This paper presents an overview of contributions in astrophysics made since 2004 through the use of long-baseline optical/infrared interferometers. Emphasis is placed on new results at near- and mid-infrared wavelengths. These results include new insights into our understanding of Cepheid and Mira variables, Young Stellar Objects, dust shells, spectroscopic binaries, and the limb-darkening of rapidly rotating stars. Plans for future work are also described.

The AMBER instrument installed at the Very Large Telescope (VLT) combines three beams from as many telescopes to produce spectrally dispersed fringes from milli-arcsecond angular scale in the near infrared. Two years after installation, first scientific observations have been carried out during the Science Demonstration Time and the Guaranteed Time mostly on bright sources due to some VLTI limitations. In this paper, we review these first astrophysical results and we show which types of completely new information is brought by AMBER. The first astrophysical results have been mainly focusing on stellar wind structure, kinematics, and its interaction with dust usually concentrated in a disk. Because AMBER has dramatically increased the number of measures per baseline, this instrument brings strong constraints on morphology and models despite a relatively poor (u,v) coverage for each object.

The nulling mode of the Keck Interferometer is being commissioned at the Mauna Kea summit. The nuller combines the two Keck telescope apertures in a split-pupil mode to cancel the on-axis starlight and coherently detect the residual signal. The nuller, working at 10 um, is tightly integrated with the other interferometer subsystems including the fringe and angle trackers, the delay lines and laser metrology, and the real-time control system. Since first 10 um light in August 2004, the system integration is proceeding with increasing functionality and performance, leading to demonstration of a 100:1 on-sky null in 2005. That level of performance has now been extended to observations with longer coherent integration times. An overview of the overall system is presented, with emphasis on the observing sequence, phasing system, and differences with respect to the V² system, along with a presentation of some recent engineering data.

The Sydney University Stellar Interferometer is a long baseline optical interferometer located in northern New South Wales, Australia. It has a North-South array of eleven fixed input siderostat stations giving a range of baselines from 5 to 640 m. Currently ten baselines from 5 to 160 m are fully operational and beam-combination and detection systems for the spectral ranges 430-520nm and 550-950nm are available. Dichroic beam-splitters have been introduced to allow simultaneous observations with both spectral systems. The original blue beam-combination system has been upgraded to improve sensitivity and to allow rapid wavelength switching. A software scheduler has been developed to automate much of the observational procedure including the acquisition of a star, fringe search and acquisition, recording of fringe scans, and the taking of photometric scans. A data pipeline for processing the observational data has been further developed to include seeing corrections and this has improved the calibration of the observational data. Preliminary results of scientific observations with both blue and red systems, including observations of single stars, binary stars and Cepheid variables are described.

We present a summary of activity at the Cambridge Optical Aperture Synthesis Telescope (COAST) group during the period 2004-2006. Our main program has focused on technical design and prototyping for future facility arrays such as the VLTI and Magdalena Ridge Observatory Interferometer, but with a small parallel effort of focused astronomical observations with COAST, in particular multi-wavelength studies of supergiants. We report on progress on these and other technical areas over the past 2 years.

The technical status of the Navy Prototype Optical Interferometer (NPOI) since the last SPIE meeting is summarized along with the current science programs. The instrument is operated in an automatic observational mode, obtaining over 10,000 stellar observations in the period, June 2004 through March 2006. The scientific program has been directed at astrometry, TPF candidate stars, binary stars and other interesting targets such as Be stars. A significant database of NPOI observations obtained in 1997-2004 is being analyzed for binaries and single stars such as rapid rotating stars: Altair and Vega.

We give an overview of recent results obtained with the GI2T interferometer. On the technical side, great improvements have been obtained on photon counting detectors, especially in terms of quantum efficiency and of photon centroiding algorithms. Piston measurements with the GI2T dispersed fringes have been made during coordinate observations with the Generalized Seeing Monitor GSM. These observations have lead to wavefront outer scale determinations. The last topic we will present concerns the polarimetric measurements done with the SPIN device on the GI2T spectrograph. We conclude this paper by a summary of the results obtained with the GI2T during its scientific life.

MIRA-I.2 is a 30m-baseline two-aperture stellar interferometer working in the visible band (from 600 to 1000 nm). In this article are presented the up-to-date progress and performance of MIRA-I.2 as well as some ongoing and future plans. The fast and coarse delay lines are now both evacuated, and the maximum OPD (optical path delay) compensations are about 16 m and 4 m long, respectively, for the fast and coarse delay lines. The current limiting magnitude is about I=4.5mag, and stars within the declination range from +8 to +51 degree is possible to be observed longer than one hour at the elevation angle of 60 degrees and higher. The OPD of the coarse delay line is modulated by about 128 micrometers around the expected fringe center with the use of PZT, and 187 fringe packets are scanned during one shot (= 60 seconds duration) to yield the mean visibility of about 10 % internal errors for each shot. The thermal environment of the building that houses the delay lines and interference optics has been improved very much, and readjustments of the optical alignment are not necessary for a whole night. The assembly and the setup of the optics to be used for the fringe tracking experiment are nearly completed.

Using the FLUOR beam-combiner installed at the CHARA Array (Mt. Wilson, CA), we have obtained highprecision visibility measurements of Vega, one of the prototypic debris-disk stars, known to be surrounded by a large amount of cold dust in a ring-like structure at 80-100 AU. The combination of short and long baselines has allowed us to separately resolve the stellar photosphere and the close environment of the star (less than 8 AU). Our observations show a significant deficit in square visibility at short baselines with respect to the expected visibility of a simple UD stellar model (ΔV² equal or equivalent to 2%), suggesting the presence of an extended source of emission around Vega. The sparse (u, v) plane coverage does not allow the discrimination between a point source and an extended circumstellar emission as the source of the extended emission. However, we show that the presence of a point-like source within the FLUOR field-of-view (1" in radius, i.e., 7.8 AU at the distance of Vega) is highly unlikely. The excess emission is most likely due to the presence of hot circumstellar dust in the inner part of Vega's debris disk, with a flux ratio of 1.29 plus or minus 0.19% between the integrated dust emission and the stellar photosphere. Complementing this result with archival photometric data in the near- and mid-infrared and taking into account a realistic photospheric model for the rapidly rotating Vega, we derive the expected physical properties of the circumstellar dust by modelling its Spectral Energy Distribution. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.

Several existing interferometers have taken large V² data sets without the use of spatial filters. The calibration of this kind of data is always seeing and alignment-dependent, and can result in significant systematic errors if several ideal calibrators are not available. I will demonstrate how high-precision unbiased estimates of V² can still be retrieved from this kind of data by using one of several simple seeing corrections. In particular, I will demonstrate the success of these techniques on data sets from the SUSI interferometer and non-redundant masking experiments, and also discuss their limitations. One of the techniques described can also aid in the calibration of low S/N spatially-filtered data.

This paper reports on efforts to control a tethered formation flight spacecraft array for NASA's SPECS mission using the SPHERES test-bed developed by the MIT Space Systems Laboratory. Specifically, advances in methodology and experimental results realized since the 2005 SPIE paper are emphasized. These include a new test-bed setup with a reaction wheel assembly, a novel relative attitude measurement system using force torque sensors, and modeling of non-ideal tethers to account for tether vibration modes. The nonlinear equations of motion of multi-vehicle tethered spacecraft with elastic flexible tethers are derived from Lagrange's equations. The controllability analysis indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor, thereby saving thruster fuel consumption. Based upon this analysis, linear and nonlinear controllers have been successfully implemented on the tethered SPHERES testbed, and tested at the NASA MSFC's flat floor facility using two and three SPHERES configurations.

