This paper is an overview of the progress in the development of a formation flying architecture concept for NASA's Terrestrial Planet Finder Interferometer project. Highlights from both system design studies and technology development efforts are briefly discussed and are supported by other papers in this conference providing greater detail. Described are the major trades, analyses, and technology experiments completed. Near term plans are also described. This paper covers progress since June 2004 and serves as an update to a paper presented at that month's SPIE conference, "Astronomical Telescopes and Instrumentation", held in Glasgow, Scotland.

The Terrestrial Planet Finder Interferometer (TPF-I) is a space-based NASA mission for the direct detection of Earth-like planets orbiting nearby stars. At the mid-infrared wavelength range of interest, a sun-like star is ~10⁷ times brighter than an earth-like planet, with an angular offset of ~50 mas. A set of formation-flying collector telescopes direct the incoming light to a common location where the beams are combined and detected. The relative locations of the collecting apertures, the way that the beams are routed to the combiner, and the relative amplitudes and phases with which they are combined constitute the architecture of the system. This paper evaluates six of the most promising solutions: the Linear Dual Chopped Bracewell (DCB), X-Array, Diamond DCB, Z-Array, Linear-3 and Triangle architectures. Each architecture is constrained to fit inside the shroud of a Delta IV Heavy launch vehicle using a parametric model for mass and volume. Both single and dual launch options are considered. The maximum separation between spacecraft is limited by stray light considerations. Given these constraints, the performance of each architecture is assessed by modeling the number of stars that can be surveyed and characterized spectroscopically during the mission lifetime, and by modeling the imaging properties of the configuration and the robustness to failures. The cost and risk for each architecture depends on a number of factors, including the number of launches, and mass margin. Quantitative metrics are used where possible. A matrix of the architectures and ~30 weighted discriminators was formed. Each architecture was assigned a score for each discriminator. Then the scores were multiplied by the weights and summed to give a total score for each architecture. The X-Array and Linear DCB were judged to be the strongest candidates. The simplicity of the three-collector architectures was not rated to be sufficient to compensate for their reduced performance and increased risk. The decision process is subjective, but transparent and easily adapted to accommodate new architectures and differing priorities.

The direct optical detection of Earth-like planets orbiting nearby stars is the goal of the Terrestrial Planet Finder Interferometer (TPF-I). At infrared wavelengths between about 7 and 17 μm, spectral absorption features in the thermal emission of such planets may indicate the presence of chemical compounds thought necessary for the existence of life. To perform nulling interferometry at these relatively long wavelengths, a long baseline telescope array is needed with an overall length between about 30 m and 200 m. The current flight design effort is concentrated on the dual chopped Bracewell architecture but the design of the flight instrument is to a large degree independent of the exact array layout. Four 4 m diameter telescopes employing a conventional three-mirror design collect the light from the star and a series of sensors and actuators direct the light to a separate beamcombiner spacecraft where a number of beam control actions take place prior to the science light detection. This paper describes the opto-mechanical systems of the telescopes and beamcombiner spacecraft including the wavefront and optical path control devices and the alignment systems. The opto-mechanical layouts of the spacecraft are outlined along with the results from preliminary thermal and vibration performance models of the structure. The layout of the beamtrain control system is also described.

The Terrestrial Planet Finder Interferometer (TPF-I) is a future NASA mission for mid-infrared astronomy in space, using formation flying to position the telescopes. A unique and significant challenge for TPF-I is control of stray light from thermally emitting objects near the starlight beam paths, such as sunshades and other warm parts of the neighboring spacecraft. A proposed strategy for stray light control in these missions is simple geometric shading of the beam-transport optics from the emitting objects, but this intrinsically limits the maximum inter-spacecraft separation. We present a preliminary study of diffractive beam propagation to set lower limits on the baffle diameters. This and other geometric constraints then lead to specific estimates of the maximum inter-spacecraft separation.

The Terrestrial Planet Finder Interferometer Project (TPF-I) has set for itself a host of challenging technical milestones along its path to demonstrating the feasibility of infrared nulling for planet detection. Our activities are focused solely upon the experimental demonstration that deep nulling in the mid-IR over a wide bandpass can be accomplished. Specifically, we have the near-term goal of demonstrating a contrast of 10^-6 at 10 μm with a 25% spectral bandwidth. To meet these goals, several areas of technical development are required. These include: single-mode infrared fibers, bright infrared sources, laser path-length and tip/tilt metrology, and improvements to null detection. Progress in each of these areas of technical development will be reviewed as well as their impact on the overarching technical milestones.

Infrared interferometric nulling is a promising technology for exoplanet detection. Nulling research for the Terrestrial Planet Finder Interferometer has been exploring a variety of interferometer architectures at the Jet Propulsion Laboratory (JPL). Three architectures have been identified as having promise for achieving deeper broadband IR null depths. Previous nulling research concentrated on layouts using dispersive elements to achieve a quasi-achromatic phaseshift across the passband. However, use of a single glass for the dispersive phase shift method inherently limits the nulling bandwidth. JPL is researching use of multiple glass types to increase null depth and bandwidth. In order to pursue nulls over much broader wavelength regions, nondispersive interferometer architectures can be employed. Toward this end, JPL has been researching two reflective architectures as nulling interferometers. The key enabling technology for this and other nondispersive field flip architectures is single mode spatial filtering devices. We have obtained results with both pinhole spatial filtering and single mode fibers.

