map
dust clouds down to a fraction of our zodiacal cloud dust brightness. PECO will acquire narrow field images
simultaneously in 10 to 20 spectral bands covering wavelengths from 0.4 to 1.0 μm and will utilize all available
photons for maximum wavefront sensing and imaging/spectroscopy sensitivity. This approach is well suited for
low-resolution spectral characterization of both planets and dust clouds with a moderately sized telescope.
We also report on recent results obtained with the laboratory prototype of a coronagraphic low order wavefront
sensor (CLOWFS) for PIAA coronagraph. The CLOWFS is a key part of PECO's design and will enable high
contrast at the very small PECO inner working angle.
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image planetary systems of
nearby stars simultaneously in a few wide spectral bands covering the visible light (0.4-0.9 μm). It achieves its
power by combining a high accuracy wavefront control system with a highly efficient Phase-Induced Amplitude
Apodization (PIAA) coronagraph which provides strong suppression very close to the star (within 2 λ/D). The
PIAA coronagraphic technique opens the possibility of imaging Earthlike planets in visible light with a smaller
telescope than previously supposed. If sized at 1.2-m, TOPS would image and characterize many Jupiter-sized
planets, and discover 2 RE rocky planets within habitable zones of the ≈10 most favorable stars. With a larger
2-m aperture, TOPS would have the sensitivity to reveal Earth-like planets in the habitable zone around ≈20
stars, and to characterize any found with low resolution spectroscopy. Unless the occurrence of Earth-like planets
is very low (η⊕ <~ 0.2), a useful fraction of the TPF-C scientific program would be possible with aperture much
smaller than the baselined 8 by 3.5m for TPF, with its more conventional coronagraph. An ongoing laboratory
experiment has successfully demonstrated high contrast coronagraphic imaging within 2 λ/d with the PIAA
coronagraph / focal plane wavefront sensing scheme envisioned for TOPS.
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image in the visible (0.4-
0.9 μm) planetary systems of nearby stars simultaneously in 16 spectral bands (resolution R≈20). For the
≈10 most favorable stars, it will have the sensitivity to discover 2RΕ rocky planets within habitable zones and
characterize their surfaces or atmospheres through spectrophotometry. Many more massive planets and debris
discs will be imaged and characterized for the first time. With a 1.2m visible telescope, the proposed mission
achieves its power by exploiting the most efficient and robust coronagraphic and wavefront control techniques.
The Phase-Induced Amplitude Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2λ/d
with nearly 100% throughput and preserves the telescope angular resolution. An efficient focal plane wavefront
sensing scheme accurately measures wavefront aberrations which are fed back to the telescope active primary
mirror. Fine wavefront control is also performed independently in each of 4 spectral channels, resulting in a
system that is robust to wavefront chromaticity.
The nulling coronagraph is one of 5 instrument concepts selected by NASA for study for potential use in the TPF-C
mission. This concept for extreme starlight suppression has two major components, a nulling interferometer to suppress
the starlight to ~10-10 per airy spot within 2 λ/D of the star, and a calibration interferometer to measure the residual
scattered starlight. The ability to work at 2 λ/D dramatically improves the science throughput of a space based
coronagraph like TPF-C. The calibration interferometer is an equally important part of the starlight suppression system.
It measures the measures the wavefront of the scattered starlight with very high SNR, to 0.05nm in less than 5 minutes
on a 5mag star. In addition, the post coronagraph wavefront sensor will be used to measure the residual scattered light
after the coronagraph and subtract it in post processing to 1~2x10-11 to enable detection of an Earthlike planet with a
SNR of 5~10.
The Microlensing Planet Finder (MPF) is a proposed Discovery mission that will complete the first census of extrasolar planets with sensitivity to planets like those in our own solar system. MPF will employ a 1.1m aperture telescope, which images a 1.3 sq. deg. field-of-view in the near-IR, in order to detect extrasolar planets with the gravitational microlensing effect. MPF's sensitivity extends down to planets of 0.1 Earth masses, and MPF can detect Earth-like planets at all separations from 0.7AU to infinity. MPF's extrasolar planet census will provide critical information needed to understand the formation and frequency of extrasolar planetary systems similar to our own.