We have conducted an exhaustive inventory of potential astrophysical "noise sources" with respect to the detection and characterisation of Earth-like planets, focusing on the specifics of the nulling interferometry technique. We have extrapolated their characterisation at these yet-to-be-attained resolution and sensitivity, from existing data and models. They range from stellar features and significant orbital motion of pegasides during exposure, to exozodiacal cloud structures, background galaxies and galactic cirrus emission. This enabled us to evaluate the offset between the various features a typical interferometric nulling field-of-view (FOV) is likely to exhibit in reality, and the simplified, planet-only, detection FOV model generally used. This work is complementary to precise nulling data model formalization and detection algorithms development (Thiebaut et al., this conference), together conducted in the frame of an ESA contract by an Alcatel Alenia Space-led consortium. As such, part of this work is under embedment into a Java open-source simulator (ORIGIN). This approach can be useful for less signal-entangling, direct imaging techniques.

We report on observations of circumstellar disks around young stars that have been obtained with the MIDI instrument, which is mounted on the VLT Interferometer and operates in the 10 μm atmospheric window. The maximum spatial resolution of 5 milli-arcsec corresponds to sub-AU scales at the distance to nearby star formation regions. Thus, we can study the disks on the spatial scales at which important processes occur, such as accretion, dust processing, and planet formation. The main results obtained so far can be summarized as follows: 1. The measured interferometric visibilities are in good qualitative agreement with those predicted by models of circumstellar disks. In particular, a predicted correlation between the strength of the far-infrared excess and the spatial structure of the disk is confirmed by direct measurements; 2. In several objects strong evidence for deviations from circular symmetry is present, indicating that an inclined disk is indeed the dominant component seen in the mid-infrared; 3. The dust properties are not uniform over the disk, but are instead a strong function of distance to the central star. The dust in the innermost disk regions is observed to be more "processed" than the dust further out, both in Herbig Ae star disks and in those around T Tauri stars.

The Palomar Testbed Interferometer (PTI) is a long-baseline, near-infrared interferometer located on Palomar Mountain, California. PTI has been conducting science operations since 1997 and continues to function both as a productive scientific instrument and as a technical testbed. Here I will review the current status and near-term plans for PTI, highlight some of the science results of the last two years, and describe the recently released PTI data archive.

A new observing mode for the Palomar Testbed Interferometer was developed in 2002-2003 which enables differential astrometry at the level of 20 micro-arcseconds (μas) for binary systems with separations of several hundred milli-arcseconds (mas). This phase-referenced mode is the basis of the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES), a search for giant planets orbiting either the primary or secondary star in fifty binary systems. We present the first science results from the PHASES search. The properties of the stars comprising binary systems are determined to high precision. The mutual inclinations of several hierarchical triple star systems have been determined. We will present upper limits constraining the the existence of giant planets in a few of the target systems.

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 a brief update on the status of this facility along with summaries of the first scientific results from the Array.

We present the latest results obtained by stellar interferometry (SI) in the field of pulsating Cephieds stars. The SI technique is particularly well suited to measure distances using the quasi-geometric pulsation parallax method known as the Baade-Wesselink (BW) method. The improvements in angular resolution and precision in the measurements have led to high precision calibration of the Cepheids period luminosity relation. On the other hand, the assumptions of the BW method can be investigated using SI: we present a calibration of the spectroscopic projection factor and the study of center to limb darkening and circumstellar envelopes. These later studies clearly demonstrate that SI has recently entered a regime where the BW implemetation is no longer limited by the precision of the angular diameter measurements but by the classical assumptions underlying the method itself. This calls for a better understanding of Cepheids, using numerical models and diverse observation techniques, including SI.

We present a brief review of recent scientific and technical advances at the Infrared Optical Telescope Array (IOTA). IOTA is a long-baseline interferometer located atop Mount Hopkins, Arizona. Recent work has emphasized the use of the three-telescope interferometer completed in 2002. We report on results obtained on a range of scientific targets, including AGB stars, Herbig AeBe Stars, binary stars, and the recent outburst of the recurrent nova RS Oph. We report the completion of a new spectrometer which allows visibility measurements at several high spectral resolution channels simultaneously. Finally, it is our sad duty to report that IOTA will be closed this year.

The ESO Very Large Telescope Interferometer (VLTI) is the first general-user interferometer that offers near- and mid-infrared long-baseline interferometric observations in service and visitor mode to the whole astronomical community. Over the last two years, the VLTI has moved into its regular science operation mode with the two science instruments, MIDI and AMBER, both on all four 8m Unit Telescopes and the first three 1.8m Auxiliary Telescopes. We are currently devoting up to half of the available time for science, the rest is used for characterization and improvement of the existing system, plus additional installations. Since the first fringes with the VLTI on a star were obtained on March 17, 2001, there have been five years of scientific observations, with the different instruments, different telescopes and baselines. These observations have led so far to more than 40 refereed publications. We describe the current status of the VLTI and give an outlook for its near future.

After two years of official operation as a facility instrument on the Very Large Telescope Interferometer (VLTI) the MID-infrared Interferometric instrument (MIDI) has provided a great wealth of new results. The number of AGNs observed and the diversity of targets requested for observations are beyond expectations and illustrate the success of the instrument concept. We will outline the scientific issues recently addressed by MIDI and present briefly the potential ones.

In this paper we report on progress at the Keck Interferometer since the 2004 SPIE meeting with an emphasis on the operations improvements for visibility science.

We summarize the status of the Keck Interferometer in V² mode, and science results recently obtained.

SIM PlanetQuest will investigate, using stellar interferometry from space, a broad range of topics in astronomy and astrophysics. Chief among its scientific goals is a search of nearby stars for terrestrial planets in the circumstellar habitable zone. Composed of three astrometric interferometers each capable of resolving approximately one billionth of a degree, SIM is poised to revolutionize our understanding of the Universe in a number of areas. With the completion of its technology development phase, SIM is now on a sure footing for the development of requirements for the flight instrument. The preliminary design of SIM is in place and many key brassboards have been built. Here we review the project status and recent progress.

A brief overview of the Darwin project in the context of the European Space Agency's Cosmic Vision program is given. The scientific goals in the context of the new approach with themes, is given. The goals are broken down into a stepwise approach first relating current ground based and immediate space based experiments (e.g. radial velocity measurements from the ground and the CNES/ESA COROT occultation mission). Then, the different approaches to how to achieving the full goal of a survey of the nearest stars is described. Then, a brief outline of steps following after the current objectives of Darwin have been reached will follow. Some focus is also given to the response of the European community on how to address these goals in a timely and technically correct fashion. This will lead up to scenarios likely to occur over the next 3 years. Darwin is developed through an active technology program, parts of which are described in other papers at this conference. A description of where the different elements fit will be given. Finally the international aspects as currently foreseen are presented.

The interferometric version of the Terrestrial Planet Finder (TPF-I) has the potential to find and characterize earth-sized planets in the habitable zones of over 250 nearby stars and to search for life using biomarkers in the atmospheres of any planets found. The scientific case for such a mission continues to be strengthened by on-going progress in the detection of planets via indirect means. This paper summarizes the status of TPF-I, illustrative scientific requirements for the mission, and its enabling technologies.

