Observations of the nucleus of NGC 4151 at 2.2 μm using the two 10-meter Keck telescopes as an interferometer show a marginally resolved source less than or equal to 0.1 pc in diameter. These observations are the first measurement of an extragalactic source with an optical/IR interferometer. These observations represent a ten-fold improvement in angular resolution when compared to previous near-infrared measurements of AGN and make it possible to test the subparsec-scale, near-infrared emission models of NGC 4151.

Dusty tori have been suggested to play a crucial role in determining the physical characteristics of active galactic nuclei (AGN), but investigation of their properties has stalled for lack of high resolution mid-IR imaging. Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a nearby AGN, was the first extragalactic object to be observed with a mid-IR interferometer, thereby obtaining the needed angular resolution to study the alleged torus. The instrument used was MIDI mounted on the ESO's VLT interferometer. The resulting 8-13 micron interferometric spectra indicated the presence of a thick (3 x 4 parsec) configuration of warm dust surrounding a hot ~1 pc component, marginally elongated in the direction perpendicular to the main orientation of the warm component. The structure of the 10 micron "silicate" absorption feature hinted at the presence of non-typical dust. In this proceeding, first the field of AGN research is briefly reviewed, with an emphasis on models of dusty tori. Second, the general properties of the key object NGC 1068 are discussed. Third, the MIDI data set is presented together with a first attempt to interpret this data in the context of tori models. Fourth, preliminary MIDI interferometric spectra of the nucleus of the nearby starbursting galaxy Circinus are presented. The apparent observed absence of both a hot component as well as a sharp absorption feature suggest that we view the torus more edge-on than is the case for NGC 1068. Finally, we briefly discuss the prospects of ESA's Darwin mission for observing nearby and distant AGN. The required capabilities for Darwin's first goal -- the search for and subsequent characterization of earth-like planets orbiting nearby stars -- are such that for its second goal -- high resolution astrophysical imaging -- the sensitivity will be similar to JWST and the angular resolution 1-2 orders better. This will allow detailed mapping of tori of low luminosity AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to redshift of 10 and beyond.

In April 2004, MIDI, the first of the two first generation instruments of the VLT interferometer, started official operation as a facility instrument on Paranal. It allows interferometric observations covering the full astronomical N band (7.8 μm - 13.5 μm). Initially, only observations with low spectral resolution λ/Δλ ~ 30 were offered. Examples for observations in this observing mode are presented in order to document the performance of the instrument. A number of new instrumental options are in preparation, which should render future observing with this instrument more sensitive, more accurate and more versatile.

The Keck Interferometer is a NASA funded project developed by the Jet Propulsion Laboratory, the William M. Keck Observatory and the Michelson Science Center at the California Institute of Technology. A technical description of the interferometer is given elsewhere in this volume. This paper will discuss the science topics and goals of the Keck Interferometer project, including a brief description of the Key Science projects, the science projects executed to date and the current availability of the interferometer for new projects. The Keck Interferometer Project consists of the Keck-Keck Interferometer, which combines the two Keck 10-meter telescopes on an 85-meter baseline, and the Outrigger Telescopes Project, a proposal to add four to six 1.8-meter telescopes that would work in conjunction with the two Kecks.

We present direct detections of the spatial extent of the circumbinary disk around HR 4049 and its companion. Observations were obtained with the ESO Very Large Telescope Interferometer using the VLT Interferometric Commissioning Instrument (VINCI) at 2 micron and the Mid Infrared Instrument (MIDI) between 8 and 12 micron. A single uniform disk model fit to the VINCI data gives an angular diameter of 27 milli-arcseconds. After taking into account the contribution from an unresolved central star we find that the observed visibilities indicate a second component with a spatial extent of 37 milli-arcseconds (which is identified as the circumbinary disk). The MIDI interferometric spectra show features which are due to PAH emission lines (8.6 and 11.3 micron). The visibilities of the emission lines indicate that the spatial extent in the lines (50 to 60 milli-arcseconds) is larger than in the continuum (35 to 45 milli-arcseconds). This leads us to propose a three emission components model to explain the interferometric observations: a central unresolved star, a 37 milli-arcseconds circumbinary disk and polar lobes emitting in the PAH bands with a size of 50 to 60 milli-arcseconds.

Atmospheric turbulence is a serious problem for ground-based interferometers. It places tight limits on both sensitivity and measurement precision. Phase referencing is a method to overcome these limitations via the use of a bright reference star. The Palomar Testbed Interferometer was designed to use phase referencing and so can provide a pair of phase-stabilized starlight beams to a second (science) beam combiner. We have used this capability for several interesting studies, including very narrow angle astrometry. For close (1-arcsecond) pairs of stars we are able to achieve a differential astrometric precision in the range 20--30 micro-arcseconds.