Deep, stable starlight nulls are needed for the direct detection of Earth-like planets and require careful control of the intensity and phases of the beams that are being combined. We are testing a novel compensator based on a deformable mirror to correct the intensity and phase at each wavelength and polarization across the nulling bandwidth. We have successfully demonstrated intensity and phase control using a deformable mirror across a 100nm wide band in the near-IR, and are in the process of building the phase 2 experiment operating at mid-IR wavelengths. This paper covers the results of our demonstration in the near-IR, as well as our current progress in the mid-IR.

The Planet Detection Testbed developed at the Jet Propulsion Laboratory is being used to test direct optical detection of an Earth-like planet using nulling interferometry. Operating at infrared wavelengths, the testbed produces four near-identical beams simulating a distant star and planet. The testbed is reconfigurable to simulate different telescope array designs that are being studied. Many of the systems which will be needed for the space application of nulling stellar interferometry are incorporated. The goal of the testbed is to simulate the planet detection process which requires both a long detection period of many hours to overcome the thermal background noise and also high instrument stability to avoid introducing noise signals that could be mistaken for a planet. Numerous control systems are needed to maintain the optical path differences to about 2 nm and maintain beam alignments in shear and tilt. The testbed emulates functions of the fringe-tracking and metrology systems envisioned for the flight system including finding and tracking the fringe, controlling vibration and allowing for changing conditions. The relationship of the testbed to flight conditions is discussed and the latest results are presented showing planet detection in the presence of bright starlight.

To detect earth-like planets orbiting around solar-type stars in the mid-infrared spectral range, a typical rejection ratio of 10⁶ of the stellar flux must be achieved. Space missions like Darwin/TPF aim at achieving such contrasts using nulling interferometry between 4 μm and 20 μm. The instrumental constraints on beam combination, spatial filtering, intensity and phase mismatches must then be accurately considered. This paper presents the first characterization results of mid-infrared waveguides for integrated optics (IO) developed in the frame of an ESA contract. Taking into account the scientific achievements already obtained with IO components in the near infrared range, results demonstrate that these technologies can also be used for future nulling devices as an alternative to bulk optics instrumentation in the mid-infrared spectral range. Good waveguiding behaviour has been obtained on dielectric waveguides based on Chalcogenide or Zinc Selenide glasses and Hollow Metallic Waveguides. The single-mode behavior, spatial filtering and polarization control capabilities of the hollow metallic channel waveguides have been also demonstrated. This paper focuses on the methods used to validate the waveguide behaviour and the first laboratory results obtained with the different technologies used in the mid-infrared.

The Achromatic Phase shifter breadboard (APS) is designed for broadband (550-750 nm) nulling. This system has already been described in detail previously^1,2, this paper describes improvements to the breadboard currently implemented and the corresponding results obtained. The breadboard improvements concern the following four points: A) Residual vibrations are compensated by active OPD control; B) Beam overlap can be optimized by an extra alignment mechanism; C) A more versatile detector/preamplifier is used; D) Adjustments of the phase shifter are now computer controlled. The paper describes the set-up and the results of these improvements on the nulling performance of the breadboard with the goal to achieve a 10⁴ nulling ratio and to validate the computer simulations of the achromatic phase shifter. The validated value for nuldepth observed is currently 4600+/-400, and ratio's above 10,000 have been found, when measuring with a higher bandwidth. Nulldepths like these have to our knowledge not been reported before for relative spectral bandwidths of this magnitude. The program continues to optimize the setup and improve the results.

Darwin is a space based interferometry mission¹ of the European Space Agency (ESA) with the aim to detect and characterise earth-like planets outside our solar system. The current Darwin baseline consists of four spacecrafts (3 telescopes). Destructive interference of the starlight is required to allow detection of much fainter planet signals. The nulling ratio required is 10⁵. For Darwin high requirements are set upon the wavefront quality of the beams. In order to be able to have destructive interference with a contrast factor of 10⁵, a wavefront quality of λ/1400 (λ=6 micrometer) is needed. With current and/or foreseen technology, it is not possible to produce the optical elements with sufficient quality to meet this requirement. This means it is vital to develop wavefront filter devices for Darwin. Most promising for this purpose are single mode fibres. For visible and near-infrared light commercially available single mode fibres are available, however they do not extend yet to wavelengths above 4 micrometer. To overcome this shortcoming new single mode fibres are developed (i.e. by Astrium and TNO/ University of Rennes) for the Darwin wavelength range (6-20 μm). To characterize and test these fibres a system is designed allowing to determine the possible star light suppression with the fibre. This system is called "Darwin Infrared Nulling Interferometer Demonstrator" (DINID). The system is designed using the in-house knowledge from previous nulling set-ups in the visible and near-infrared wavelength range. It will permit to test fibres around 4 and 9 micrometer and includes an optical path difference control in order to compensate drifts. This paper describes the basis on which the set-up is designed.