The Terrestrial Planet Finder Coronagraph (TPF-C) is a deep space mission designed to detect and characterize Earth-like planets around nearby stars. TPF-C will be able to search for signs of life on these planets. TPF-C will use spectroscopy to measure basic properties including the presence of water or oxygen in the atmosphere, powerful signatures in the search for habitable worlds. This capability to characterize planets is what allows TPF-C to transcend other astronomy projects and become an historical endeavor on a par with the discovery voyages of the great navigators.

PRIMA, the Phase-Referenced Imaging and Micro-arcsecond Astrometry facility for the Very Large Telescope Interferometer, is now nearing the end of its manufacturing phase. An intensive test period of the various sub-systems (star separators, fringe sensor units and incremental metrology) and of their interactions in the global system will start in Garching as soon as they are delivered. The status and performances of the individual sub-systems are presented in this paper as well as the proposed observation and calibration strategy to reach the challenging goal of high-accuracy differential astrometry at 10 μas level.

We report on experiments in multi-wavelength phase referencing using the Navy Prototype Optical Interferometer (NPOI). In these experiments we use the unique capability of the NPOI to simultaneously observe 16 spectral channels covering 512-850 nm on multiple baselines simultaneously. We present observations of the well-known Be star ζ Tauri using custom filters which allow us to isolate the Hα line in a single spectral channel while the other channels observe the stellar continuum. Since the central star is unresolved, we can use the data in the continuum channels to calibrate the spectral line data. Using the phase information recovered in this way, it is possible for the first time to use standard techniques to construct simple images of the line-emitting region around the star.

Observational modes in which simultaneous high spatial and spectral information are recovered, without the complexity and expense of a dispersed detection system, have been discussed for some time. Sometimes called Double Fourier/Spatio-Spectral Interferometry (DFSSI), these methods fuse the concepts of Fourier Transform Spectrometry with high spatial resolution interferometry. The basic underlying principle comes from the idea that different spectral components, yielding different fringe frequencies, can be separated out in the fringe spectrum for individual study. However in practice, seeing fluctuations have the effect of shifting and blurring together the fringe frequencies making it difficult to isolate discrete spectral components. DFSSI has not been widely exploited in astronomical interferometry, due in part to such considerations. Here we propose a closely-related, although distinct technique which is the analog of DFSSI implemented in the spatial (delay) space rather than the time (frequency) domain. We propose the name Double-Fourier Spatio-Spectral Decoding to distinguish it from the latter. The technique relies on careful calibration of the fringe envelope shape, which is a function of the shape of the overall bandpass of the interferometer. We show that for astrophysical systems with interesting variations in spatial structure for neighboring spectral regions (such as stars with emission-line winds) that it is possible to untangle separate spatial and spectral components without a multi-channel dispersed fringe detector. The principle has been demonstrated successfully with observations of the prototype emission-line object P Cygni at the CHARA array.

VLTi Spectro-Imager (VSI) is a proposition for a second generation VLTI instrument which is aimed at providing the ESO community with the capability of performing image synthesis at milli-arcsecond angular resolution. VSI provides the VLTI with an instrument able to combine 4 telescopes in a baseline version and optionally up to 6 telescopes in the near-infrared spectral domain with moderate to high spectral resolution. The instrument contains its own fringe tracker in order to relax the constraints onto the VLTI infrastructure. VSI will do imaging at the milli-arcsecond scale with spectral resolution of: a) the close environments of young stars probing the initial conditions for planet formation; b) the surfaces of stars; c) the environment of evolved stars, stellar remnants and stellar winds, and d) the central region of active galactic nuclei and supermassive black holes. The science cases allowed us to specify the astrophysical requirements of the instrument and to define the necessary studies of the science group for phase A.

Our objective is the development of mid-infrared imaging at the VLTI. The related science case study demonstrates the enormous capability of a new generation mid-infrared beam combiner. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI by increasing the number of recombined beams up to four. MIDI is a very successful instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar environments by using a wide mid-infrared band coverage extended to L, M, N and Q spectral bands. The four beam combination of MATISSE provides an efficient UV-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide for the first time aperture synthesis images in the mid-infrared spectral regime. The mid-infrared spectral domain is very relevant for the study of the environment of various astrophysical sources. Our science case studies show the wide field of applications of MATISSE. They will be illustrated in the first part of this presentation through the perspective of imaging the circumstellar environments/discs of young stellar objects. The MATISSE characteristics will be given in a second part of the presentation.

The ESA Darwin space mission will require a ground based precursor to i/ demonstrate nulling interferometry in an operational context and ii/ carry out some precursor science, such as the characterization of the level of exozodiacal light around the main Darwin targets. These are the stated objectives of the GENIE nulling instrument that was studied for the VLTI. We argue here that the same objectives can be met in a more efficient way by an antarctic-based nulling experiment. The ALADDIN mission concept is an integrated L-band nulling breadboard with relatively modest collectors (1m) and baseline (40m). Because of its privileged location, this is suffcient to achieve a sensitivity (in terms of detectable zodi levels) which is 1.6 to 3.5 times better than GENIE at the VLTI, bringing it below the 20-zodi threshold value identified to carry out the Darwin precursor science. The integrated design enables top-level optimization and full access to the light collectors for the duration of the experiment, while reducing the complexity of the nulling breadboard.

We present the adaptive optics assisted, near-infrared VLTI instrument - GRAVITY - for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. Precision astrometry and phase-referenced interferometric imaging will realize the most advanced vision of optical/infrared interferometry with the VLT. Our most ambitious science goal is to study motions within a few times the event horizon size of the Galactic Center massive black hole and to test General Relativity in its strong field limit. We define the science reference cases for GRAVITY and derive the top level requirements for GRAVITY. The installation of the instrument at the VLTI is planned for 2012.

We discuss the specific potential of a long baseline optical interferometer at the Antarctica Dome C site for high limiting magnitude observations. First we compute the potential limiting magnitude for cophasing an interferometer with N diameter D apertures. Combining this with a computation of the isopistonic angle and of the density of stars, we evaluate that an interferometer with 1.5 m telescopes performing off axis fringe tracking will provide a minimum sky coverage of 50% in the direction of the galactic pole, while at Paranal, this could be achieved only if the individual apertures are 10 times larger. This is due to the fact that most of optical seeing at Dome C is produced in the first 30 meters above the ground. This makes the Dome C an unique site on earth to permit optical interferometers to reach high magnitudes and therefore contribute to extra galactic science and cosmology.

Dome C is probably the best accessible site on earth for infrared interferometry, but siting an interferometer on the Antarctic plateau poses significant technical problems. EOS Technologies has studied how existing interferometric telescopes can be adapted to the Antarctic environment, having completed a design study for the Pathfinder for an International Large Optical Telescope (PILOT), and has proposed a unique technique for manufacturing delay lines on site, from prefinished coil stock. Modifications to EOST's standard 2m class telescopes are discussed, including lubrication options and differential expansion of materials assembled at room temperature and cooled to -70°C, as well as continuous, high precision delay line construction, using patented rotary sizing technology.