We present near infrared aperture synthesis maps of the well known binary star Capella (alpha Aur) produced with the upgraded IOTA interferometer on top of Mount Hopkins, Arizona. Michelson interferograms were obtained simultaneously on three interferometer baselines in the H-band between 2002 November 12 and 16. The simultaneous observation of fringes on three baselines permitted a single phase closure to be estimated along with three visibility amplitudes. Hybrid Mapping techniques were then used to reconstruct the source brightness distribution with a beam size of 5.4 x 2.6 mas, which allows for the resolution of the stellar surfaces of the Capella giants. The maps provide the first demonstration of imaging with phase closure on the IOTA instrument.

We review the theory of rotating stars, first developed 80 years ago. Predictions include a specific relation between shape and angular velocity and between surface location and effective temperature and effective gravity. Seen at arbitrary orientation rapidly rotating stars will display ellipsoidal shapes and possibly quite asymmetric intensity distributions. The flattening due to rotation has recently been detected at PTI and VLTI. With the increasing baselines available in the visible and the implementation of closure phase measurements at the NPOI it is now possible to search for the surface brightness effects of rotation. Roche theory predicts only large scale deviations from the usual centro-symmetric limb-darkened models, ideal when the stellar disks are only coarsely imaged as now. We report here observations of Altair and Vega with the NPOI using baselines that detect fringes beyond the first Airy zero in both objects. Asymmetric, non-classical intensity distributions are detected. Both objects appear to be rotating at a large fraction of their breakup velocity. Vega is nearly pole on, accounting for its low apparent rotational velocity. Altair's inclination is intermediate, allowing high S/N detection of all the predicted features of a Roche spheroid. We describe how these objects will test this fundamental theory and how Vega's role as a standard will need reinterpretation.

We present preliminary results from an ongoing survey for multiplicity among the bright stars using the Navy Prototype Optical Interferometer (NPOI). While the NPOI has previously concentrated on producing "visual" orbits of known close speckle and spectroscopic binaries, we have now embarked on a broader survey to detect new binary/multiple systems. We first present a summary of previous NPOI observations of known binary and multiple systems to illustrate the instrument's detection sensitivity for binaries at large magnitude differences over the range of angular separation detectable by the NPOI (currently 3 - 300 mas). We then discuss early results of the survey of bright stars north of declination -20°. This survey, which compliments previous surveys of the bright stars by speckle interferometry, initially emphasizes stars in a proposed Terrestrial Planet Finder (TPF) target list. To date, 29 of the 60 brightest TPF candidate stars (V ≤ 4.3) have each been observed on multiple nights. Preliminary analysis of these data indicates the possible detection of stellar companions to several of these stars.

The Palomar Testbed Interferometer has observed several binary star systems whose separations fall between the interferometric coherence length (a few hundredths of an arcsecond) and the typical atmospheric seeing limit of one arcsecond. Using phase-referencing techniques we measure the relative separations of the systems to precisions of a few tens of micro-arcseconds. We present the first scientific results of these observations, including the astrometric detection of the faint third stellar component of the kappa Pegasi system.

The coronagraphic techniques serving to reject most light from a star, when trying to image a nearby planet, can be pushed with an adaptive holographic element. Located after the coronagraph, it can in principle remove most of the residual star light by adding a phase-shifted holographic reconstruction of it . The scheme is also usable within each sub-aperture of a diluted hypertelescope array, sufficiently large to resolve details of an exo-Earth. A possible panoramic version of the previously mentioned Exo-Earth Imager is shaped as a virtual bubble of 400 km diameter , consisting of thousands of 3-meter mirrors, free-flying and arranged co-spherically. The half-size focal sphere is explored by beam combiners, one for each exo-Earth observed within tens of parsecs. Each beam-combiner includes a kilometer-sized corrector of spherical aberration at F/2, which is also diluted and consisting of small free-flyers. The instrument is expected to provide direct coronagraphic images of exo-Earths, resolved in 50x50 resels, with enough dynamic range obtained in 30mn exposures to search colored features and their seasonal variations, indicative of photosynthetic life .

Extragalactic Astronomy is one of the new domains being successfully addressed by the new generation of optical long baseline interferometers. The recent results on the unresolved Seyfert 1 nucleus NGC4151 by Keck Interferometer team and on the partially resolved Seyfert 2 nucleus NGC1068 by VLTI team confirm our current understanding of the Active Galactic Nuclei phenomenology. Once an extra level in increased angular resolution is reached by new observatories (e.g., Optical Hawaiian Array for Nanoradian Astronomy or Overwhelmingly Large Array), it will become possible to study the Broad Line Region of Seyfert 1 nuclei. In this paper, we present key results that interferometry could obtain on the topic, how they compare to an existing technique - reverberation mapping, and we describe a new observational technique that will lead to a tomographic picture of this region.