The Terrestrial Planet Finder Coronagraph (TPF-C) demands extreme wave front control and stability to achieve its goal of detecting earth-like planets around nearby stars. We describe the performance models and error budget used to evaluate image plane contrast and derive engineering requirements for this challenging optical system. We show that when the coronagraph is coupled to an 8^th-order band-limited mask, the performance is limited by shearing of the starlight beam across imperfect optics (a.k.a. beam walk), and that this in turn demands tight rigid body pointing, sub-milliarcsecond fine guiding, high-quality optics, and sub-micron positional stability of the optics including the secondary mirror. Additionally we show that the stability of low-order aberrations (focus, astigmatism, coma, and trefoil) is required to be ~ 2-4 Angstroms, while higher-order modes must remain stable to a few picometers.

The Terrestrial Planet Finder Coronagraph (TPF-C) is a future NASA mission to search for earth-like planets around nearby stars. The tremendous sensitivity needed primarily rests on the stability of the wavefront over several hours. One way this wavefront can change is by lateral motion of the beam (beam walk) across the mirror surfaces due to mispointing of the entire telescope or of individual mirrors. A deformable mirror (DM) corrects the exit pupil wavefront initially to extremely high quality, but beam walk changes the wavefront presented to that DM, so that its setting is no longer ideal. This paper present a simple method to estimate the power spectral density of these wavefront changes.

Current plans call for the first Terrestrial Planet Finder mission, TPF-C, to be a monolithic space telescope with a coronagraph for achieving high contrast. Our group at Princeton pioneered the concept of shaped pupils for high contrast imaging and planet detection. In previous papers we introduced a number of families of optimal shaped pupils in square, circular, and elliptical apertures. Here, we show our most promising designs and present our laboratory results for the elliptical shaped pupil. We are currently achieving better than 10⁻⁷ contrast at 10 λ/D and 10⁻⁵ contrast at 4 λ/D, without wavefront control. We describe the deep ion etching manufacturing process to make free standing masks. We also discuss what is limiting contrast in the laboratory and our progress in wavefront correction.

The Terrestrial Planet Finder Coronagraph (TPF-C) for observing and characterizing exo-solar planets requiring star light suppression to 10^-10 level demands optical aberrations and instrument stability to sub-nm levels. Additionally, wavefront polarization has to be tightly controlled over the 8m x 3.5m primary mirror aperture and 500nm - 800nm minimum bandwidth because the Deformable Mirror (DM) employed to control the wavefront can not correct simultaneously for the different wavefronts presented by two orthogonal uncorrected polarization fields. Further, leakage of cross polarization fields introduced by the various optical surfaces can degrade the image contrast. The study reported here shows mirror coating designs that reduce the phase difference between orthogonal polarizations reflected by a mirror surface to less than 0.6 deg over the bandwidth and aperture which may encounter a maximum angle of incidence of about 12 deg at a curved mirror. Such designs mitigate the contrast degradation due to cross polarization leakage. Simulations show that required contrast levels can be achieved with such coatings.

The direct detection at visible wavelengths of Earth-like planets around nearby stars requires starlight suppression by a factor of 10¹⁰ - 10¹¹ at offsets of order 100 mas. It has been shown that perfect suppression is possible in principle, using a combination of a band-limited focal plane coronagraphic mask and a pupil plane Lyot stop. Errors in the transmission amplitude and phase of the mask degrade the performance. These errors can be corrected completely at a given wavelength and polarization using deformable mirrors (DMs) operating in the pupil plane of the system. Both the errors and correction have different chromatic dependences, however, and the DM correction becomes ineffective as the optical bandwidth is increased. The mask errors can be divided into 2 classes: (1) errors that are uncorrelated with the mask pattern, arising, for example, from the surface roughness of the mask substrate, and (2) errors that are correlated with the mask pattern. We present the results of analysis of random errors and simulate the effects of systematic errors using a specific example mask design. In both cases we find that the contrast required by TPF-C imposes very challenging demands on the design and fabrication of the masks. Several potential mitigation approaches are discussed.

The edge generated stray-light from corner boundary conditions, interactions with the lower mask structure, and surface plasmon polaritons that may limit Terrestrial Planet Finder Coronagraph performance are characterized. Previously a number of stray light sources, unaccounted for by the ideal thin mask theory used to design the pupil-plane masks, were identified. In this paper we illustrate and quantify the most important outstanding stray-light sources in the near-field in order to improve the model of pupil-plane mask transmission used by the Integrated Telescope Model. Corner spikes, caused by the need to bring the ideal top-hat field into compliance with the boundary conditions set forth by Maxwell's equations, form the strongest source of stray-light, accounting for up to a 1λ shift in the effective opening width per edge. Undercutting mask edges by 20^° reduces this source of stray-light by more than a factor of five. Interactions between light and the lower mask structure, a secondary effect, account for only a few percent of the stray-light in the TE polarization but account for up to 50% of the stray-light in the TM polarization due to surface plasmon polaritons. Surface plasmon polaritons, surface waves that run for tens of microns and radiate at corners, form the final stray-light source. On thin masks they may account for up to a 1λ shift in the effective opening width; however, their effects can be easily mitigated by choosing a poor surface plasmon material, such as Chrome. The results presented here are being used to facilitate end-to-end system modeling through the Integrated Telescope Model.

