Proceedings Volume 4860

High-Contrast Imaging for Exo-Planet Detection

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Proceedings Volume 4860

High-Contrast Imaging for Exo-Planet Detection

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Volume Details

Date Published: 3 March 2003
Contents: 8 Sessions, 40 Papers, 0 Presentations
Conference: Astronomical Telescopes and Instrumentation 2002
Volume Number: 4860

Table of Contents

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Table of Contents

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  • Space Systems: Existing Systems, Upgrades, Instruments, and/or Proposed Future Systems I
  • Space Systems: Existing Systems, Upgrades, Instruments, and/or Proposed Future Systems II
  • Poster Session
  • Ground Systems: Existing Systems, Upgrades, Instruments, and Proposed Future Systems
  • Poster Session
  • Ground Systems: Existing Systems, Upgrades, Instruments, and Proposed Future Systems
  • Poster Session
  • Enabling Technology for High-Contrast Imaging
  • Poster Session
  • Analysis, Theory, and Techniques for High-Contrast Imaging I
  • Analysis, Theory, and Techniques for High-Contrast Imaging II
  • Poster Session
  • Additional Paper from Conference 4839
Space Systems: Existing Systems, Upgrades, Instruments, and/or Proposed Future Systems I
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Coronagraphy with HST/NICMOS: detectability is a sensitive issue
Glenn Schneider, Murray D. Silverstone
HST provides an unparalleled venue for high contrast imaging which has enabled new observational domains in exo-planet and debris disk imaging. Unburdened by atmospheric 'seeing', NICMOS and STIS achieve very low levels of background contamination from the wings of stellar point spread functions (PSF). Coronagraphy provides additional contrast gains approaching an order of magnitude at small angular distances from occulted stars. The stability of the platform allows scattered and diffracted light to be further reduced by two additional orders of magnitude through PSF-subtraction. The non-destructive read-out modes of the NICMOS detectors permit sampling the PSF, with its strong radial brightness gradient, over a dynamic range exceeding 5x107 in a single spacecraft orbit. In H-band, sub-stellar companions of ΔH ~ 8+2 x (angular separation in arcseconds) are unambiguously detected in twenty minutes of integration. Raw sensitivity metrics, such as presumtively static Strehl ratios, are often invoked in comparing the performance of different instrumental systems but belie the true detectability levels which are dominated by systemic non-repeatable PSF variations (not photon statistics). Such variations can give rise to false detections of companions (and circumstellar disks) and introduce very significant photometric errors. The ability to rotate the HST field with high precision about the target axis and acquire temporally stable reference PSFs readily permits the identification and rejection of rotationally-invariant optical artifacts. We discuss the repeatable, quantifiable performance limits routinely reached by HST (currently unachievable on ground-based systems), for which PSF stability is critical.
Imaging forming planetary systems: the HST/STIS legacy and prospects for future missions
Bruce Woodgate, Carol Grady, Charles W. Bowers, et al.
The STIS coronagraph performance is measured as a function of angle from the star, reaching 1×10-8 per resolution element compared to the total stellar flux at 2 arcsec with optimal PSF subtraction. Highly structured envelopes, disks, jets and planetary clearings have been detected. Estimates of planet detectability with NGST show that Jupiter mass objects around the nearest stars should be detectable.
Advanced camera for surveys coronagraph on the Hubble Space Telescope
The Advanced Camera for Surveys (ACS) was installed in the Hubble Space Telescope (HST) during Servicing Mission 3B in early 2002. It includes a High Resolution Camera (0.025 arcsec/pixel) with a selectable coronagraphic mode. Because it was added after construction began on the instrument, this classical Lyot coronagraph must operate on the uncorrected, spherically aberrated beam from HST. The occulting spot is placed at the circle of least confusion, requiring relatively large spot diameters of 1.8 and 3.0 arcseconds. The Lyot stop is placed at the correcting mirror that compensates for spherical aberration. Despite this non-optimal configuration, the diffraction pattern is reduced by a factor of 5-10x, bringing it below the level of light scattered by the mid-spatial-frequency errors in the HST optics. Together with PSF subtraction, the PSF wings can be reduced by over 1000x. The coronagraph will provide high-resolution, high-contrast imaging over a large wavelength range (200-1000 nm).
Optical Planet Discoverer: how to turn a 1.5-m telescope into a powerful exo-planetary systems imager
Optical Planet Discoverer (OPD) is a 1.5m class space telescope concept working as a visible nulling-interferometer imager. It is designed to detect Jupiter-like planets orbiting main sequence stars 10pc away in a few minutes of integration and carry out a low resolution (~20) spectroscopy of their atmosphere. OPD would fit in the budget envelope of a discovery class mission. It would serve as an efficient precursor to a Visible Terrestrial Planet Finder (VTPF), a scaled-up 4m class version based on the same optical scheme and allowing direct detection of 10pc Earthlike planets in a few hours. We detail here OPD's optical principle layout, which is primarily driven by an integrated stellar light attenuation of 1e-6 in the final focal plane. The optical concept is based on a double-shearing nulling interferometer followed by an array of single-mode waveguides. The waveguides array ensures high residual starlight suppression - as already demonstrated at the 1e-6 level by preliminary JPL visible LASER nulling experiments - together with diffraction limited imaging of the circumstellar environment over a 2 arcsec field. During the observations, the telescope is spun around the line of sight to allow for proper detection of fixed planetary signatures against residual off-axis speckle patterns at the 1e-9 level. Use of the single-mode waveguide array to filter out scattered starlight eliminates the requirements for pristine λ/4000 rms wavefronts anywhere in the optical train. With OPD, stringent phase requirements apply only to scales larger than 5 cm - the equivalent size of the pupil regions to be recombined and nulled in a given fiber, so that phase specifications can be met using low order active optics.