As a near-infrared (NIR) wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC/NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging of galactic nuclei including the center of the Milky Way. Recent optical/IR imaging surveys can now quite successfully be used to search for star-galaxy pairs that are suitable for interferometric observation with LINC NIRVANA. These objects can then be used to efficiently investigate galaxy interaction, nuclear activity, and star formation in distant galaxies. In the NIR these investigations will be carried out at scales below 100~pc for z<0.05 and at scales below 500~pc at z<2. The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are most likely super massive black holes. Observations with LINC NIRVANA will allow to simultaneously investigate the stellar dynamics of the entire central cluster, the determination of the amount of extended mass within the cusp region, and to monitor the activity of the 3 million solar mass black hole at the position of Sagittarius A* at separations of only about 10 light hours or 15 Schwarzschild radii.

Young Stellar Objects (YSOs) play a central role in the understanding of stellar and planet formation, and progress in spatial resolution and sensitivity of long infrared interferometers made such instruments particularly well suited to probe the inner part of the disk where essential physical processes and interactions are believed to take place. The first astrophysical results obtained on young stars arising from this technique are already giving a handful of informations about the structure of the extended component. However, model-fitting methods used to reduce the data prevent from definitive and unambiguous interpretations. Interferometric observations of Herbig Ae/Be stars is one of the most striking example. Whereas first results seemed to be in good agreement with accretion disk model, latest observations tend to favor the presence of a uniform ring with a inner radius set by dust sublimation temperature. Direct imaging of close environments around YSOs with infrared (IR) interferometers will allow to disentangle between current suggested models and to improve one step further the scenarios of stellar formation. Within this framework, we anticipate observations of YSOs with the VLTI and we investigate the potential of AMBER to recover images. Modelling their circumstellar environment, we simulate realistic observations of Herbig Ae/Be and TTauri stars. By using reconstruction technique specially dedicated to infrared interferometry and to sparse ($u,v$) data coverage, we analyze the quality of the recovered images, and we emphasize the critical points to take into account in the image reconstruction process. We conclude that it requires at least three nights of observations to perform imaging of YSOs with AMBER on the VLTI.

Interferometry with the Very Large Telescope Interferometer (VLTI) will allow imaging of the Galactic Center and the nuclei of extragalactic sources at an angular resolution of a few milliarcseconds. VLTI will be a prime instrument to study the immediate environment of the massive black hole at the center of the Milky Way. With the MID infrared Interferometric instrument (MIDI) for example the enigmatic compact dust embedded MIR-excess sources within the central parsec should be resolvable. Further the observations of external galactic nuclei will allow unprecedented measurements of physical parameters (i.e. structure and luminosity) in these systems. With the exception of a few 'self-referencing' sources these faint-target observations will benefit from the available off-axis wavefront-correction system STRAP, working on suitable guide stars (GS). To fully exploit the use of VLTI within this context, the following questions have to be addressed among others: How feasible is blind-pointing on (faint) science targets? Are VLTI observations still efficiently feasible if these faint science targets exceed the usual angular distance (≤1 arcmin) to a GS candidate, enabling a standard closed-loop tip-tilt correction? How is the fringe-tracking procedure affected in densely populated regions such as the Galactic Center? What preparatory steps have to be performed to successfully observe these non-standard targets with the VLTI? In this contribution, we present aspects for the preparation of VLTI observations, which will be conducted in the near future. Considering these example observations of the Galactic Center region, several details of observing modes are discussed, which are necessary to observe such science targets. The final goal is the definition of observational strategies that are optimized for the discussed classes of targets, which provide properties touching the limits of VLTI observability.

Over the last year, a set of well defined science requirements has been established for the Terrestrial Planet Finder (TPF) mission. They consist of top level specifications, such as the number and characteristics of stars to be observed, the planetary sizes and orbital phase spaces to be searched for, the desired completeness of the search, etc. For each of the concurrent observing techniques considered - thermal infrared nulling interferometry and optical coronagraphy-, dedicated spectroscopy requirements have also been formulated. On the interferometry side, the most promising design studied so far consists of a free flyer assembly of four 4m class telescopes. It basically allows to thoroughly search for planets in the habitable zone of ~ 160 nearby main sequence F,G and K dwarfs in 1 year of continuous integration (~ 2 years of operation). Over 1.5 year of subsequent observation, this design would also enable low resolution (20) spectroscopic characterization of up to 10 exo-planetary atmospheres in the [6.5 - 17] micron range, assuming 260K exo-planets with Earth albedo, and at least half the Earth area are present around the target stars. With only minor additions to the nulling design, and taking advantage of a spatial resolution 10 to 50 times higher than JWST, the free flyer design would also provide fantastic contributions in the fields of comparative planetology, the study of very young stellar objects and high-z galaxies.

Optical and infrared interferometers operating with high spectral resolution can provide velocity-resolved information on the milliarcsecond and sub-milliarcsecond scale. This enables completely new observational approaches to many open problems in stellar and circumstellar astrophysics. Fiber coupling of the output from a relatively simple beam combiner to existing spectrographs offers a cost-effective way to implement interferometric high-resolution spectroscopy at the European Southern Observatory's Very Large Telescope Interferometer.