Studies by Mark Swain and a colleague at the Max Planck Institut fur Astronomie, coupled with results from past and ongoing projects at Harvey Mudd College, strongly suggest that it may be possible to achieve imaging performance comparable to the Hubble Space Telescope at relatively low cost using available, commercial products. This is achievable by placing a 2.4 m telescope, with readily available adaptive optics, on a 30 m tower located at a high-elevation geological "dome" in Antarctica. An initial project surveyed relevant tower design approaches, then generated and evaluated six concept designs for telescope towers. Using data for typical and extreme wind at Dome C to generate wind loads, finite element analysis yielded lateral deflections at the top of 0.3 mm for typical winds and 12.1 mm for extreme gusts, with the lowest resonant frequency at 0.7 Hz; some tower concepts are innovative and allow for easy shipment, setup, and relocation. A subsequent project analyzed a tower designed by Hammerschlag and found fundamental resonance frequencies at 4.3 Hz for bending and 5.9 Hz for torsion; this project also designed and simulated an active telescope control system that maintained 17 milliarcsecond pointing error for the telescope atop the tower during typical wind conditions.

The Keck Interferometer Nuller (KIN) is now largely in place at the Keck Observatory, and functionalities and performance are increasing with time. The main goal of the KIN is to examine nearby stars for the presence of exozodiacal emission, but other sources of circumstellar emission, such as disks around young stars, and hot exoplanets are also potential targets. To observe with the KIN in nulling mode, knowledge of the intrinsic source spectrum is essential, because of the wide variety of wavelengths involved in the various control loops - the AO system operates at visible wavelengths, the pointing loops use the J-band, the high-speed fringe tracker operates in the K-band, and the nulling observations take place in the N-band. Thus, brightness constraints apply at all of these wavelengths. In addition, source structure plays a role at both K-band and N-band, through the visibility. In this talk, the operation of the KIN is first briefly described, and then the sensitivity and performance of the KIN is summarized, with the aim of presenting an overview of the parameter space accessible to the nuller. Finally, some of the initial observations obtained with the KIN are described.

The Keck Interferometer Nuller is designed to detect faint off-axis mid-infrared light a few tens to a few hundreds of milliarcseconds from a bright central star. The starlight is suppressed by destructive combination along the long (85 m) baseline, which produces a fringe spacing of 25 mas at a wavelength of 10 μm, with the central null crossing the position of the star. The strong, variable mid-infrared background is subtracted using interferometric phase chopping along the short (5 m) baseline. This paper presents an overview of the observing and data reduction strategies used to produce a calibrated measurement of the off-axis light. During the observations, the instrument cycles rapidly through several calibration and measurement steps, in order to monitor and stabilize the phases of the fringes produced by the various baselines, and to derive the fringe intensity at the constructive peak and destructive null along the long baseline. The data analysis involves removing biases and coherently demodulating the short-baseline fringe with the long-baseline fringe tuned to alternate between constructive and destructive phases, combining the results of many measurements to improve the sensitivity, and estimating the part of the null leakage signal which is associated with the finite angular size of the central star. Comparison of the results of null measurements on science target and calibrator stars permits the instrumental leakage - the "system null leakage" - to be removed and the off-axis light to be measured.

Water vapor is the dominant source of randomly-changing atmospheric dispersion on timescales of seconds to minutes in the near- and mid-infrared. The dispersion changes are sufficient to limit the performance of the Keck Nuller unless steps are taken to measure and compensate for them. Here we present the first measurements of water vapor differential column fluctuations with the mid-infrared Keck Nuller and its near-infrared fringe tracker, taken in October 2005, and discuss theoretical and practical aspects of our dispersion feedforward implementation. The data show much larger fluctuations than were seen in median Mauna Kea conditions measured at radio wavelengths, and probably account for the generally poor performance of the Nuller during the observing run. The measurements in the two bands show strong correlations, indicating that the planned feedforward of the near-infrared value to stabilize the dispersion in the mid-infrared will substantially reduce the residual dispersion fluctuations seen by the Nuller.

The Terrestrial Planet Finder Interferometer (TPF-I) concept is being studied at the Jet Propulsion Laboratory and the TPF-I Planet Detection Testbed has been developed to simulate the detection process for an earthlike planet orbiting a star within about 15 pc. The testbed combines four beams of infrared light simulating the operation of a dual chopped Bracewell interferometer observing a star and a faint planet. This paper describes the results obtained this year including nulling of the starlight on four input beams at contrast ratios up to 250,000 to 1, and detection of faint planet signals at contrast ratios with the star of 2 million to 1.

The ESA-Darwin mission is devoted to direct detection and spectroscopic characterization of earth-like exoplanets in the thermal infrared domain by nulling interferometry in space. This technique yields the rejection of starlight so as to make detectable the faintly emitting planet in the neighborhood. In that context, Alcatel Alenia Space has developed a nulling breadboard for ESA in order to perform the rejection of an unresolved on-axis source. This device, the Multi Aperture Imaging Interferometer (MAII) demonstrated high rejection capability at a relevant level for exoplanets, in single-polarized and mono-chromatic conditions. In this paper we report on our late investigations using the MAII focussed on modal filtering. The dependence of the nulling ratio on the degeneracy of the guided modes in the modal filter is put into evidence.

The NASA Terrestrial Planet Finder Interferometer (TPF-I) and ESA Darwin missions are designed to directly detect mid-infrared photons from earth-like planets around nearby stars. The technique of nulling interferometry is used to suppress the light from the parent star, typically 10⁷ times brighter than the planet, with an angular offset of 10-100 mas. There are two classes of noise: photon shot noise from the stellar leakage, local- and exo-zodiacal dust, and instability noise from variations in the instrument response. Shot noise requires that the instrument null depth is at least 10^-5. The instability noise requires a null depth of ~10^-6, corresponding to control of the optical paths at ~1 nm rms, and control of relative intensities at ~0.2%; it is these requirements that currently drive the design of the instrument. This paper describes a new technique that effectively removes the effect of instability noise. This relaxes the nulling requirement by a factor ~10 and makes planet detection robust to instrument variations. At the same time, the integration period needed to detect and characterize planets is reduced and the angular resolution of the array is significantly improved. Analysis and simulations are presented, and implications for the array architecture are discussed.

We present a new type of nulling interferometer that makes use of polarization properties to have on-axis destructive interference. The proposed design, which only involves commercial components and no achromatic device, is also suitable for internal modulation. This type of interferometer should enable a high rejection ratio in a theoretically unlimited spectral band. We implemented that concept on a two-beam white-light interferometer and we present here the first experimental results.

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 conducting experiments in the mid-IR wavelengths. This paper covers the current results and in the mid-IR.

We present VLTI-MIDI (the Mid-Infrared Interferometric Instrument at ESO's Very Large Telescope Interferometer) observations of MWC349 A, which are a prime example of the power of combined spatial and spectral resolution for addressing complex astrophysical phenomena. Previous observations of the peculiar emission line star MWC349A suggest that it is a young massive star in the short-lived phase of already having dissipated its parent cloud, but still being surrounded by the accretion disk, which is seen nearly edge-on. It is believed that the unique hydrogen recombination line maser / laser activity of MWC349A from mm to infrared wavelengths is also a consequence of this viewing geometry. We have taken 13 measurements with MIDI at the VLTI (Very Large Telescope Interferometer) in the GRISM mode covering the N band (8 to 13 microns) at a spectral resolution R ≈ 230. The wavelength dependence of the continuum visibility agrees with model calculations for circumstellar dust disks. In addition, the signatures of at least a dozen emission lines have been identified in the interferometric data. We present a first analysis of visibility amplitudes and differential phase data. In particular we show that a simple model can represent the SED and visibility amplitude of the continuum flux. Also the visibilities for the hydrogen and forbidden lines are discussed.