Extra-solar planetary imager (ESPI) for space-based Jovian planetary detection
Richard G. Lyon, Daniel Y. Gezari, Gary J. Melnick, et al.
The Extra-Solar Planetary Imager (ESPI) is envisioned as a space based, high dynamic range, visible imager capable of detecting Jovian like planets. Initially proposed as a NASA Midex (NASA/Medium Class Explorer) mission (PI:Gary Melnick), as a space-based 1.5 x 1.5 m2 Jacquinot apodized square aperture telescope. The combination of apodization and a square aperture telescope reduces the diffracted light from a bright central source increasing the planetary to stellar contrast over much of the telescope focal plane. As a result, observations of very faint astronomical objects next to bright sources with angular separations as small as 0.32 arcseconds become possible. This permits a sensitive search for exo-planets in reflected light. ESPI is capable of detecting a Jupiter-like planet in a relatively long-period orbit around as many as 160 to 175 stars with a signal-to-noise ratio > 5 in observations lasting maximally 100 hours per star out to ~16 parsecs. We discuss the scientific ramifications, an overview of the system design including apodizing a square aperture, signal to noise issues and the effect of wavefront errors and the scalability of ESPI with respect to NASA's Terrestrial Planet Finder mission.
UMBRAS: a matched occulter and telescope for imaging extrasolar planets
Alfred B. Schultz, Ian J.E. Jordan, Mark Kochte, et al.
We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets. Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits. In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 kms from the telescope, on the sky instead of by moving the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope. In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power (solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.
Space Systems: Existing Systems, Upgrades, Instruments, and/or Proposed Future Systems II
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Eclipse architecture
The Eclipse instrument concept is being optimzed to produce direct images of cool Jovian planets around several hundred candidate nearby stars, and consists of matched large telescope, wavefront sensing and control, and coronagraphic camera modules. Designed to fully comply with a Discovery Program budget and schedule, it operates in a wavelength band from 550 nm to 950 nm, and includes an unobscured telescope 1.8m in diameter. Spaceborne direct exoplanet imaging is now practical through deformable mirror (DM) technlogy that permits a hundred-fold or more quasi-static correction of a Hubble Space Telescope level primary mirror surface error at the mid-spatial frequencies that scatter starlight over its planet. Due to the stability of the spaceborne environment, wavefront error sensing can be accomplished in calibration runs preceding planet observation. Eclipse development is aided by a JPL testbed including many attributes of the flight article, and by validation of diffractive propagation algorithms that define system performance. As such, Eclipse both may serve as a pathfinder for envisioned larger instruments for terrestial planet finding, and provide the first sensitive survey of nearby planetary systems. Eclipse is in the process of definition and design, and the results shown here may be modified with further analysis and design. The emphasis of this paper is our approach to the Eclipse Space Element, and the definition of a checklist of features for coronagraphic design. The ground element, and specific devices and algorithms, and the next stages of design will be subjects of future papers. It is not sufficient to design a system capable of providing 10-9 contrast. A coronagraphic system must have sufficient stability that this level of contrast can be maintained over a reasonable observing interval.
Optical systems engineering for a terrestrial planet finder coronagraph
Optical coronagraphy is a promising possibility for finding and characterizing Earth-like planets that orbit nearby stars. This approach begins with a large unobscured conventional telescope, but significant modifications are needed to achieve adequate suppression of the glare of the star. Three techniques are under consideration for suppression of the aperture diffraction which redirects starlight into the planet's pixel; once this is satisfied, extraordinary precision and stability in the wavefront are needed as well to suppress scatter of the starlight into that pixel. We discuss the central choices in the setting of error budgets, a summary of key allocations in that budget, optical model results that demonstrate the operation and performance of the system, and key hardware requirements.
Infrared coronagraph for the Terrestrial Planet Finder Mission: I. Overview and design concept
NASA wants to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, perform comparative planetology studies, and obtain general astrophysics observations. The detection of a 30th magnitude planet located within 80 milli-arcseconds of a 5th (Visual) magnitude star is an exceptionally challenging objective. Observations in the thermal infrared (7-17 mm) are somewhat easier since the planet is 'only' 15m fainter than the star at these wavelengths, but many severe challenges must still be overcome, including: Designing a spacecraft, a telescope and an IR coronagraph for star-planet separations equal to λ/D;(i) Providing a stable (~30K) thermal environment for the optics and isolating them from vibration sources; (ii)Developing a deployment scheme for a 28-m space telescope that can fit in an existing launch vehicle; (iii) Minimizing telescope mass to enable launch to L2 or a driftway orbit with a single launch vehicle; (iv) Generating a manufacturing plan that will permit TPF to be developed at a reasonable cost and schedule; (v) Identifying the key enabling technologies for TPF. This paper describes the IR Coronagraph we designed during our recent TPF Mission Architecture study in an effort to meet these challenges.