We briefly summarize the status of dynamic model atmospheres for carbon rich red giants and present some recent results of comparisons between synthetic spectra and observations. We then discuss synthetic intensity profiles and visibilities for such stars. In particular, the effects of atmospheric dynamics on different spectral bands and for different model parameters are studied.

The prime objective of GENIE (Ground-based European Nulling Interferometry Experiment) is to obtain experience with the design, construction and operation of an IR nulling interferometer, as a preparation for the DARWIN / TPF mission. In this context, the detection of a planet orbiting another star would provide an excellent demonstration of nulling interferometry. Doing this through the atmosphere, however, is a formidable task. In this paper we assess the prospects of detecting with nulling interferometry on ESO's VLTI, low-mass companions in orbit around their parent stars. With the GENIE science simulator (GENIEsim) we can model realistic detection scenarios for the GENIE instrument operating in the VLTI environment, and derive detailed requirements on control-loop performance, IR background subtraction and the accuracy of the photometry calibration. We analyse the technical feasibility of several scenarios for the detection of low-mass companions in the L'-band.

The perfect site for astronomical interferometry would be one where the wind speeds throughout the atmosphere were very low, there was little atmospheric turbulence (especially at high altitudes), the seismic activity was negligible and the climate very stable. Perhaps surprisingly, these conditions describe the Antarctic plateau perfectly. At Dome C, for example, the average wind speed at ground level is just 2.7 metres/sec. This, combined with low wind speeds at all altitudes up to the stratosphere, leads to a dramatic increase in coherence times. Despite the extreme cold, the Antarctic plateau is a relatively benign environment. At thermal infrared wavelengths the high elevation (typically over 3000 m), intense cold and exceptional dryness also combine to give greatly increased background-limited sensitivities relative to other sites. This increased sensitivity can be used to further enhance interferometer performance, or can be traded against mirror size to allow for a smaller instrument of the same sensitivity.

The Antarctic Planet Interferometer is a concept for an instrument designed to detect and characterize extrasolar planets by exploiting the unique potential of the best accessible site on earth for thermal infrared interferometry. High-precision interferometric techniques under development for extrasolar planet detection and characterization (differential phase, nulling and astrometry) all benefit substantially from the slow, low-altitude turbulence, low water vapor content, and low temperature found on the Antarctic plateau. At the best of these locations, such as the Concordia base being developed at Dome C, an interferometer with two-meter diameter class apertures has the potential to deliver unique science for a variety of topics, including extrasolar planets, active galactic nuclei, young stellar objects, and protoplanetary disks.

We propose a science demonstrator for the Antarctic Plateau Interferometer. It is a comparatively much simpler system than API but dedicated to the goal of obtaining the first low-resolution spectra in the thermal infrared of a few "hot Jupiter" type exoplanets. It would provide a unique platform to acquire operational experience on antarctic stellar interferometry, and build up an extensive database on the relevant site properties, as a preparation for API.

Turbulence in the earth's atmosphere severely limits the resolution and sensitivity of astronomical observations. The vertical distribution of turbulence in the atmosphere has a profound effect on the residuals after correction by an active instrument such as adaptive optics or a fringe tracking interferometer. It has already been shown that the South Pole has turbulence profiles unlike those at any other site, dominated by ground layer turbulence, with low free air seeing. This paper examines the meteorology, climatology and atmospheric physics that produces these conditions. Combining meterological observations at remote sites with models of atmospheric turbulence allows quantitative extrapolation to the likely conditions at sites now under development and consideration that may provide the ultimate ground based site for near and mid-infrared interferometry. The high plateau sites in Antarctica will likely enable complete sky coverage for adaptive optics and interferometry in the near infrared with natural guide stars.

The Micro Arcsecond X-ray Imaging Mission (MAXIM) has been proposed to NASA in response to the Black Hole Imager line in the newly created "Beyond Einstein" program. Meeting the scientific goals of event horizon imaging has created a new set of technical challenges for the Maxim team. Certainly the most difficult of these is the need for 0.1 micro-arcsecond resolution imaging in the x-ray. We will review the key scientific challenges and show how they flow down into instrument requirements. We will then review our baseline design, discuss the array of technical challenges facing Maxim, and present the current status of our technology.

The Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) is a space-based imaging and spectral ("double Fourier") interferometer with kilometer maximum baseline lengths for imaging. This NASA "vision mission" will provide spatial resolution in the far-IR and submillimeter spectral range comparable to that of the Hubble Space Telescope, enabling astrophysicists to extend the legacy of current and planned far-IR observatories. The astrophysical information uniquely available with SPECS and its pathfinder mission SPIRIT will be briefly described, but that is more the focus of a companion paper in the Proceedings of the Optical, Infrared, and Millimeter Space Telescopes conference. Here we present an updated design concept for SPECS and for the pathfinder interferometer SPIRIT (Space Infrared Interferometric Telescope) and focus on the engineering and technology requirements for far-IR double Fourier interferometry. We compare the SPECS optical system requirements with those of existing ground-based and other planned space-based interferometers, such as SIM and TPF-I/Darwin.