In 2002, the Fiber Linked Unit for Optical Recombination (FLUOR) has been moved from the Infrared Optical Telescope Array (IOTA) to the CHARA Array. We present here the main upgrades that followed the installation, the new features installed, including spectral dispersion, and the current capabilities of the instrument.

Atmospheric turbulence is a major impediment to ground-based optical interferometry. It causes fringes to move on ms time-scales, forcing very short exposures. Because of the semi-random phase shifts, the traditional approach averages exposure power spectra to build signal-to-noise ratio (SNR). This incoherent average has two problems: (1) A bias of correlated noise is introduced which must be subtracted. The smaller the visibility/the fainter the target star, the more diffcult bias subtraction becomes. SNR builds only slowly in this case. Unfortunately, these most difficult small visibility baselines contain most of the image information. (2) Baseline phase information is discarded. These are serious challenges to imaging with ground based optical interferometers. But if we were able to determine fringe phase, we could shift and integrate all the short exposures. We would then eliminate the bias problem, improve the SNR, and we would have preserved most of the phase information. This coherent averaging becomes possible with multi-spectral measurements. The group delay presents one option for determining phase. A more accurate approach is to use a time-dependent model of the fringe. For the most interesting low-visibility baselines, the atmospheric phase information can be bootstrapped from phase determinations on high-visibility baselines using the closure relation. The NPOI, with 32 spectral channels and a bootstrapping configuration, is well-suited for these approaches. We will illustrate how the fringe modeling approach works, compare it to the group-delay approach, and show how these approaches can be used to derive bias-free visibility amplitude and phase information. Coherent integration provides the highest signal-to-noise (SNR) improvement precisely in the situations where SNR builds most slowly using incoherent averaging. Coherent integration also produces high-SNR phase measurements which are calibration-free and thus have high real uncertainties as well. In this paper we will show how to coherently integration on NPOI data, and how to use baseline visibilities and calibrate coherently integrated visibility amplitudes.

LINC-NIRVANA is the interferometric near-infrared imaging camera for the Large Binocular Telescope (LBT). Being able to observe at wavelength bands from J to K (suppported by an adaptive optics system operating at visible light) LINC-NIRVANA will provide an unique and unprecedented combination of high angular resolution (~ 9 milliarcseconds at 1.25μm), wide field of view (~ 100 arcseconds² at 1.25μm), and large collecting area (~ 100m²). One of the major contributions of the 1. Physikalische Institut of the University of Cologne to this project is the development and provision of the Fringe and Flexure Tracking System (FFTS). In addition to the single-eye adaptive optics systems the FFTS is a crucial component to ensure a time-stable wavefront correction over the full aperture of the double-eye telescope, a mandatory pre-requisite for interferometric observations. Using a independent HAWAII 1 detector array at a combined focus close to the science detector, the Fringe and Flexure Tracking System analyses the complex two-dimensional interferometric point spread function (PSF) of a suitably bright reference source at frame rates of up to several hundred Hertz. By fitting a parameterised theoretical model PSF to the preprocessed image-data the FFTS determines the amount of pistonic phase difference and angular misalignment between the wavefronts of the two optical paths of LINC-NIRVANA. For every exposure the corrective parameters are derived in real-time and transmitted to a dedicated piezo-electric fast linear mirror for simple path lengths adjustments, and/or to the adaptive optics systems of the single-eye telescopes for more complicated corrections. In this paper we present the basic concept and currect status of the opto-mechanical design of the Fringe and Flexure Tracker, the operating principle of the fringe and flexure tracking loops, and the encouraging result of a laboratory test of the piston control loop.

Current and future opportunities for interferometric observations of the Galactic Center in the near- and mid-infrared (NIR/MIR) wavelength domain are highlighted. Main emphasis is being put on the Large Binocular Telescope (LBT) and the Very Large Telescope Interferometer (VLTI). The Galactic Center measurements of stellar orbits and strongly variable NIR and X-ray emission from Sagittarius A* (SgrA*) at the center of the Milky Way have provided the strongest evidence so far that the dark mass concentration at this position is associated with a super massive black hole. Similar dark mass concentrations seen in many galactic nuclei are most likely super massive black holes as well. High angular resolution interferometric observations in the NIR/MIR will provide key information on the central massive black hole and the stellar cluster it is embedded in. These observations have already started: Recent results on the luminous dust enshrowded star IRS3 using MIDI at the VLTI are presented and future scientific possibilities in the GC using MIDI at the VLTI in the MIR and GRAVITY in the NIR are highlighted. As a 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 galactic nuclei including the center of the Milky Way.

During the last two years we have used the Palomar Testbed Interferometer to observe several explosive variable stars, including V838 Monocerotis, V1663 Aquilae and recently RS Ophiuchi. We observed V838 Monocerotis approximately 34 months after its eruption, and were able to resolve the ejecta. Observations of V1663 Aql were obtained starting 9 days after peak brightness and continued for 10 days. We were able to resolve the milliarcsecond-scale emission and follow the expansion of the nova photosphere. When combined with radial-velocity information, these observations can be used to infer the distance to the nova. Finally we have resolved the recurrent nova RS Oph and can draw some preliminary conclusions regarding the emission morphology.

One of the concepts of radio interferometry which is very difficult to apply to the visible domain is phase closure. The main difficulty is the spatial requirement, namely that all pencil beams will interfere with all other beams on a flat detector. We use a pair-wise combination method using anamorphic stretching of the beams. All beams are lined up, imaged through a cylindrical lens into a square where each beam is now spread into a parallel line. The comb of lines is made to interfere with a copy of itself rotated at 90°. A rotation shear interferometer is employed for that stage, and the cross pattern of apertures is imaged on the detector. The diagonal shows interference of each beam with itself, for intensity calibration purposes. An extended source clearly reduces contrast on some off-axis patterns, in a symmetric manner. We have already tested two designs in the laboratory using lasers and white light.

The Keck Interferometer links the two 10m Keck Telescopes located atop Mauna Kea in Hawaii. It was the first 10m class, fully AO equipped interferometer to enter operation. Further, it is the first large interferometer to implement a nuller, whereby the on axis light from a bright point source (e.g. a star) can be removed interferometrically, allowing study of light from nearby, low contrast sources (e.g. exo-zodiacal dust). This paper describes the control system we have implemented to enable operation of the Keck interferometer nuller. We give a general overview of the control system, plus details of how control differs from the already implemented and operational, standard visibility science mode of the interferometer. The nuller is challenging in its requirements for control because of the necessary control precision and the complexity of the number of points of control. We have implemented some novel control methods to meet these requirements and we describe those here.

The VLTI has been operating for about 5 years using the VINCI instrument first, and later MIDI. In October 2005 (Period 76) the first Science Operations with the AMBER instrument started, with 14 Open Time proposals in the observing queues submitted by the astronomical community. AMBER, the near-infrared/red focal instrument of the VLTI, operates in the bands J, H, and, K (i.e. 1.0 to 2.5 micrometers) with three beams, thus enabling the use of closure phase techniques. Light was fed from the 8m Unit Telescopes (UT). The Instrument was offered with the Low Resolution Mode (JHK) and the Medium Resolution Mode in K-band on the UTs. We will present how the AMBER VLTI Science Operations currently are performed and integrated into the general Paranal Science Operations, using the extensive experience of Service Mode operations performed by the Paranal Science operations and in particular applying the know-how learned from the two years of MIDI Science Operations. We will also be presenting the operational statistics from these first ever Open Time observations with AMBER.