The Terestrial Planet Finder (TPF) mission to search for exo-solar planets is extremely challenging both technically and from a performance modeling perspective. For the visible light coronagraph (the C) approach, the requirements for 1e10 rejection of star light to planet signal has not yet been achieved in laboratory testing and full-scale ground testing provides additional challenges to overcome. Therefore, end-to-end performance modeling will be relied upon to fully predict system performance. One of the key technologies developed for achieving the rejection ratios uses shaped pupil masks to selectively cancel starlight in planet search regions by taking advantage of the diffraction. Modeling results published to date have been based upon scalar wavefront propagation theory to compute the residual star and planet images. This ignores the 3D structure of the mask and the coupled EM fields resulting when light interacts with matter. Secondly it ignores a most important engineering question which is how well the proposed wavefront control system can correct any effects introduced by mask/ light interactions. To address this problem we incorporate results from vector propagation through the masks. These fields, computed by the Finite Difference Time Domain (FDTD) method, are coupled into a TPF coronagraph integrated model and propagated end-to-end through the optical system. In this paper we build upon two recently published papers (refs 1,2) and evaluate this additional disturbance to the far field image, discuss the interface with surface-to-surface propagators and set up the formulism for polarization effects. A follow-on paper, part II, results will be presented with a surface-to-surface Fourier-based propagator coupled to the difference field models which include corrections from a wavefront control system.

Relative to ground-based telescopes, the James Webb Space Telescope (JWST) will have a substantial sensitivity advantage in the 2.2-5μm wavelength range where brown dwarfs and hot Jupiters are thought to have significant brightness enhancements. To facilitate high contrast imaging within this band, the Near-Infrared Camera (NIRCAM) will employ a Lyot coronagraph with an array of band-limited image-plane occulting spots. In this paper, we provide the science motivation for high contrast imaging with NIRCAM, comparing its expected performance to that of the Keck, Gemini and 30 m (TMT) telescopes equipped with Adaptive Optics systems of different capabilities. We then describe our design for the NIRCAM coronagraph that enables imaging over the entire sensitivity range of the instrument while providing significant operational flexibility. We describe the various design tradeoffs that were made in consideration of alignment and aberration sensitivities and present contrast performance in the presence of JWST's expected optical aberrations. Finally we show an example of a two-color image subtraction that can provide 10^-5 companion sensitivity at sub-arcsecond separations.

An optical vortex may be characterized as a dark core of destructive interference in a beam of spatially coherent light. This dark core may be used as a filter to attenuate a coherent beam of light so an incoherent background signal may be detected. Applications of such a filter include: eye and sensor protection, forward-scattered light measurement, and the detection of extra-solar planets. Optical vortices may be created by passing a beam of light through a vortex diffractive optical element, which is a plate of glass etched with a spiral pattern, such that the thickness of the glass increases in the azimuthal direction. An optical vortex coronagraph may be constructed by placing a vortex diffractive optical element near the image plane of a telescope. An optical vortex coronagraph opens a dark window in the glare of a distant star so nearby terrestrial sized planets and exo-zodiacal dust may be detected. An optical vortex coronagraph may hold several advantages over other techniques presently being developed for high contrast imaging, such as lower aberration sensitivity and multi-wavelength operation. In this manuscript, I will discuss the aberration sensitivity of an optical vortex coronagraph and the key advantages it may hold over other coronagraph architectures. I will also provide numerical simulations demonstrating high contrast imaging in the presence of low-order static aberrations.

This paper compares two well-known types of interferometer arrays for optical aperture synthesis. An analytical model for both types describes the expected output, in terms of photon counts. The goal is to characterize the performance of both types of array for blind imaging of a wide-field or extended object that would be partially resolved by a single elementary aperture. The spectrum of the source is assumed to be constant over the source and in time, but broad-banded. The light levels are such that only a few photons per pixel or bin are received. The simulated interferometer responses are discussed. The process of reconstructing the source from the 'recorded' responses is presented, but not discussed in this paper. It turns out that both types of interferometer are capable of imaging a partially resolved source with high spatial frequencies present all over the source.

In contrast to standard Michelson interferometry, the idea of entry pupil processing is to somehow convert light gathered at each telescope (of a multi-spacecraft array) into data, then process the data from several telescopes to compute the mutual coherence values needed for image reconstruction. Some advantages are that weak beams of collected light do not have to be propagated to combiners, extreme precision relative path length control among widely separated spacecraft is unnecessary, losses from beam splitting are eliminated, etc. This paper reports our study of several entry pupil processing approaches, including direct electric field reconstruction, optical heterodyne systems and intensity correlation interferometry using the Hanbury Brown-Twiss effect. For all these cases and for amplitude interferometry, we present image plane signal-to-noise (SNR) results for exo-planet imaging, both in the case of planet emissions and for imaging the limb of planets executing a transit across their stars. We particularly consider terrestrial-class planets at a range of 15 pc or less. Using the SNR and related models, we assess the relative advantages and drawbacks of all methods with respect to necessary aperture sizes, imager sensitivity, performance trends with increasing number of measurement baselines, relative performance in visible and in IR, relative positioning and path length control requirements and metrology requirements. The resulting comparisons present a picture of the performance and complexity tradeoffs among several imaging system architectures. The positive conclusion of this work is that, thanks to advances in optoelectronics and signal processing, there exist a number of promising system design alternatives for exo- planet imaging.