The Darwin and Terrestrial Planet Finder missions, represent the European Space Agency (ESA) and NASA's interest in ultimately searching for and when found studying planets similar to the Earth-like planets in our own Solar System. As such they may be technologically very challenging space missions but recent developments points towards robust solutions. In this talk, we compare the technologies, the available solutions, and the current status in both projects. We put the emphasis on the optical technologies required, and address both main possibilities considered for planet finding, i.e. a nulling interferometer and a coronograph. We outline the strategies for selecting the appropriate technology for each element of the missions. Finally we also address the synergy in the technologies required for other missions, as well as other applications except purely scientific.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near-to-mid-infrared spectral region (3-8µm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets: Σ What are the characteristics of the known extrasolar giant planets? Σ What are the characteristics of the extrasolar zodiacal clouds around nearby stars? Σ Are there giant planets around classes of stars other than those already studied? We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5m telescopes on a 12.5m boom feeding a Mach-Zender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution.

The Stellar Imager (SI) is a far-horizon or "Vision" mission in the NASA Sun-Earth Connection (SEC) Roadmap, conceived for the purpose of understanding the effects of stellar magnetic fields, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best possible forecasting of solar/stellar activity and its impact on life in the Universe. The science goals of SI require an ultra-high angular resolution, at ultraviolet wavelengths, on the order of 0.1 milliarcsec and thus baselines on the order of 500 meters. These requirements call for a large, multi-spacecraft (>20) imaging interferometer, utilizing precision formation flying in a stable environment, such as in a Lissajous orbit around the Sun-Earth L2 point. SI's resolution (several 100 times that of HST) will make it an invaluable resource for many other areas of astrophysics, including studies of AGN's, supernovae, cataclysmic variables, young stellar objects, QSO's, and stellar black holes. In this paper, we present an update on the ongoing mission concept and technology development studies for SI. These studies are designed to refine the mission requirements for the science goals, define a Design Reference Mission, perform trade studies of selected major technical and architectural issues, improve the existing technology roadmap, and explore the details of deployment and operations, as well as the possible roles of astronauts and/or robots in construction and servicing of the facility.

The design and performance of a Fizeau interferometer with long focal length and large field of view are discussed. The optical scheme presented is well suited for very accurate astrometric measurements from space, being optimised, in terms of geometry and aberrations, to observe astronomical targets down to the visual magnitude mV=20, with a measurement accuracy of 10 microarcseconds at mV=15. This study is in the context of the next generation astrometric space missions, in particular for a mission profile similar to that of the Gaia mission of the European Space Agency. Beyond the accuracy goal, the great effort in optical aberrations reduction, particularly distortion, aims at the optimal exploitation of data acquisition done with CCD arrays working in Time Delay Integration mode. The design solution we present reaches the astrometric goals with a field of view of 0.5 square degrees.

This overview paper is a progress report about the system design and technology development of two interferometer concepts studied for the Terrestrial Planet Finder (TPF) project. The two concepts are a structurally-connected interferometer (SCI) intended to fulfill minimum TPF science goals and a formation-flying interferometer (FFI) intended to fulfill full science goals. Described are major trades, analyses, and technology experiments completed. Near term plans are also described. This paper covers progress since August 2003 and serves as an update to a paper presented at that month's SPIE conference, "Techniques and Instrumentation for Detection of Exoplanets."

We investigate whether the design of the DARWIN nulling interferometer can be simplified, while maintaining or even improving its performance, by accepting somewhat higher levels of stellar leakage. We establish detailed requirements on stellar suppression for a nulling interferometer, considering a realistic DARWIN stellar target sample. The dominating noise source, represented by the local zodiacal cloud, is essentially constant for all target stars while the stellar leakage decreases as the inverse of the distance squared. This means that such stellar leakage has an effect on the integration times of near-by target stars, while for more distant targets its influence decreases significantly. We assess the impact of different types of nulling profiles and identify those stars for which the detection sensitivity can be maximised.

A number of stellar systems that can be searched for presence of Earth-like planets in a given mission lifetime is a key figure of merit for planet hunting stellar interferometers. We have developed a method to calculate the number of stellar systems that can be searched and characterized. Using this method we have evaluated the performance of a number of architectures. We conclude that simpler second-order null architectures outperform more complicated fourth-order null architectures. We also quantify the advantages of the variable length formation-flying configurations vs. fixed length structurally connected configurations.