Infrared coronagraph for the Terrestrial Planet Finder Mission: II. Instrument design and performance
NASA plans to launch a Terrestrial Planet Finder (TPF) mission in 2014 to detect and characterize Earth-like planets around nearby stars, to perform comparative planetology studies, and to obtain general astrophysics observations. As part of our recently completed TPF Mission Architecture study for NASA/JPL we developed the conceptual design for a Large Aperture IR Coronagraph that meets these mission objectives. This paper describes the optical design of the telescope and the coronagraph to detect and characterize exo-solar planets. The telescope design was optimized to provide a well-corrected image plane that is large enough to feed several instruments and control scattered light while accommodating packaging for launch and manufacturing limitations. The coronagraph was designed to provide a well corrected field of view with a radius > 5 arcsec around the star it occults in the 7-17 microns wavelength region. A design for this instrument as well as results of a system simulation model are presented. The methodology for wavefront error correction and control of scattered and diffracted light are discussed in some detail as they are critical parameters to enable detecting planets at separations of down to ~λ/D.
Integrated system modeling for evaluating the coronagraph approach to planet detection
A coronagraphic telescope is one of the several approaches proposed for finding and characterizing extrasolar planets for the Terestrial Planet Finder (TPF) mission. The coronagraph approach permits one to directly image planets but puts demanding requirements on the optical system performance. The planet flux, in the visible spectrum, is typically 1e10 times less than the star flux. Imaging the planet requires extremely tight tolerancing of the optical system and rigorous management of the disturbance environment. To investigate many of the complex system issues, the Ball Integrated Telescope Model (ITM) was configured to do performance modeling and system trades. The individual discipline models in structural dynamics, optics, controls, signal processing, detector physics, and disturbance modeling are seamlessly integrated into one cohesive model to support efficient system level trades and analysis. The core of the model is formed by the optical toolbox implemented in MATLAB and realized in object-oriented Simulink environment. This paper describes the ITM architecture and concludes with results obtained for two potential TPF coronagraph designs. Disturbance models input into the coupled structural/optical modeling are used to explore some of the system sensitivities.
Phenomenology of extra-solar planets in reflected light and system level requirements for detection and characterization
During our NASA sponsored study of candidate architectures for the Terrestrial Planet Finder mission we estimated the values of observable properties that would be accessible to an instrument intended to detect starlight reflected by a planet in the habitable zone of the system. These properties include architecture and wavelength independent geometrical properties such as angular separation between the star and planet, and timescales associated with orbital motion. Properties that do depend on the detection technique and wavelength include the brightness of the planet, its contrast relative to the star, and variability associated with diurnal and seasonal phenomena. The search space for a reflected light TPF is the range of these parameters calculated for a sample of 200 main sequence stars whose stellar properties make them potential targets. A scientific investigation such as that described by the TPF Science Working Group then leads to requirements on the sensitivity of the system, angular resolution, suppression of starlight and operational efficiency. We will describe our star sample, the search space of planetary observables and apparent system requirements.
Poster Session
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Jovian Planet Finder optical system
The Jovian Planet Finder (JPF) is a proposed NASA MIDEX mission to place a highly optimized coronagraphic telescope on the International Space Station (ISS) to image Jupiter-like planets around nearby stars. The optical system is an off-axis, unobscured telescope with a 1.5 m primary mirror. A classical Lyot coronagraph with apodized occulting spots is used to reduce diffracted light from the central star. In order to provide the necessary contrast for detection of a planet, scattered light from mid-spatial-frequency errors is reduced by using super-smooth optics. Recent advances in polishing optics for extreme-ultraviolet lithography have shown that a factor of >30 reduction in midfrequency errors relative to those in the Hubble Space Telescope is possible (corresponding to a reduction in scattered light of nearly 1000x). The low level of scattered and diffracted light, together with a novel utilization of field rotation introduced by the alt-azimuth ISS telescope mounting, will provide a relatively low-cost facility for not only imaging extrasolar planets, but also circumstellar disks, host galaxies of quasars, and low-mass substellar companions such as brown dwarfs.
Optical designs of snapshot interferometer for coronagraphy
Sophie Gillet, Pierre Riaud, Anthony Boccaletti, et al.
Following the idea developed in (Boccaletti et al. 2000), a snapshot imaging interferometer is proposed as an alternative to the NASA Origin project.
ExPO: a Discovery-class apodized square aperture (ASA) extrasolar planet observatory
Daniel Y. Gezari, Peter Nisenson, Costas D. Papaliolios, et al.