We present a study of the LBT nulling interferometer (LBTI) performance considered from the control systems point of view. Focusing first on the fast path length corrector controller within the LBTI, we show that a simple modification of the controller algorithm reduces the modal tracking error by 50% or more depending on the system dynamics. WE show that this translates into a null depth improvement by a factor larger than 5. Next, we consider coupling the LBTI real-time controller to that of the AO to take advantage of the high order information that is available on the LBTI phase sensor and we show that here again, the global performance can be improved, albeit more modestly by 20%-30%.

We describe a plan to study the radial velocity of low mass stars and brown dwarfs using a combination of interferometry and multichannel dispersive spectroscopy, Externally Dispersed Interferometry (EDI). The EDI technology allows implementation of precision velocimetry and spectroscopy on existing moderate-resolution echelle or linear grating spectrograph over their full and simultaneous bandwidth. We intend to add EDI to the new Cornell TripleSpec infrared simultaneous JHK-band spectrograph at the Palomar Observatory 200" telescope for a science-demonstration program that will allow a unique Doppler-search for planets orbiting low mass faint M, L and T type stars. The throughput advantage of EDI with a moderate resolution spectrograph is critical to achieving the requisite sensitivity for the low luminosity late L and T dwarfs.

Direct detection of planets around nearby stars requires the development of high-contrast imaging techniques, because of their very different respective fluxes. This led us to investigate the new coronagraphic approach based on the use of a four-quadrant phase mask (FQPM). Combined with high-level wavefront correction on an unobscured off-axis section of a large telescope, this method allows high-contrast imaging very close to stars. Calculations indicate that for a given ground-based on-axis telescope, use of such an off-axis coronagraph provides a near-neighbor detection capability superior to that of a traditional coronagraph utilizing the full telescope aperture. A near-infrared laboratory experiment was first used to test our FQPM devices, and a rejection of 2000:1 was achieved. We next built an FQPM instrument to test the feasibility of near-neighbor observations with our new off-axis approach on a ground-based telescope. In June 2005, we deployed our instrument to the Palomar 200-inch telescope, using existing facilities as much as possible for rapid implementation. In these initial observations, stars were rejected to about the 100:1 level. Here we discuss our laboratory and on-sky experiments, and the results obtained so far.

The Keck Interferometer Nuller (KIN) will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. A successful pre-ship review was held for the KIN in June 2004, after which the KIN was shipped to the Keck Observatory. The integration of the KIN's many sub-systems on the summit of Mauna Kea, and initial on-sky testing of the system, has occupied the better part of the past year. This paper describes the KIN system-level configuration, from both the hardware and control points of view, as well as the current state of integration of the system and the measurement approach to be used. During the most recent on-sky engineering runs in May and July 2005, all of the sub-systems necessary to measure a narrowband null were installed and operational, and the full nulling measurement cycle was carried out on a star for the first time.

Mid-infrared (8-13μm) nulling is a key observing mode planned for the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. By destructively interfering and thereby canceling the on-axis light from nearby stars, this observing mode will enable the characterization of the faint emission from exo-zodiacal dust surrounding these stellar systems. We report here the null leakage error budget and pre-ship results obtained in the laboratory after integration of the nulling beam combiner with its mid-infrared camera and key components of the Keck Interferometer. The mid-infrared nuller utilizes a dual-polarization, modified Mach-Zehnder (MMZ) beam combiner in conjunction with an atmospheric dispersion corrector to achieve broadband achromatic nulling.

One of the science drivers for the Extremely Large Telescope (ELT) is imaging and spectroscopy of exo-solar planets located as close as 20mas to their parent star [1]. The application requires a well thought-out design of the high contrast imaging instrumentation. Several working coronagraphic concepts have already been developed for the monolithic telescope with the diameter up to 8 meter. Nevertheless the conclusions made about the performance of these systems cannot be applied directly to the telescope of the diameter 30-100m. The existing schemes are needed to be reconsidered taking into account the specific characteristics of a segmented surface. We start this work with the classical system - Lyot coronagraph. We show that while the increase in telescope diameter is an advantage for the high contrast range science, the segmentation sets a limit on the performance of the coronagraph. Diffraction from intersegment gaps sets a floor to the achievable extinction of the starlight. Masking out the bright segment gaps in the Lyot plane although helps increasing the contrast, does not solve completely the problem: the high spatial frequency component of the diffractive light remains. We suggest using the Lyot stop which acts on the light within gaps in order to produce the uniform illumination in the Lyot plane. We show that for the diffraction limit regime and a perfect phasing this type of coronagraph achieves a sufficient star light extinction.