Avalanche photodiodes offer many advantages for photon counting in the visible and near IR. However, as with all pulse counting systems, finite response times result in missed pulses as signal levels are increased. Further, APDs build up a pulse by accelerating electrons through large potential drops before being quenched which can result in significant heating with increasing signal levels and subsequent loss of quantum efficiency. Both these effects, heating and deadtime, result in a significantly non-linear response at high signal levels. We report here a combination of simple models of the thermal behavior of the detectors and the finite nature of counting electronics that allows us to account for these efects. We also demonstrate a simple method for measuring off line the parameters of this model. With a relatively few free parameters we are able to restore linearity very close to detector saturation. A silver lining is that the combined loss of quantum efficiency (heating) and detected pulses (deadtime) provides a factor of two gain in incoming signal levels before saturation.

The Michigan Infrared Combiner (MIRC) has been designed for two primary goals: 1) imaging with all six CHARA telescopes simultaneously in the near-infrared, 2) direct detection of "hot Jupiter" exoplanets using precision closure phases. In September 2005, MIRC was commissioned on-sky at the CHARA Array on Mt. Wilson, CA, successfully combining light from 4 telescopes simultaneously. After a brief overview of MIRC features and design philosophy, we provide detailed description of key components and present results of laboratory tests. Lastly, we present first results from the commissioning run, focusing on engineering performance. We also present remarkable on-sky closure phase results from the first night of recorded data with the best-ever demonstrated closure phase stability and precision (ΔΦ = 0.03 degrees).

We present four alternative designs for a near-infrared science beam combiner for the Magdalena Ridge Observatory Interferometer. The candidate designs considered are: (1) a four way pupil-plane combiner, fed by a "fast switchyard" that would be reconfigured at a few minute intervals to select different subsets of six input beams; (2) a four way image-plane combiner, fed by a similar fast switchyard; (3) an eight way pupil plane combiner; and (4) a six way image plane combiner. The criteria by which fully-optimised versions of the designs will be compared include: realistic signal-to-noise and imaging speed, stability and ease of alignment and calibration, cost (including detector costs), and technical and schedule risk to the MROI project.

We present a test bench designed to study the performances of interferometric recombination systems, mainly for direct imaging applications (hypertelescope principle). It aims at comparing the aperture synthesis, Fizeau and densified pupils beam combination schemes. It allows identification of the technical requirements like photometry and cophasing correction of the future imaging recombiners for large arrays. A densified assembly has been designed in the visible wavelengths, using a multi-apertures mask associated with a wavefront sensor. It allows pupil rearrangement and spatial filtering by using single mode fibers. The technical specifications and the conception of the fiber densifier are described here, with a particular attention to the correction of the differential chromatic dispersion.

We present a flexible code created for imaging from the bispectrum and V². By using a simulated annealing method, we limit the probability of converging to local chi-squared minima as can occur when traditional imaging methods are used on data sets with limited phase information. We present the results of our code used on a simulated data set utilizing a number of regularization schemes including maximum entropy. Using the statistical properties from Monte-Carlo Markov chains of images, we show how this code can place statistical limits on image features such as unseen binary companions.

We present a formal comparison of the performance of algorithms used for synthesis imaging with optical/infrared long-baseline interferometers. Five different algorithms are evaluated based on their performance with simulated test data. Each set of test data is formatted in the OI-FITS format. The data are calibrated power spectra and bispectra measured with an array intended to be typical of existing imaging interferometers. The strengths and limitations of each algorithm are discussed.

The Magdalena Ridge Observatory Interferometer (MROI) is a ten element optical and near-infrared imaging interferometer being built in the Magdalena mountains west of Socorro, NM at an altitude of 3230 m. The interferometer is being designed and built by a collaboration which includes the New Mexico Institute of Mining and Technology (NMT) as the prime contractor and center for the technical team, and the University of Cambridge, Physics Department at the Cavendish Laboratory, which participates in the design and executes work packages under contract with NMT. This manuscript serves as a status update on MROI, and will present progress and milestones toward the observatory's first fringes in 2008.

SIM is an astrometric mission that will be capable of 1 microarcsec relative astrometric accuracy in a single measurement of ~1000 sec. The search for terrestrial planets in the habitable zone around nearby stars is one of the main science goals of the project. In 2001, NASA through the peer review process selected 10 key projects, two of which had as its goal, the search for terrestrial planets around nearby stars. The two teams, one led by G. Marcy (UC Berkeley) and one lead by M. Shao (JPL), have an extensive preparatory science program underway. This paper describes the status of this activity as well as the technology status of SIM's narrow angle astrometry capability, to reach 1 μas in a single epoch measure and its ability to average multiple epoch measurements to well below 1 μas.

During the last few years, considerable effort has been directed towards large-scale (> $1 Billion) missions to detect and characterize earth-like planets around nearby stars, such as the Terrestrial Planet Finder Interferometer and Darwin missions. However, technological issues such as formation flying, cryocooling, obtaining sufficient null depth for broadband signals, and control of systematic noise sources will likely prevent these missions from entering Phase A until at least the end of the present decade. Futhermore, a large mission like TPF-I will also need the endorsement of the next Astronomical Decadal Survey to obtain a Phase A start in the next decade. Thus, given the present circumstances, we can expect TPF-I to launch no earlier than about 2020 or even as late as 2025. Presently more than 168 planets have been discovered by precision radial velocity survey techniques, and little is known about the majority of them. A simplified nulling interferometer operating in the near- to mid-infrared (e.g. ~ 3-8 microns), like the Fourier Kelvin Stellar Interferometer (FKSI), can characterize the atmospheres of a large sample of the known planets. Many other scientific problems can be addressed with a system like FKSI, including the imaging of debris disks, active galactic nuclei, and low mass companions around nearby stars. We discuss the rationale, both scientific and technological, for a competed mission in the $450-600 Million range, of which FKSI is an example. Such a mission is essential to develop our community and keep the larger community, including young scientists, engaged in the long-term effort towards the detection of Earth-like planets.

The Stellar Imager (SI) is a UV-Optical, Space-Based Interferometer designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the Universe in general and asteroseismic imaging of stellar interiors. SI is identified as a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar System Connection (SSSC) Roadmap and as a candidate for a "Pathways to Life Observatory" in the Exploration of the Universe Division (EUD) Roadmap (May, 2005). SI will revolutionize our view of many dynamic astrophysical processes: its resolution will transform point sources into extended sources, and snapshots into evolving views. SI's science focuses on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI's prime goal is to enable long-term forecasting of solar activity and the space weather that it drives. SI will also revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes in the Universe. The results of the SI "Vision Mission" Study are presented in this paper. Additional information on the SI mission concept and related technology development can be found at URL: http://hires.gsfc.nasa.gov/si/.

SIM-PlanetQuest is a NASA astrophysics mission that is implementing the National Research Counsel's recommended Astrometric Interferometry Mission (AIM) to develop the first, in-space, optical, long-baseline Michelson Stellar Interferometer for performing micro-arcsecond-level astrometry. This level of astrometric precision will enable characterization of planetary systems around nearby stars and enable a number of key investigations in astrophysics including calibration of the cosmological distance scale, stellar and galactic structure and evolution, and dark matter/energy distribution. This paper provides an update on the SIM-PlanetQuest Mission covering the results of the 2005 mission redesign and the recent completion of the last in a series of technology "gates." The SIM-PlanetQuest mission redesign was directed by NASA to recover eroded mass and power margins and to meet specific implementation cost targets. The resulting mission redesign met all redesign objectives with minimal impact to mission science performance. This paper provides the mission redesign objectives and describes the resulting mission and system design including changes in science capability. SIM-PlanetQuest also completed the last of eight major technology development gates that were established in 2001 by NASA, completing the enabling technology development. The technology development program, the last gate, and its significance to the project's flight verification and validation (V&V) approach are briefly described (covered in more detail in a separate paper at this conference). An update on project programmatic status and plans is also provided.