The Extrasolar Planet Observatory (ExPO) is envisioned as a Discovery-class space telescope for the direct detection and characterization of extra-solar planets. ExPO would also demonstrate the feasibility of a number of technologies which could be critical to the ultimate success of the Terrestrial Planet Finder mission. ExPO would detect a wide range of planet types in the visible and near IR, and do spectrophotometry and spectroscopy on many of the detected objects. The apoodized square aperture coronagraphic space telescope is designed to resolve faint companions near much brighter point-like sources by achieving very high dynamic range imaging at separations as small as 0.1 arcsec.
Ground Systems: Existing Systems, Upgrades, Instruments, and Proposed Future Systems
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TRIDENT: an infrared camera optimized for the detection of methanated substellar companions around nearby stars
A near-infrared camera in use at the Canada-France-Hawaii Telescope and at the 1.6m telescope of the Observatoire du Mont-Mégantic is described. The camera is based on a Hawaii-1 1024×1024 HgCdTe array detector. Its main feature is to acquire three simultaneous images at three wavelengths (simultaneous differential imaging) across the methane absorption bandhead at 1.6 micron, enabling an accurate subtraction of the stellar point spread function (PSF) and the detection of faint close methanated companions. The instrument has no coronagraph and features a fast (1 MHz) data acquisition system without reset anomaly, yielding high observing efficiencies on bright stars. The performance of the instrument is described, and it is illustrated by CFHT images of the nearby star Ups And. TRIDENT can detect (3 sigma) a methanated companion with Delta H=10 at 0.5” from the star in one hour of observing time. Non-common path aberrations between the three optical paths are the limiting factors preventing further PSF attenuation. Reference star subtraction and instrument rotation improve the detection limit by one order of magnitude.
High-contrast imaging with Gaussian-shaped pupils
John H. Debes, Jian Ge, Caylin Mandelowitz, et al.
Gaussian aperture pupil masks can in theory achieve the contrast requisite for directly imaging an extrasolar planet. We outline the process of fabricating and testing a GAPM for use on the Penn State near-IR Imager and Spectrograph (PIRIS) at the Mt. Wilson 100" telescope. We find that the initial prototype observations are quite successful, achieving a contrast similar to a traditional Lyot coronagraph without blocking any light from a central object and useful for finding faint companions to nearby young solar analogues. In the lab we can reproduce the expected PSF to within an order of magnitude and with new designs achieve ~5×10-5 contrast at 10λ/D. We find that small inaccuracies in the mask fabrication process and insufficient correction of the atmosphere contribut ehe most degradation to contrast. Finally we compare the performance of GAPMs and Lyot coronagrphs of similar throughput.
Can we really go for direct exo-planet detection from the ground?
Markus Feldt, Thomas F. E. Henning, Stefan Hippler, et al.
We are currently investigating the possibilities for a high-contrast, adaptive optics assisted instrument to be placed as a 2nd-generation instrument on ESO's VLT. This instrument will consist of an 'extreme-ao' system capable of producing very high Strehl ratios, a contrast-enhancing device and two differential imaging detection systems. It will be designed to collect photons directly coming from the surface of substellar companions - ideally down to planetary masses - to bright, nearby stars and disentangle them from the stellar photons. We will present our current design study for such an instrument and discuss the various ways to tell stellar from companion photons. These ways include the use of polarimetric and/or spectroscopic information as well as making use of knowledge about photon statistics. Results of our latest simulations regarding the instrument will be presented and the expected performance discussed. Derived from the simulated performance we will also give details about the expected science impact of the planet finder. This will comprise the chances of finding different types of exo-planets - notably the dilemma of going for hot planets marginally separated from their parent stars or cold, far-away plamnets delivering very little radiation, the scientific return of such detections and follow-up examinations, as well as other topics like star-formation, debris disks, and planetary nebulae where a high-resolution, high-contrast system will trigger new break-throughs.
Poster Session
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Performance of a coronagraph on the AEOS 3.6-m adaptive optics telescope
Patrick T. Ryan, Scott Milster, Jack D. Drummond
Adaptive optics (AO) is useful in correcting the blurring effects of the atmosphere responsible for most energy in the halo of the point spread function. A coronagraph can further enhance faint companion searches by reducing the diffraction rings surrounding a well corrected image peak. Here, we use a coronagraph on the Advanced Electro-Optical System (AEOS) at the Maui Space Surveillance Complex (MSSC) to test these benefits. The spatial characteristics of the scattered energy produced at AEOS are explored from the viewpoint of searching for faint stellar companions. The benefit of using AO is found to be 3 to 4.5 stellar magnitudes for a ten second integration time at 1 to 2 microns, with the most benefit at radial distances less than one arcsecond. More advanced AO/coronagraphic systems should be able to produce even better results. Using AO and a coronagraph should be strongly considered when attempting to image faint companions.
Ground Systems: Existing Systems, Upgrades, Instruments, and Proposed Future Systems
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The adaptive optics point-spread function at moderate and high Strehl ratios
Anand Sivaramakrishnan, Philip E. Hodge, Russell B. Makidon, et al.