"Extreme" adaptive optics systems are optimized for ultra-high contrast applications, such as ground-based extrasolar planet detection. The Extreme Adaptive Optics Testbed at UC Santa Cruz is being used to investigate and develop technologies for high-contrast imaging, especially wavefront control. We use a simple optical design to minimize wavefront error and maximize the experimentally achievable contrast. A phase shifting diffraction interferometer (PSDI) measures wavefront errors with sub-nm precision and accuracy for metrology and wavefront control. Previously, we have demonstrated RMS wavefront errors of <1.5 nm and a contrast of >10⁷ over a substantial region using a shaped pupil without a deformable mirror. Current work includes the installation and characterization of a 1024-actuator Micro-Electro-Mechanical-Systems (MEMS) deformable mirror, manufactured by Boston Micro-Machines for active wavefront control. Using the PSDI as the wavefront sensor we have flattened the deformable mirror to <1 nm within the controllable spatial frequencies and measured a contrast in the far field of >10⁶. Consistent flattening required testing and characterization of the individual actuator response, including the effects of dead and low-response actuators. Stability and repeatability of the MEMS devices was also tested. Ultimately this testbed will be used to test all aspects of the system architecture for an extrasolar planet-finding AO system.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.

A possible system for imaging an exo-planet uses a telescope, coronagraphic mask, and Lyot stop. However, the optics must be nearly flawless for a planet to be imaged, since imperfections cause the image to have speckles which mask the planet. The speckle pattern can be suppressed by removing its phase with an adaptive optic and by reimaging it through a lens with a small, central obscuration. We show how phase diversity can be used to determine the phase.

This paper considers the Hanbury Brown-Twiss effect and its application to astrometry in the service of extra-solar planet detection, particularly terrestrial planets at a range of 15 pc or less. The system considered comprises several modest-sized telescopes (light collectors) each equipped with photodetection apparatus and the means to record the photodetector signal time-history. At some convenient location, the cross-correlations of the individual light collector photodetection histories is computed to yield, in turn, a collection of values for the magnitudes of the mutual coherence of the target scene at various measurement baselines. With this type of observation system, we show that if there are known guide stars within the picture frame, the computed coherence magnitudes may be used to infer the apparent motion of the target star. Provided sufficiently large measurement baselines, the resolution of the target star motion can be very fine. We first compute the signal-to-noise (SNR) ratio of a single coherence magnitude measurement and then, using simple models of the telescope array and the target star gravitational perturbation due to a terrestrial planet, we compute the SNR for determination of the planet orbit parameters, up to the determinacy afforded by astrometric measurements. We have provided expressions for the region in the (planetary mass-orbital semi-major axis) plane for which SNR is above a desired value. With these results, we can determine the sensitivity and range of the overall instrument for astrometry in planet detection. Moreover, one can assess the relative advantages of this technique in comparison with amplitude interferometry.

The light from exoplanets is expected to be partially polalized and the image intensity becomes different with the polarization direction. Based on this expectation we have reported the laboratory experiment of two-channel nulling stellar coronagraph for direct imaging of exoplanets, where a differential imaging with respect to mutually orthogonally polarized light is conducted. We show that this differential technique is also useful for obtaining objective spectra of exoplanets. Several experimental results on the differential objective spectrometer are reported.

The Terrestrial Planet Finder Coronagraph (TPF-C) is a future NASA mission to search for earth-like planets around nearby stars. Detecting a planet that is almost 10 billion times fainter than its parent star is extremely difficult, and it has been shown that polarization effects can cause stellar leakage which threatens that sensitivity goal. Building on our earlier work, we now show the combination of basic polarization effects with a representative coronagraph masking system, the eighth order linear field mask and Lyot stop, results in adequate performance.

Current techniques for actuating spacecraft in formation flying systems such as NASA's Terrestrial Planet Finder (TPF) use propellant-based systems. While maintaining relative orientation, propellant can become a critical consumable which can limit the mission lifetime. Additionally, propellant can cause optimal contamination, plume impingement, thermal emission, and vibration excitation. A novel technique called Electromagnetic Formation Flight (EMFF) can be used to eliminate propellant-based systems to control the relative degrees of freedom for TPF. The EMFF system consists of electromagnets in concert with reaction wheels and is used to replace the consumables. Solar energy, a renewable resource provides power for EMFF. This paper investigates the design for TPF using EMFF. The results show that EMFF is a viable option for TPF and compares favorably in terms of mass to propellant-based systems.

Conventional adaptive optics systems correct the wavefront by adjusting a deformable mirror based on measurements of the phase aberration taken in a pupil plane. The ability of this technique, known as phase conjugation, to correct aberrations is normally limited by the maximum spatial frequency of the DM. In this paper we show that conventional phase conjugation is not able to achieve the dark nulls needed for high-contrast imaging. Linear combinations of high frequencies in the aberration at the pupil plane "fold" and appear as low frequency aberrations at the image plane. After describing the frequency folding phenomenon, we present an alternative optimized solution for the shape of the deformable mirror based on the Fourier decomposition of the effective phase and amplitude aberrations.

Pupil-mapping is a technique whereby a uniformly-illuminated input pupil, such as from starlight, can be mapped into a non-uniformly illuminated exit pupil, such that the image formed from this pupil will have suppressed sidelobes, many orders of magnitude weaker than classical Airy ring intensities. Pupil mapping is therefore a candidate technique for coronagraphic imaging of extrasolar planets around nearby stars. Pupil mapping is lossless and preserves the full angular resolution of the collecting telescope. Prior analyses based on pupil-to-pupil ray-tracing indicate that a planet fainter than 10^-10 times its parent star, and as close as about 2λ/D, could be detectable. In this paper, we describe the results of careful diffraction analysis of pupil mapping systems. These results reveal a serious unresolved issue. Namely, high-contrast pupil mappings distribute light from very near the edge of the first pupil to a broad area of the second pupil thereby dramatically amplifying diffraction-based edge effects resulting in a limiting attainable contrast of about 10⁻⁵. We provide two hybrid designs that provide partial solutions to this problem but a complete resolution remains open.