This paper describes the current status of the technical program aiming to demonstrate the viability of a formation-flying mid-infrared nulling interferometer architecture for the Terrestrial Planet Finder (TPF) program. Until recently the TPF project was considering four architectures, with the goal of selecting one in the 2006 timeframe. In April 2004, the project office opted instead to follow a path leading to a small (4x6m) visible-light coronagraph, to launch around 2014, and a formation-flying interferometer (FFI), to launch before 2020. The FFI is proposed to satisfy the full TPF science goal to completely survey 150 stars for evidence of terrestrial planets similar to Earth, while the coronagraph will perform a survey of 30-50 stars at visible wavelength. FFI trade studies conducted since mid-2003 have focused on key factors driving overall flight segment mass and performance, including launch vehicle packaging, deployment approach, thermal design (particularly the thermal shield configuration), structural design, and formation flying approach. This paper summarizes the results of the recent design trades, with discussion of the primary requirements that drive the baseline design concept. Analyses supporting the baseline design are summarized, and areas for future study are discussed.

The MIT Space Systems Laboratory and Payload Systems Inc. has developed the SPHERES testbed for NASA and DARPA as a risk-tolerant medium for the development and maturation of spacecraft formation flight and docking algorithms. The testbed, which is designed to operate both onboard the International Space Station and on the ground, provides researchers with a unique long-term, replenishable, and upgradeable platform for the validation of high-risk control and autonomy technologies critical to the operation of distributed spacecraft missions such as the proposed formation flying interferometer version of Terrestrial Planet Finder (TPF). In November 2003, a subset of the key TPF-like maneuvers has been performed onboard NASA's KC-135 microgravity facility, followed by 2-D demonstrations of two and three spacecraft maneuvers at the Marshall Space Flight Center (MSFC) in June 2004. Due to the short experiment duration, only elements of a TPF lost in space maneuver were implemented and validated. The longer experiment time at the MSFC flat-floor facility allows more elaborate maneuvers such as array spin-up/down, array resizing and array rotation be tested but in a less representative environment. The results obtained from these experiments are presented together with the basic estimator and control building blocks used in these experiments.

Metrology and pointing will be enabling technologies for a new generation of astronomical missions having large and distributed apertures and delivering unprecedented performance. The UV interferometer Stellar Imager would study stellar dynamos by imaging magnetic activity on the disks of stars in our Galaxy. The X-ray interferometer Black Hole Imager would study strong gravity physics and the formation of jets by imaging the event horizons of supermassive black holes. These missions require pointing to microarcseconds or better, and metrology to nm accuracy of optical elements separated by km, for control of optical path difference. This paper describes a metrology and pointing system that meets these requirements for the Stellar Imager. A reference platform uses interferometers to sense alignment with a guide star. Laser gauges determine mirror positions in the frame of the reference platform, and detector position is monitored by laser gauges or observations of an artificial star. Applications to other astronomical instruments are discussed.

This paper describes the broad goals and the current status of the Space Interferometry Mission (SIM). SIM was endorsed in the 1990 decadal report of the Astronomy and Astrophysics survey committee of the National Research Council. The SIM mission would be the first long baseline interferometer in space. The goals of SIM represent not factors of two or three improvement in astronometric accuracy, but two to three orders of magnitude improvement. The current most accurate astrometric measurements are from the Hipparcos satellite launched by ESA in 1990. Hipparchos achieved slightly better than 1 milliarcsec global astrometric accuracy. SIM's goal is 4 microarcsec accuracy for global astrometry (for a nominal 5 yr mission) and 1 microarcsec for single measurement narrow angle accuracy. The narrow angle precision translates to the ability to measure the "wobble" of stars with an error of 0.14 uas, if the target is observed 50 times during the 5 year mission. The paper gives an overview of the type of scientific questions SIM will address, concentrating on the planet detection aspects of SIM.

Optical interferometry will open new vistas for astronomy over the next decade. The Space Interferometry Mission, operating unfettered by the Earth's atmosphere, will offer unprecedented astrometric precision that promises the discovery of Earth-class extra-solar planets as well as a wealth of important astrophysics. Optical interometers also present severe technological challenges: laser metrology systems must perform with sub-nanometer precision; mechanical vibrations must be controlled to nanometers requiring orders of magnitude distrubance rejection; a multitude of actuators and sensors must operate flawlessly and in concert. The Jet Propulsion Laboratory along with its industry partners, Northrop Grumman Space Technology, and Lockheed Martin, are addressing these challenges with a technology development program that is nearing completion. Emphasis is shifting from technology demonstration to technology transfer to the flight team that wil build and launch the space system.