Optical interferometry will open new vistas for astronomy over the next decade. The Space Interferometry Mission (SIM-PlanetQuest), 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 interferometers 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 disturbance rejection; a multitude of actuators and sensors must operate flawlessly and in concert. The Jet Propulsion Laboratory, with the support of Lockheed Martin Advanced Technology Center (LM ATC) and Northrop Grumman Space Technology (NGST), has addressed these challenges with a technology development program that is now complete. Technology transfer to the SIM flight team is now well along and the project is proceeding toward Preliminary Design Review (PDR) with a quickening pace.

Future space-based optical interferometers, such as the Space Interferometer Mission Planet Quest (SIM), require thermal stability of the optical wavefront to the level of picometers in order to produce astrometric data at the micro-arc-second level. In SIM, the internal path of the interferometer will be measured with a small metrology beam whereas the starlight fringe position is estimated from a large concentric annular beam. To achieve the micro-arc-second observation goal for SIM, it is necessary to maintain the optical path difference between the central and the outer annulus portions of the wavefront of the front-end telescope optics to a few tens of picometers. The Thermo-Opto-Mechanical testbed (TOM3) was developed at the Jet Propulsion Laboratory to measure thermally induced optical deformations of a full-size flight-like beam compressor and siderostat, the two largest optics on SIM, in flight-like thermal environments. A Common Path Heterodyne Interferometer (COPHI) developed at JPL was used for the fine optical path difference measurement as the metrology sensor. The system was integrated inside a large vacuum chamber in order to mitigate the atmospheric and thermal disturbances. The siderostat was installed in a temperature-controlled thermal shroud inside the vacuum chamber, creating a flight-like thermal environment. Detailed thermal and structural models of the test articles (siderostat and compressor) were also developed for model prediction and correlation of the thermal deformations. Experimental data shows SIM required thermal stability of the test articles and good agreement with the model predictions.

This paper describes the high precision Instrument Pointing Control System (PCS) for the Stellar Interferometry Mission (SIM) - Planet Quest. The PCS system provides front-end pointing, compensation for spacecraft motion, and feedforward stabilization, which are needed for proper interference. Optical interferometric measurements require very precise pointing (0.03 as, 1-σ radial) for maximizing the interference pattern visibility. This requirement is achieved by fine pointing control of articulating pointing mirrors with feedback from angle tracking cameras. The overall pointing system design concept is presented. Functional requirements and an acquisition concept are given. Guide and Science pointing control loops are discussed. Simulation analyses demonstrate the feasibility of the design.

The Laser Interferometer Space Antenna (LISA) is a joint NASA/ESA space mission aimed to detect gravitational waves in the 3×10^-5 to 1Hz frequency range. Expected sources for LISA include super massive black hole mergers (SMBH), galactic neutron star and white dwarf binaries, and extreme mass ratio inspirals (EMRI). The three LISA spacecraft will travel in a heliocentric orbit trailing or leading earth by about 20°. The distance between the spacecraft will be about 5 million km or 16s light travel time. Laser interferometry will measure the distance with pm/√Hz accuracy. This report focuses on the technology for LISA interferometry.

The European DARWIN mission aims at detection and characterization of Earth-like exo-planets as well as at aperture synthesis imaging. The method to be applied is nulling interferometry in the mid-infrared wavelength regime. The DARWIN instrument consists of a flotilla of free-flying spacecraft, one spacecraft carrying the optics for beam recombination and three or more spacecraft carrying the large collector telescopes. We provide a trade-off of different configuration, payload, and mission concepts. We discuss various two and three-dimensional aperture configurations with three or four telescopes, beam routing schemes, phase modulation methods, and beam recombination and detection schemes as well as different launch vehicle configurations, launch scenarios, and orbits. We trade the different DARWIN concepts by assessing the performance in terms of science return, development risk, and planning.

This paper reviews recent progress with technology being developed for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. TPF-I is in the early phase of its development. The science requirements of the mission are described along with the current design of the interferometer. The goals of the nulling and formation-flying testbeds are reviewed. Progress with TPF-I technology milestones are highlighted.

We have designed and built a testbed demonstrating an angle sensor that measures the relative angular position between two free-flying spacecraft when used in conjunction with a distance-metrology system. In flight, one spacecraft would carry an LED beacon while the other would carry the sensor system. Our fixed, staring testbed sensor demonstrated a 10 degree capture range with 0.1 arcsec resolution over the inner 1 arcmin of field, and an update rate of over 100 Hz. The testbed showed this performance for simulated spacecraft separations of 100 to 1000 meters.

Interferometer performances are linked to the measurement and the correction of telescope aberrations. For cophasing the large number of beams required by the DARWIN mission with the specified requirements (realtime piston/tip/tilt correction and measurement of higher orders up to spherical aberration), focal-plane approach has been selected due to its simple opto-mechanical device. Several focal-plane algorithms, developed at ONERA and gathered in the stand-alone MASTIC tool, were validated by experiment with a dedicated breadboard on the laboratory test bench BRISE. Our study shows the correct behaviour of the algorithms for linearity and repeatability; specific requirements are reached for piston/tip/tilt and higher order aberrations. These results confirm the validity of focal-plane sensors for the cophasing of multiple-aperture telescopes.

We present a new four-telescope integrated optics (IO) beam combiner in the near-infrared H band, and preliminary photometric and interferometric measurements obtained in laboratory. The combiners tested and characterized in our experiments are at the heart of the VSI/VITRUV instrument, whose goal is to combine four to six telescopes of the VLTI. In this paper, we describe the combiners which incorporate phase-shifting devices and their characterization through the analysis of polarization properties, instrumental visibilities and phases. Our results were obtained with an eight-telescope laboratory interferometer, specially developed to simulate the VLTI. These results demonstrate one more time that the integrated optics technology is particularly well suited for interferometric combination of multiple beams, and therefore to achieve aperture synthesis imaging with the VSI/VITRUV instrument.

This paper presents the development and tests in the thermal infrared of Integrated Optics (IO) technology in preparation of ESA's space mission Darwin. This mission aims to detect and characterize earth-like planets orbiting solar-type stars, using nulling interferometry in the spectral range 6 - 20 μm. Since typically 1:1e6 rejection of starlight is required, wavefront modal filtering is mandatory. Thus, mid-infrared single-mode IO is being developed in the framework of the ESA-funded "Integrated Optics for Darwin" project. Beyond its wavefront filtering capabilities, an IO component may support various optical functions, and is thus likely to ease instrumental design. This paper addresses the manufacturing process and the characterization tests results of newly developed IO devices. Investigated solutions are dielectric waveguides based on Chalcogenide glasses and Hollow Metallic Waveguides. In a first phase, the pre-selected technological solutions were validated and modal behavior of the manufactured devices was demonstrated, both through polarization and spectral analysis. Preliminary nulling ratios up to 5000 have been obtained with an IO modal filter in the 6 - 20 μm range. In a second phase of the project, the development of more complex IO functions was attempted. The methods used to validate the waveguide behavior and interferometric capabilities are also discussed. After achieving 1:1e5 polychromatic extinctions with similar solutions in the near IR, the presented results further underline the credibility of a mid-infrared IO concept for Darwin.