Correction of the in.uence of phase corrugation in the pupil plane is a fundamental issue in achieving high dynamic range imaging. It can be done in real-time with a deformable mirror, but also by post-detection data processing. We present here an imaging system which, thanks to non-redundant pupil remapping and spatial filtering by single mode fibers, allows perfect calibration of the Modulation Transfer Function (MTF). Using phase closure, we have then precise knowledge of the complex values filling the spatial frequency plane of the telescope. We do show that such a system would be free from phase perturbations, photon noise limited, and would allow processed imaging with very high dynamic range. This at the price of loosing the convolution operation between the focal plane and the object.

Three-dimensional common-path interferometer is proposed to obtain achromatic nulling for the on-axial source; the off-axial source remains detectable. The 3D interferometer involves ±90^o polarization rotations in each interferometer arm. That results in the achromatic 180^o phase shift, so that the on-axial source interferes destructively. Depending on the source axial position, the light energy is split by different ratios between the Bright and the Nulled interferometers outputs. For the linearly polarized on-axial source, all the energy at nearly 100% is directed to the Bright port. For the off-axial source, the light is split by the ratio at nearly 50% / 50 % between the Bright and the Nulled ports. The common-path scheme compensates effectively the optical path difference (OPD) and it remains stable to mechanical vibrations. Theory, simulations and preliminary breadboard experiments are shown to be in reasonable agreement.

The search for planetary transits in star light-curves can be improved in an non standard way applying appropriate filtering of the systematic effects just after the detection step. The procedure has been tested using a set of light curves simulated in the context of the CoRoT space mission. The level of the continuum in the detection curves is significantly lowered when compared to other standard approaches, a property we use to reduce false alarm. Ambiguities may originate in unexpected effects that combine instrumental and environmental factors. In a large set of synchronous light curves collective behaviours permit to identify systematic effects against which the detected events are compared. We estimate a significance of our detections and show that with our procedure the number of true detections is increased by more than 80% (22 events detected over the 36 injected ones). In spite of its simplicity, our method scores quite well (average results) when compared to the other methods used for the CoRoT "blind test" exercice by Moutou et al.¹

The goal of the Terrestrial Planet Finder (TPF) mission is to detect and characterize terrestrial exoplanets at visible wavelengths. One approach combines an 8m by 3.5m aperture telescope with a coronagraph (TPF-C) to obtain the required planet to parent star contrast. The proposed design places severe constraints on alignment tolerances and requires optics of the highest possible quality. The integration, test and verification of the observatory will require extraordinary procedures. This paper is an initial attempt to outline a plausible program to verify before launch the in-orbit performance requirements.

We present here results from an experimental and theoretical study in the use of graded focal-plane occulting masks to improve high-contrast astronomical imaging at near-infrared wavelengths. The study includes investigations of both high-energy beam sensitive (HEBS) glass (a product of Canyon Materials, San Diego, CA) and binary notch-filter technologies to create precision graded occulting masks. In conjunction with this investigation, we conduct computer simulations showing expected high-contrast levels for various graded masks being considered for installation in the PHARO camera of the Palomar 200-inch (5m) Hale Telescope Adaptive Optics (AO) system. Our results demonstrate that the implementation of a graded exponential mask in the Palomar system should improve high-contrast sensitivities by about 2.4-mag in K-band (2.0-2.4 μm), for 0.75-1.5 arcsec separations. We also demonstrate that both HEBS and binary notch-filter technologies present adequate platforms for necessary occulting requirements. We conclude with a discussion of theinsights our study yields for planned space-based high-contrast observatories such as NASA's planned Terrestrial Planet Finder Coronagraph (TPF-C) and the proposed Eclipse mission.

Large stellar telescope is indispensable for astronomy. Aperture synthesis is a well-known technique to simulate a large space telescope by an array of small telescopes. Condition for aperture synthesis is that the light of the telescopes have to be combined coherently. Therefore, an interferometric Fringe Sensor (FS) to detect and stabilize the Optical Path Difference (OPD) between light from the different telescopes is required. Conventional Fringe Sensor for Space Interferometer utilizes either Quadrature Stabilization or Double Synchronous Detection to find and control OPD=0. OPD demodulation based on Quadrature Stabilization is sensitive to change in the visibility V of the interferometric signal, while Double Synchronous Detection requires an active modulation of the OPD to generate the required carrier signal. To overcome these problems, TNO develops a Fringe Sensor based on a 3x3 Fiber Optic (FO) coupler. A breadboard demonstrator operating around 830 nm is built. A piezo stretcher and a translation stage are used to generate the OPD. High-speed OPD measurement down to 0.15 nm is demonstrated. The influence of the visibility V of the interferometric signal is also investigated. Even for V=0.2, an OPD modulation of 0.4 nm can still be detected.