The Space Interferometry Mission (SIM) will perform global astrometry (full sky), local wide-angle (15 degree) and narrow-angle (1 degree) observations to search extra-solar planets, and can calibrate stellar and galactic evolution theories. The astrometric accuracy of the SIM mission depends on spectral characteristics of the optics, detectors and targets. This paper will discuss the photometric throughput of the SIM instrument, and analyze effects of wavefront errors, optical mismatches and control biases as a function of wavelength. The color dependence models of the instrument optics including mirrors, lenses, field-stop and beam-splitter are presented. The performances of different detectors with a variety of coatings are compared. A model of the SIM fringe spectrometer is created. For early and late types of stars, brightness dependency errors are analyzed for different combinations of optics and detectors. Visibility loss due to imperfect optics is investigated in detail. Based on the models of instrument and estimated visibilities, the astrometric accuracies for various kinds of stars are evaluated. It is important to emphasize that not only light sources, mirrors, lenses, field stop and detectors are all wavelength dependent, but also fringe visibility loss, wavefront error, optics control error, etc. are all a function of wavelengths. For the first time the estimate of SIM performance is based on spectral analysis of all factors above, rather than monochromatic approximations of detected fringes, or simply adopted constants. This paper summarizes the astrometric accuracies for a wide range of stars and various combinations of optical design and detector configurations. It has been verified that SIM has astrometric accuracy of about 4 μas for targets with different spectra.

The Space Interferometer Mission (SIM) flight instrument will not undergo a full performance, end-to-end system test on the ground due to a number of constraints. Thus, analysis and physics-based models will play a significant role in providing confidence that SIM will meet its science goals on orbit. The various models themselves are validated against the experimental results of severl "picometer" testbeds. In this paper we describe a set of models that are used to predict the magnitude and functional form of a class of field-dependent systematic errors for the science and guide interferometers. This set of models is validated by comparing predictions with the experimental results obtained from the MicroArcsecond Metrology (MAM) testbed and the Diffraction testbed (DTB). The metric for validation is provided by the SIM astrometric error budget.

We present a study of how the TPF interferometer sensitivity relates to vibration level and control system performance. The error budgets for the TPF interferometer have two parts: the null floor, i.e. the suppression of starlight leakage due to instrument imperfections and performance limits; and null variation, i.e. planet-mimicking signal variations due to instrument errors and performance variations. The first impacts the statistical noise and thus the integration time; the second represents a planet sensitivity floor which limits further improvement with integration time. Oliver Lay (JPL) has developed an extensive analysis tool known as the Interferometer Performance Model (IPM) for managing both budgets. The budgets we use have many of the same features, but are less well-developed; the requirements are similar. Here we develop an example implementation of the TPF interferometer control system, analyze the system performance using an integrated model, and show that it meets the TPF planet sensitivity requirements using our performance budgets. We summarize key requirements and lessons arising from this exercise.

The Large Binocular Telescope (LBT) will be a unique interferometric facility when it is completed in 2005. The telescope incorporates two, 8.4-meter diameter primary mirrors on a single mounting. With 14.4 meter center-to-center spacing, this interferometer provides the equivalent collecting area of a 12-meter telescope, and, depending on the beam combination scheme, the spatial resolution of a 14.4 or 22.8-meter telescope. We report on the status of two initial interferometric instruments planned for the LBT. A group based at the University of Arizona is constructing LBTI, a thermal infrared beam combiner focusing on nulling, but allowing thermal imaging as well. This instrument will search for and measure zodiacal light in candidate stellar systems in preparation for the Terrestrial Planet Finder (TPF) and Darwin missions. There is also a program to search for young Jupiters. A second group, based in Heidelberg, Arcetri, Cologne, and Bonn, is building LINC-NIRVANA, a near-infrared Fizeau-mode beam combiner with multi-conjugated adaptive optics (MCAO). Fizeau interferometry preserves phase information and allows true imagery over a wide field of view. Using state-of-the-art detector arrays, coupled with advanced atmospheric correction strategies, LINC-NIRVANA will enable a broad variety of scientific programs that require the ultimate in sensitivity, field-of-view, and spatial resolution.

The Mauna Kea Observatory offers a unique opportunity to build a large and sensitive interferometer. Seven telescopes have diameters larger than 3 meters and are or may be equipped with adaptive optics systems to correct phase perturbations induced by atmospheric turbulence. The maximum telescope separation of 800 meters can provide an angular resolution as good as 0.25 milli-arcseconds in the J band. The large pupils and long baselines make 'OHANA very complementary to existing large optical interferometers. From an astrophysical point of view, it opens the way to imaging of the central part of faint and compact objects such as active galactic nuclei and young stellar objects. On a technical point of view, it opens the way to kilometric or more arrays by propagating light in single-mode fibers. First instruments have been built and tested successfully at CFHT, Keck I and Gemini to inject light into single-mode fibers thus partly completing Phase I of the project. Phase II is now on-going with the prospects of the first combinations of Keck I - Keck II in 2004 and Gemini - CFHT in 2005.

The Magdalena Ridge Observatory Interferometer will be the first facility-class optical interferometer optimized strictly for an imaging science mission. The array in its final form is envisaged to comprise ten 1.4 m aperture movable telescopes in a Y configuration, baselines extending from 8 to 400 meters, delay lines capable of tracking well-placed sources for 6 continuous hours, fringe-tracking in the near-infrared , and undertake science observations at both near-infrared and optical wavelengths. The science reference mission includes studies of young stellar objects, a full range of stellar astrophysics, and imaging studies of the nearest and brightest 100 active galactic nuclei. We will be staffing up to our full complement of personnel in New Mexico over the next year. Our goal for first fringes on the first baseline is 2007.