We describe the symmetries present in the monochromatic point-spread function (PSF) corrected by an adaptive optics (AO) system to Strehl ratios of about 60% or greater. We expand the PSF in powers of the Fourier transform of the phase disturbance over an arbitrarily shaped and apodized entrance aperture. We show that for traditional unapodized aperture geometries, bright speckles pinned to the bright Airy rings are part of an antisymmetric first order perturbation of the perfect PSF (we make no assumptions about the symmetries of the aperture). This first order term redistributes power within each bright ring, but contributes no power in regions where the perfect image would have no light because it is modulated by the square root of the perfectly corrected PSF. It also vanishes at the center of the image. There are two symmetric second degree terms, one is negative at the center, and is also modulated by the perfect Airy field strength--it reduces to the Marechal approximation at the center of the PSF. The other second degree term is non-negative everywhere, zero at the image center, and is responsible for the extended halo--it limits the dynamic range in the dark portions of the image. These features can be exploited by appropriate telescope and instrument design, observing strategies, and data reduction methods to improve the dynamic range of AO observations.
Poster Session
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Waffle mode error in the AEOS adaptive optics point-spread function
Russell B. Makidon, Anand Sivaramakrishnan, Lewis C. Roberts Jr., et al.
Adaptive optics (AO) systems have improved astronomical imaging capabilities significantly over the last decade, and have the potential to revolutionize the kinds of science done with 4-5m class ground-based telescopes. However, provided sufficient detailed study and analysis, existing AO systems can be improved beyond their original specified error budgets. Indeed, modeling AO systems has been a major activity in the past decade: sources of noise in the atmosphere and the wavefront sensing WFS) control loop have received a great deal of attention, and many detailed and sophisticated control-theoretic and numerical models predicting AO performance are already in existence. However, in terms of AO system performance improvements, wavefront reconstruction (WFR) and wavefront calibration techniques have commanded relatively little attention. We elucidate the nature of some of these reconstruction problems, and demonstrate their existence in data from the AEOS AO system. We simulate the AO correction of AEOS in the I-band, and show that the magnitude of the `waffle mode' error in the AEOS reconstructor is considerably larger than expected. We suggest ways of reducing the magnitude of this error, and, in doing so, open up ways of understanding how wavefront reconstruction might handle bad actuators and partially-illuminated WFS subapertures.
Large sparse-aperture densified-pupil hypertelescope concept for ground-based detection of extrasolar planets
A concept is presented for a 10-meter sparse aperture hypertelescoep to detect extrasolar planets by direct imaging from the ground through the turbulent atmosphere. The telescope achieves high dynamic range with good image quality very close to bright stellar sources using pupil densification techniques and real-time atmospheric correction. Active correction of the perturbed wavefront is greatly simplified by several unique design features of the telescope: 1) use of an array of 19 small subaperture flat mirrors, 2) mounting the flats on a steerable parabolic truss structure, 3) operating in the near-IR, and 4) making the subaperture flats comparable in size to the seeing cells. These features relax the requirements on the wavefront sensing and control system. This paper describes the general concept. The details of design and implementation will be addresed separately.
Simulations of high-order curvature adaptive optics
CFHT recently considered an upgrade of its curvature adaptive optics system to perform high dynamic range imaging. This requires high Strehl ratios and very stable PSF, which is usually achieved with high order AO systems providing a dense sampling and correction across the pupil. Such systems are conventionally thought of as Shack-Hartman/piezostack devices because, in theory the scalign law for noise propagation goes as N.log(N) where N is the number of actuators. However, it is difficult to demonstrate this behavior in practice, and as systems get more compelx, their efficiency drops to lower levels than expected. Simulations of such systems are usually optimistic because they fail to take the complexity of a real system's flaws into account.
Enabling Technology for High-Contrast Imaging
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Four-quadrant phase mask: analytical calculation and pupil geometry
Of the many novel coronagraphic and nulling techniques that have been suggested to improve image contrast for exoplanet detection, one of the most promising is the Quadrant Phase Mask suggested by Rouan et al. Analysis of this optical system has previously been performed by discrete Fourier transform methods, that result in systematic errors due to the implicit assumptions of the methods and mathematical singularities in the transform of the phase mask. In this paper, we describe an analytical treatment of this optical system that treats these singularities explicitly. We calculate the leakage of a Quadrant Phase Mask Coronagraph with these analytical techniques, and show that a Quadrant Phase Mask rejects all on-axis light for an unaberrated, unobscured circular aperture and is therefore a nearly perfect coronagraph. We demonstrate why the Quadrant Phase Mask coronagraph suffers degraded performance with an obscured aperture, and propose modifications to the pupil geometry to mitigate this problem.
Achromatic four-quadrant phase mask using the dispersion of form birefringence
Dimitri Mawet, C. Lenaerts, Vincent Moreau, et al.