Nulling interferometry is the baseline technique for the DARWIN planet finding mission of the European Space Agency. Using this technique it will be possible to cancel, by destructive interference, the light from the bright star and look directly at its surrounding planets and eventually discover life on them. To achieve this goal wavefront errors need to be reduced to a very high degree in order to achieve the required nulling quality. Such a high wavefront quality can only be achieved with adequate wavefront filtering measures. Single mode fibers in general have excellent mode filtering capabilities, but they were not recently available for the broad infrared wavelength region of Darwin (4-20 um). Within an ESA technology development project, TNO has designed and tested an infrared single mode fiber based on chalcogenide glasses that has been manufactured by the University of Rennes. Several tests are carried out to characterize the materials used and the IR single mode fiber. Far field intensity distribution measurement at 10.6 um reveals the single mode operation of the manufactured fiber. Influence of coating, length, light coupling and bending of the fiber are also investigated.

The New Worlds Observer, currently studied under a NASA Institute for Advanced Concepts grant, will be a pinhole camera in space designed to directly detect and study extrasolar terrestrial planets. An apodized occultor or pinhole creates an image of the planetary system in the focal plane far away, where a second telescope craft orbits to detect the light. In this study we simulate the expected signal of NWO to find the optimal configuration and specifications of the two craft. The efficiency of direct detection through photometric imaging depends strongly on occulter and telescope size, while preliminary studies on absorption biomarker detection and photometric variability measurements are summarized.

Space-based nulling interferometers will play a major role in the search for exoplanets, as both NASA and ESA plan missions for the near future. Current architectures produce the requisite broadband π phase shift in one arm of each nulling telescope pair by means of a system of "field flip" optics that may involve one of a number of sophisticated technologies (periscope, phase plates, through-focus, or other). The two beams, of equal intensity but conjugate phase, are then combined, perhaps in a modified Mach-Zehnder (MMZ) or similar beam combiner of high configurational symmetry. A novel approach has recently been proposed, however, in which the achromatic π phase shift is supplied by two applications of the innate π/2 phase shift between transmitted and reflected beams in a beam splitter. This simply requires using the traditionally bright output port of the MMZ as a nulled port; adaptive nulling can be used to ease the tolerances on matching the moduli of reflection and transmission coefficients. The rather substantial systems benefit that accrues is that the external phase shifting ("field flipping") optics may be entirely eliminated. Here, I discuss the feasibility of this "self-nulling" beam combiner scheme.

The development of stellar coronagraphs for exoplanet detection requires apodized occulting masks to effectively remove the light from the central star while allowing planet light to propagate past. One possible implementation, a gray-scale mask, includes the placement of micron-scale neutral density light absorbing patterns using High Energy Beam Sensitive (HEBS) glass. A second implementation, binary masks, uses micron-scale diffractive/reflective patterns. Coronagraph performance will be influenced by wavefront phase shifts introduced by the masks, hence accurate characterization of the fundamental optical properties, namely optical density (OD), phase advance/delay and optical constants of the material is needed for occulter design, development and modeling. In this paper we describe an interferometric apparatus that measures wavefront phase advance/delay through grey-scale and binary masks as functions of wavelength and optical density, which is also measured. Results for HEBS gray-scale masks will be presented along with ellipsometric measurements of optical constants.

Knowledge of wavefront amplitude is as important as the knowledge of phase for a coronagraphic high contrast imaging system. Efforts have been made to understand various contributions to the amplitude variation in Terrestrial Planet Finder's (TPF) High Contrast Imaging Testbed (HCIT). Modeling of HCIT with as-built mirror surfaces has shown an amplitude variation of 1.3% due to the phase-amplitude mixing for the testbed's front-end optics. Experimental measurements on the testbed have shown the amplitude variation is about 2.5% with the testbed's illumination pattern having a major contribution to the low order amplitude variation.

SAO has set up a testbed to study coronagraphic techniques, starting with Labeyrie's multi-step speckle reduction technique. This technique expands the general concept of a coronagraph by incorporating a speckle corrector (phase and/or amplitude) in combination with a second occulter for speckle light suppression. Here we are describing the initial testbed configuration. In addition, the testbed will be used to test a new approach of the phase diversity method to retrieve the speckle phase and amplitude. This method requires measurements of the speckle pattern in the focal plane and slightly out-of-focus. Then we will calculate a phase of the wave from which we can derive a correction function for the speckle corrector. Furthermore we report results from a parallel program which studies new manufacturing methods of soft-edge occulter masks. Masks were manufactured using the spherical caps method. Since the results were not satisfying we also investigated the method of ion beam milling of masks. Here we will present the outline of this method. Masks manufactured with both methods will be fully characterized in our mask tester before their use in the testbed.

Phase mask coronagraphy is a technique aiming at accommodating both high dynamic and high angular resolution imaging of faint sources around bright objects such as exo-planets orbiting their parent stars or host galaxies around Active Galactic Nuclei. We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements consisting of optimized surface-relief gratings. The first one is an evolution of the Four Quadrant Phase Mask coronagraph, which resolves the π phase shift chromaticity problem: the Four Quadrant Zeroth Order Grating (4QZOG). The second one is a totally new design consisting of an optical vortex induced by a space-variant grating: the Annular Groove Phase Mask (AGPM) coronagraph is fully symmetric and free from any "shaded zones". Some manufacturing hints are given.