According to the "hypertelescope" imaging mode, stellar interferometers could provide direct snapshot images. Whereas the Fizeau imaging mode is useless when the aperture is highly diluted, a "densified-pupil" or "hypertelescope" imaging mode can concentrate most light into the high-resolution central interference peak, allowing direct imaging of compact sources and stellar coronagraphy for exoplanets finding. The current VLTI is able to combine light from 2 to 3 telescopes coherently, but the combination of 4 to 8 beams is foreseen in subsequent phases. In order to exploit the full forthcoming VLTI infrastructure, a next generation instrument has been proposed (VIDA) in 2002 for very high-resolution snapshot imaging with UTs and/or ATs. This paper presents a new attractive design studied for this instrument using single mode optical fibers and allowing a multi-field imaging mode. We also give the expected performances with a coronagraph, computed from numerical simulations including cophasing and adaptive optics residual errors.

The PRIMA facility will implement dual-star astrometry at the VLTI. We have formed a consortium that will build the PRIMA differential delay lines, develop an astrometric operation and calibration plan, and deliver astrometric data reduction software. This will enable astrometric planet surveys with a target precision of 10μas. Our scientific goals include determining orbital inclinations and masses for planets already known from radial-velocity surveys, searches for planets around stars that are not amenable to high-precision radial-velocity observations, and a search for large rocky planets around nearby low-mass stars.

We are studying an optical concept aiming at recombining four mid-infrared telescope beams, where interference fringes are sampled in the pupil plane. Such a principle is perfectly adapted for reconstructing images by aperture synthesis with teh VLTI. It could be used for building a new generation 10 μm instrument, but instead of doing a totally new instrument, we propose the design of an optical module that can supply the surrent MIDI-VLTI instrument with 4 beams. The combined use of this module together with the MIDI instrument is the project called APreS-MIDI. Such an instrument at the VLTI focus will have an unique and very strong astrophysical potential.

The science objectives of VITRUV is to investigate the morphology of compact astrophysical objects in optical wavelengths like the environment of AGN, star forming regions, stellar surfaces. This instrument will take full advantage of the VLTI site with 4 very large telescopes and 4 auxiliary telescopes. The instrument concept is to built aperture synthesis images like the millimeter-wave radiointerferometer of the IRAM Plateau de Bure. VITRUV coupled to the VLTI will have similar and even better resolution than ALMA. The astrophysical specifications although not yet finalized will be a temporal resolution of the order of 1 day, spectral resolution from 100 to 30,000, image dynamic from 100 to 1,000, a field of view of 1 arcsec for an initial wavlength coverage from 1 to 2.5 microns that could be extended from 0.5 to 5 microns. The technology that is contemplated at this stage is integrated optics.

The Very Large Telescope Interferometer (VLTI) on Cerro Paranal (2635 m) in Northern Chile reached a major milestone in September 2003 when the mid infrared instrument MIDI was offered for scientific observations to the community. This was only nine months after MIDI had recorded first fringes. In the meantime, the near infrared instrument AMBER saw first fringes in March 2004, and it is planned to offer AMBER in September 2004. The large number of subsystems that have been installed in the last two years - amongst them adaptive optics for the 8-m Unit Telescopes (UT), the first 1.8-m Auxiliary Telescope (AT), the fringe tracker FINITO and three more Delay Lines for a total of six, only to name the major ones - will be described in this article. We will also discuss the next steps of the VLTI mainly concerned with the dual feed system PRIMA and we will give an outlook to possible future extensions.

Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 μm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 μm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control.

We describe recent science projects that the Navy Prototype Optical Interferometer (NPOI) scientific staff and collaborators are pursuing. Recent results from the wide angle astrometric program and imaging programs (rapid rotators, binaries and Be stars) will be summarized. We discuss some of the technology that enables the NPOI to operate routinely as an observatory astronomical instrument.

Georgia State University's Center for High Angular Resolution Astronomy (CHARA) operates a multi-telescope, long-baseline, optical/infrared interferometric array on Mt. Wilson, California. We present an update on the status of this facility along with a sample of preliminary results from current scientific programs.

Closure-phase science and technology are dominant features of the recent activity at IOTA. Our science projects include imaging several spectroscopic binary stars, imaging YSOs including Herbig AeBe stars, detecting asymmetries in a large sample of Mira stars, and measuring water shells around Miras. Many technology projects were pursued in order to make these science observations possible. These include installation of a third-generation integrated-optics 3-beam combiner (IONIC), completion of the real-time control system software, installation of fringe-packet tracking software, use of narrow sub-H band filters, validation of the phase-closure operation, development of CPLD control of the science camera (PICNIC) and star-tracker camera (LLiST), installation of a new star-tracker camera, expansion of the observing facility, and installation of new semi-automated optical alignment tools.