A 2-meter by 4-meter aperture DART (dual anamorphic reflector telescope) system has been designed and fabricated using thin stretched mesh reflectors. The system concept consists of a pair of single curvature reflectors with curvature in orthogonal directions relative to each other and is being developed for future ultra-lightweight space applications. The current design is an extension of a 1-meter aperture system previously prototyped and successfully tested in the FarIR. The 2m x 4m system is a laboratory prototype with areal density of less than 10kg/m2 for each reflector. The new design demonstrates the advantageous scaling properties of the single curvature reflector concept. The 2m x 4m system was configured and tested in the RF over several frequencies from 5.8 - 8.2 GHz. This paper documents the structural configuration, test preparation, test results, and analysis correlation. Test results show the DART system to be a high directivity antenna (46.5 dB), very low cross-polarization (-33 dB), and good off-axis properties. Test results were in good agreement with analytical predictions of the performance. Generally, the DART system easily achieves the surface accuracy requirements at 8.2 GHz.
The control system architecture and vibration mitigation approach for a Terrestrial Planet Finder (TPF) mission based on structurally connected interferometers are defined. The spacecraft configurations investigated and associated control and operational requirements are presented. Disturbance sources are identified and their relevance assessed. Results of dynamics analysis are presented, as well as a description of the dynamic models and simulations used to predict on-orbit performance. An assessment of the maturity of the technologies for control and vibration mitigation is provided. Analysis results indicate that pointing and path-length control requirements for a TPF mission based on structurally connected interferometers with baselines from 9 to 80 meters can be achieved using a conventional spacecraft attitude control system combined with active pointing and path-length control and a vibration mitigation approach that does not rely on structural damping.
The Galactic Exoplanet Survey Telescope (GEST) will observe a 2 square degree field in the Galactic bulge to search for extra-solar planets using a gravitational lensing technique. This gravitational lensing technique is the only method employing currently available technology that can detect Earth-mass planets at high signal-to-noise, and can measure the abundance of terrestrial planets as a function of Galactic position. GEST's sensitivity extends down to the mass of Mars, and it can detect hundreds of terrestrial planets with semi-major axes ranging from 0.7 AU to infinity. GEST will be the first truly comprehensive survey of the Galaxy for planets like those in our own Solar System.
This paper describes the design of the space IR telescope Facility (SIRTF) as the project enters the detailed design phase. SIRTF is the fourth of NASA's Great Observatories, and is scheduled for launch in December 2001. SIRTF provides background limited imaging and spectroscopy covering the spectral range from 3 to 180 micrometers , complementing the capabilities of the other great observatories - the Hubble Space Telescope (HST), the Advanced X-ray Astrophysics Facility, and the Compton Gamma Ray Observatory. SIRTF will be the first mission to combine the high sensitivity achievable forma cryogenic space telescope with the imaging and spectroscopic power of the new generation of IR detector arrays. The scientific capabilities of this combination are so great that SIRTF was designated the highest priority major mission for all of US astronomy in the 1990s.
We consider a possible precursor interferometer to Terrestrial Planet Finder. The precursor called Planet Discoverer Interferometer (PDI) would search for broadband 10 μm radiation from possible terrestrial planets orbiting stars out to a distance of 8-10pc and at an angular separation of at least 0.1 arcseconds. There are about 20 stars of types A,F,G and K around which an Earth-analog might be detected. PDI would be able to confirm such planets by seeing their orbital motion. PDI would also be able to observe 5 μm radiation from the more massive and younger gas-giant planets around stars up to distances ∼ 150 pc, separated from their star by more than 0.05 arc seconds. It would also see the re-radiated thermal radiation of Jupiter-like planets at temperatures above ∼130K. The device would be a 15m long truss with four SIRTF-like telescopes. It would need to be in a SIRTF-like Earth-trailing orbit, and would be radiatively cooled. A very preliminary design suggest that PDI could fit into the shroud of a Delta II rocket. Similar preliminary calculations suggest that the total lifetime cost of such a mission would be under $300M. Detailed studies of this concept are in process.
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