We describe an alternative design for the 4-quadrant phase mask coronagraph described recently by Rouan et al. 2000. Based on the same principle, i.e. producing a very efficient nulling by mutually destructive interferences of the coherent light from the main source, our mask realises the pi phase shift using some properties of ZOGs (Zeroth Order Gratings) and according to an original scheme respecting the 4-quadrant symmetry. When the period of the one-dimension grating structure is smaller than the wavelength of the incident light, the structure becomes birefringent. The effective refractive indices depend on the wavelength. Using this feature, we can design a mask whose nulling e±ciency is maintained within a wide wavelength range. Numerical simulations were made according to the RCWT (Rigorous Coupled Wave Theory).
Estimated performances of a four-quadrant phase mask coronagraph for planet detection
The concept we recently introduced of a coronagraph using a four-quadrant phase mask has been the subject of detailed model calculations and of laboratory validation proving its great potential in planet finding. A nulling factor of 12500 is already demonstrated in the laboratory (Riaud et al., this conference). We first remind the principle of the 4QC: a destructive interference between the two equal fractions of the amplitude with opposite signs produces a very efficient nulling of the_star light. We propose to install such a device on several ground-based and space instruments, including present (NAOS/CONICA) or future (Planet Finder) VLT instruments and MIRI, the mid-IR camera of the NGST. The present paper focus on the question of direct imaging of exoplanets using this type of device. Indeed, one advantage of the four-quadrant coronagraph is to permit probing the vicinity of a star down to smaller angular distances than a classical Lyot mask. We examine the sources of uncertainties in different cases of optimized ground-based and space experiments and different situations of planet/star couples, using as far as possible realistic models of planetary evolution. On the VLT, even with an extremely powerful adaptive optics system, the speckle noise will be the main limitation: contrast in magnitude as large as Dm = 15 are however possible in the K band. The combination of a 4QC and differential imaging at two wavelength is likely the most promising concept for direct planet Detection from the ground. On the other hand, we show that with a 4QC on MIRI, a classical Jupiter is indeed detectable from space and at 20 μm for a star closer than 10 pc, while the more favorable cases of a young (hot) giant planet allows detection at 6 μm for a star belonging to the closest star forming region at 50 pc.
High-dynamic-range imaging with the phase knife coronagraph: theory and first laboratory results
Lyu Abe, Armando Domiciano de Souza Jr., Farrokh Vakili
We report experimental results from the Phase Knife Coronagaph which validate our previous theoretical and numerical simulations, prove the physical principle of it and set realistic limitations to the nulling properties of the coronagraph. The optical set-up, phase knives manufacturing technique and different aspects of the instrumental limitations are given. The first results attain easily a 3000 nulling effect obtained both on single and double simulated stars. Optical and mechanical stabilities are discussed and future steps to be carried out are outlined.
Improvement of coronagraphy using optimal apodizations
Remi Soummer, Claude Aime, Andre Ferrari, et al.
This communication deals with apodization and coronagraphy. For apodization alone, we present analytical studies of point spread functions obtained with a rectangular or a circular telescope apodized by cross-linear or circular prolate spheroidal functions, respectively. Results are compared with sonine apodizations investigated by Nisenson and Papaliolios (2001) for the Apodized Square Aperture project. For coronagraphy, prolate functions are the optimal apodizers for rectangular and circular apertures. With the Roddier & Roddier phase mask technique, these apodizations can produce a total extinction of the star light. For Lyot coronagraphy, the extinction, not complete, is surprisingly good for a reasonable mask size. A comparison between coronagraphy and apodization alone for different telescope aperture shape is in favor to the use of a coronagraph with a circular aperture.
Disturbance reduction system: testing technology for precision formation control
William M. Folkner, Sasha Buchman, Robert L. Byer, et al.
The Disturbance Reduction System (DRS) is a space technology demonstration within NASAs New Millennium Program. DRS is designed to validate system-level technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for formation-flying interferometers. DRS is based on a freely-floating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. Colloidal microthrusters will be used to control the spacecraft position within a few nanometers, over time scales of tens to thousands of seconds. For testing the level of acceleration noise on the test mass, a second test mass will be used as a reference. The second test mass will also be used as a reference for spacecraft attitude. The spacecraft attitude will be controlled to an accuracy of a few milliarcseconds using the colloidal microthrusters. DRS will consist of an instrument package and a set of microthrusters, which will be attached to the European Space Agencys SMART2 spacecraft with launch scheduled for August 2006.
Poster Session
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Nulling stellar coronagraph using liquid-crystal device
Naoshi Murakami, Naoshi Baba, Tsuyoshi Ishigaki, et al.
Nulling stellar coronagraph has been proposed to detect faint objects very close to a bright point-like star, especially extra-solar planets. The principle of the nulling stellar coronagraph is to cause destructive interference for the light from a star. There have been proposed several methods for nulling interferometry. The key point of the nulling interferometry is the way to produce π-phase shift over wide range of wavelength. Here we propose a method for realizing achromatic π-phase shift utilizing polarization interference. The phase difference between two light beams that pass through different polarizers is π radians when these polarizers are placed between mutually orthogonal polarizer and analyzer. We adopt a ferroelectric liquid-crystal (FLC) device to convert the polarization direction of the incident beam. The FLC device is regarded as a birefringent device with retardation π, namely a half wave plate. The FLC device forms four-quadrant structure and is placed between the polarizer and the analyzer. By fixing the optic axes of the four-quadrant FLC suitably, it can rotate the incident linearly polarized light in parts by plus/minus 45°.
Novel approach to the fabrication of noncircular apodized aperture ultrasmooth mirrors for exo-planet detection
We suggest new approaches towards direct detection, in the near term, of Earth-like planets around Sun-like stars. Optical detection of such 'exo-planets' at visual wavelengths requires telescopes mirrors >1m diameter with very accurate figures, extremely smooth surfaces, and highly unusual shapes. The availability of such mirrors, with reasonable fabrication times and at affordable cost, is an issue of major concern. We describe how composite mirror technology is being developed to meet these very challenging requirements. A number of non-circular mirrors of modest (0.5 to 1.2m) aperture have already been made. We present data on the development and status of a low cost process that can make mirrors of corornagraphic quality with arbitrary shapes, very highly smooth surface, and ultra-low areal density.
Eclipse apodization: realization of occulting spots and Lyot masks
Daniel W. Wilson, Paul D. Maker, John T. Trauger, et al.
A key element in the Eclipse coronagraph is the apodized occulting spot. For exo-planet detection in the presence of a sun-like star, the specification for intensity transmittance at the center of the spot is less than 1×10-8. It must also taper smoothly with a desired functional form to avoid diffraction artifacts in the angular region of planet detection. From a fabrication point of view, these requirements are very challenging. A candidate technology for fabricating such spots is electron-beam exposure of high-energy beam sensitive (HEBS) glass (a product of Canyon Materials, San Diego, CA). In this work, we have calibrated HEBS glass optical density as a function of electron-beam exposure and attained optical densities up to 7.66 without saturation. We then fabricated occulting spots having various functional forms including circular Gaussian, one-dimensional sinc2, and circular sinc2. Preliminary quantitative analysis of the circular sinc2 occulting spot is encouraging.
Analysis, Theory, and Techniques for High-Contrast Imaging I
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Residual wave front phase estimation in the reimaged Lyot plane for the Eclipse coronagraphic telescope
Coronagraphs for extra-solar planet detection remove diffracted stellar light through the combination of a coronagraphic mask and a Lyot stop. When the entrance pupil contains a nearly perfect wave front, most of the stellar light is absorbed at the mask. Light scattered around the spot due to mid- and high-spatial frequency phase errors in the pupil appears at the Lyot plane as speckles whose amplitudes are proportional to the local wave front phase residuals. The speckles scale with optical wavelength but are not radially smeared. The Eclipse deformable mirror (DM) can be used to modify the Lyot amplitude distribution, providing a simple means of estimating the residual phase content and controlling the wave front. To reduce the detrimental noise carried by uncontrollable high-spatial frequency wave front components, the Lyot plane signal is filtered at the science plane to pass only the controllable spatial frequencies that contribute to the dark hole. The Lyot stop is then reimaged onto a detector. We demonstrate through simulations that this approach significantly improves the signal-to-noise ratio of the planet measurement.
Optimal shaped pupil coronagraphs for extrasolar plant finding
We examine several different approaches to achieving high contrast imaging of extrasolar planets. Rather than controlling the diffracted light by masking the star's image as in a classical coronagraph, we use the pupil's transmission function to focus the starlight. There are two broad classes of pupil coronagraphs examined in this paper: apodized pupils with spatially varying transmision functions and shaped pupils, whose transmission values are either 0 or 1. The latter are much easier to manufacture to the needed tolerances. This paper introduces several new shaped pupils and applies integration time and other metrics to them as well as to apodized pupils. These new designs can achieve nearly as high a throughput as the best apodized pupils and perform significantly better than the apodized square aperture design. The new shaped pupils enable searches of 50% -100% of thedetectable region, suppress the star's light to below 10-10 of its peak value and have inner working distances as small as 2.8 λ/D.
Comparative optical analysis of extrasolar planetary imaging techniques
Richard G. Lyon, Jan M. Hollis, John E. Dorband
We discuss intermediary results of our on-going study of filled aperture coronagraphic imaging methods for detection and characterization of extra-solar planets from a space-based telescope. Chosen are three high contrast imaging methods which are currently under investigation as part of NASA's Terrestrial Planet Finder study. Developed are analytical and computational models for three techniques that include: (i) an apodized square aperture telescope, (ii) Lyot stop coronagraph, and (iii) Spergel/Kasdin pupil. Each of the techniques is quantitatively compared and contrasted utilizing the in-pixel contrast ratio, signal-to-noise, and detection zone as metrics. The results are parameterized with respect to planetary to angular separation, planet to stellar luminosity ratio, and aperture or baseline size. We ultimately desire a compact formalism to compare and contrast different techniques, for extra-solar planetary detection, on equal footing, from an optical detection theory point of view.
Analysis, Theory, and Techniques for High-Contrast Imaging II
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Extreme wave front sensing accuracy for the Eclipse coronagraphic space telescope
The Eclipse coronagraphic telescope will allow for high contrast imaging near a target star to facilitate planet finding. One key element will be its high accuracy, high authority deformable mirror (DM) that controls the wave front error (WFE) down to an acceptable level. In fact, to achieve the desired contrast ratio of nine orders of magnitude (in intensity) to within 0.35 arcseconds of the target star, the WFE in the telescope must be controlled to level below 1Å rms within the controllable bandwidth of the DM. To achieve this extreme wave front sensing (WFS) accuracy, we employ a focus-diverse phase retrieval method extended from the Next Generation Space Telescope baseline approach. This method processes a collection defocused point-spread functions, measured at the occulting position in the Eclipse optical system, into a high accuracy estimate of the exit-pupil WFE. Through both simulation and hardware experiments, we examine and establish the key data requirements, such as the defocus levels and imaging signal-to noise level, that are necessary to obtain the desired WFS accuracy and bandwidth.
Eclipse telescope design factors
Very high contrast imagery, required for exoplanet image acquisition, imposes significantly different criteria upon telescope architecture than do the requirements imposed upon most spaceborne telescopes. For the Eclipse Mission, the fundamental figure-of-merit is a stellar contrast, or brightness reduction ratio, reaching a factor of 10-9 or better at star-planet distances as close as the 4th Airy ring. Factors necessary to achieve such contrast ratios are both irrelevant and largely ignored in contemporary telescope design. Although contemporary telescoeps now meet Hubble Space Telescope performance at substantially lower mass and cost than HST, control of mid-spatial-frequency (MSF) errors, crucial to coronagraphy, has not been emphasized. Accordingly, roughness at MSF has advanced little since HST. Fortunately, HST primary mirror smoothness would nearly satisfy Eclipse requirements, although other aspects of HST are undesirable for stellar coronagraphy. Conversely, the narrow field required for Eclipse eases other drivers of traditional telescope design. A systematic approach to telescope definition, with primary and sub-tier figures-of-merit, will be discussed in the context of the Eclipse Mission.
Poster Session
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Active optics concepts for hypertelescope aberration control and pupil densification
Kjetil Dohlen, Pascal Dargent, Marc Ferrari, et al.
One of the instrumental concepts under study for large baseline interferometers for high resolution astronomical imaging, in particular applied to exoplanet search and characterisation, is the hypertelescope (HT). Mainly considered for space deployment, this sparse array of mirror segments supported either by a struss structure or by free-flying micro satellites form a giant, diluted primary mirror. The focal plane instrumentation, including pupil densification optics, is located in the primary focus instrument space craft (ISC). Baselines considered for first-generation HTs are of the order of 100 m, but one can envisage kilometric arrays capable of unprecedented angular resolution. Pointing with such a telescope poses orbital navigation problems. Letting the entire array perform a slow sky-scanning motion and navigating the ISC within the primary focal plane in order to follow the image of the object may solve these problems. The ISC must therefore be equipped with aberration correction optics capable of covering a sufficiently large primary field of view, of the order of a few degrees. In this paper we present optical and mechanical concepts for combined aberration correction and pupil densification using multimode deformable mirror (MDM) and mechanically amplified piezo actuator technologies. Among the advantages of such a system over large monolithic corrector optics is the relaxation of piston alignment requirements for primary segments.
Spectral contrast enhancement techniques for extrasolar planet imaging
Tilak Hewagama, Richard B. Barclay, Tina C. Chen, et al.
We use analytical models to characterize the optical response of observing systems and explore spectroscopic techniques that exploit the planetary spectral signature to enhance the planet-to-star contrast in imaging. Radiative transfer model calculations of gas-giant extrasolar planets are employed in selecting spectral regions where the planetary signal shows enhancement with respect to the stellar spectrum. We specifically discuss the use of frequency switching, in conjunction with pupil plane techniques (shaping, apodizing), to optimize the detection of the planetary signal of potential planetary systems for both ground-based and space-borne observations. Expected properties of known extrasolar planets motivate the use of the near-IR spectral region. This work was motivated by the focused technology development of a MEMS tunable filter, and the application of such devices to frequency-switched imaging spectroscopy.
Additional Paper from Conference 4839
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Ground-based coronagraphy: performance of a four-quadrant phase mask
Elena Puga, Markus Feldt, Thomas F. E. Henning, et al.
Detection of faint companions near bright stars requires the usage of high dynamic range instrumentation. The four quadrant phase mask is a quite efficient nulling device for the light of on-axis stars as shown by simulations. We conducted a test of the true performance of this concept starting with the manufacturing of the optical element, continuing with the installation in the telescope and the usage of the Adaptive Optics system. A four quadrant phase mask was installed in the 3.5m telescope on Calar Alto and several tests with both an artificial source and natural stars were conducted. Tests in order to detect faint companions around HD 140913, TRN 1 and HD 161797 were successful for the last target and also, although almost serendipitously, in the case of HD144004. The main limitations found for the phase mask cancelling effect at relatively low Strehl ratios (16%-63%) were the residual tip-tilt of our system and the control of placement of the mask in the optical train.