A vital function of the space interferometer foreseen in the DARWIN mission is the so-called "nulling" operation. The challenge of nulling is making the null in the interferometric signal sufficiently deep to cancel the light from the bright star during the collection of light from its surrounding planets. The performance of the nulling is limited by the wavefront quality of the beams. The wavefront error can be reduced by filtering. One promising concept for nulling wavefront filtering is using a single mode fiber. For the wavefront filtering in the DARWIN mission, the fiber has to cover the operational wavelength range of 4-20 μm. Furthermore, a minimal insertion loss is required to ensure a minimum exposure time. This results in the separation of the complete wavelength range into several separate wavelength bands in the nulling system. Within an ESA project, a chalcogenide glass fiber based on the Te-As-Se (TAS) composition is selected to be used for the short wavelength band. TNO has designed and tested several TAS fibers that have been manufactured by the University of Rennes. Single mode operation is demonstrated. Furthermore, the effect of bending the fiber and light coupling are investigated. For the long wavelength band up to 20 μm, Tellurium based glass is proposed. Different samples of various composition based on Te glass are manufactured. Accurate temperature control to avoid crystallization is found to be essential for the manufacturing process. For the bulk material, a transmission window up to 20 μm is measured.

We report our progress in development of extremely coherent single mode fiber bundle arrays for high contrast imaging of extrasolar planets with TPF-C. These bundles with 32x32 fiber arrays are designed to reject scattered light while improving the image contrast by three orders of magnitude in the TPF-C visible nulling coronagraph, studied as one of the five TPF-C instrument concepts. We have developed 10x10 fiber prototype fiber bundle arrays using a combination of precision V-grooves on double side polished silicon wafers, single mode fibers and custom made lenslet arrays. The initial lab results are presented.

A breadboard is under development to demonstrate the calibration of spectral errors in microarcsecond stellar interferometers. Analysis shows that thermally and mechanically stable hardware in addition to careful optical design can reduce the wavelength dependent error to tens of nanometers. Calibration of the hardware can further reduce the error to the level of picometers. The results of thermal, mechanical and optical analysis supporting the breadboard design will be shown.

The delay lines currently under development for the MRO Interferometer will provide up to 380m of optical delay with only 3 reflections. We describe the novel aspects of the delay line design which include using the inside walls of the vacuum pipes as "rails", active shear compensation, and replacing dragged cables with contactless power transfer and communication. We describe the results of tests of various of these design concepts, and progress on the design and construction of the prototype trolley.

We propose a new family of achromatic phase shifters that uses the modulated total internal reflection (TIR) phenomenon. These components can be seen as enhanced Fresnel rhombs for infrared applications like nulling interferometry and polarimetry. The TIR phenomenon comes with a differential phase shift between the polarization components of the incident light. Modulating the index transition at the TIR interface allows compensating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over broad bands. The modulation can be induced by a thin film of a well-chosen medium or a subwavelength grating whose parameters are specially optimized. We present results from theoretical simulations together with preliminary fabrication outcomes.

EOS Technologies and EOS Space Systems have developed a unique and cost effective telescope, enclosure and transport system for imaging interferometry applications, using experience gained over a decade of telescope, enclosure and site equipment design and construction. Telescope and enclosure design details, a unique transport system for relocation for telescopes between stations in an interferometric array, and deployment to difficult sites are shown.

The Space Interferometry Mission (SIM) is a microarcsecond interferometric space telescope that requires picometer level precision measurements of its truss and interferometer baselines. Single-gauge metrology errors due to non-ideal physical characteristics of corner cubes reduce the angular measurement capability of the science instrument. Specifically, the non-common vertex error (NCVE) of a shared vertex, double corner cube introduces micrometer level single-gauge errors in addition to errors due to dihedral angles and reflection phase shifts. A modified SIM Kite Testbed containing an articulating double corner cube is modeled and the results are compared to the experimental testbed data. The results confirm modeling capability and viability of calibration techniques.

The DARWIN mission of ESA will search for earth-like exo-planets orbiting suitable target stars in our solar neighborhood, and will allow direct low resolution spectroscopy of exo-planetary atmospheres. The optical enabling technology for DARWIN is high-contrast destructive Nulling interferometry of stellar light, necessitating utmost symmetry of optical beam trains. We report the system driven design of a cryogenic optical delay line compatible with the extremely tight requirements imposed by optical symmetry. After analysis of requirements and system aspects, we describe the actual design implementations and our validation scheme. We conclude with an outlook on integration of this ODL into a cryogenic ground-based testbed for DARWIN Nulling interferometry.

The Space Interferometry Mission(SIM) will measure optical path differences (OPDs) with an accuracy of tens of picometers, requiring precise calibration of the instrument. In this article, we present a calibration approach based on fitting star light interference fringes in the interferometer using a least-squares algorithm. The algorithm is first analyzed for the case of a monochromatic light source with a monochromatic fringe model. Using fringe data measured on the Micro-Arcsecond Metrology(MAM) testbed with a laser source, the error in the determination of the wavelength is shown to be less than 10pm. By using a quasi-monochromatic fringe model, the algorithm can be extended to the case of a white light source with a narrow detection bandwidth. In SIM, because of the finite bandwidth of each CCD pixel, the effect of the fringe envelope can not be neglected, especially for the larger optical path difference range favored for the wavelength calibration. We eliminate the fringe envelope effect by "projecting away" the fringe envelope, i.e. working in a subspace orthogonal to the envelope signal. The resulting fringe envelope parameters are needed for subsequent OPD estimation in SIM. We show the sensitivities to various errors. The algorithm is validated using both simulation and the fringe data obtained on the MAM test bed.

TNO, in cooperation with Micromega-Dynamics, SRON, Dutch Space and CSL, has designed a compact breadboard cryogenic delay line (figure 1) for use in future space interferometry missions. The breadboard (BB) delay line is representative of a flight mechanism. The delay line has a single stage voice coil actuator for Optical Path Difference (OPD) control, driving a two-mirror cat's eye. Magnetic bearings provide frictionless and wear free operation with zero-hysteresis. The development test programme, including operation at 100 K has been completed. The verification test programme is currently being carried out by Alcatel Alenia Space (in cooperation with Sageis-CSO) and will include functional testing at 40 K. A short design description and the intermediate results of the verification test programme are reported in this paper.

A corner cube (CC) articulation model has been developed to evaluate the SIM internal metrology (IntMet) optical delay bias (with the accuracy of picometer) due to the component imperfections, such as vertex offset, reflection coating index error, dihedral error, and surface figure error at each facet. This physics-based and MATLAB-implemented geometric optics model provides useful guidance on the flight system design, integration, and characterization. The first portion of this paper covers the CC model details. Then several feature of the model, such as metrology beam footprint visualization, roofline straddling/crossing analysis, and application to drive the sub-system design and the error budget flow-down, are demonstrated in the second part.

The primary goal of DARWIN is to detect earth-like extrasolar planets and to search for biomarkers. This is achieved by means of nulling interferometry, using three free-flying telescopes and a Beam-Combiner (BC) hub. DARWIN will be able to perform astrophysical imaging with high spectral and spatial resolution. Should one of Darwin's telescope flyers fail, then Darwin's capability of detecting earth-sized exo-planets is dramatically reduced. However, with only two telescopes the imaging mode can continue operating with minimal performance degradation, thus ensuring mission success. This work describes a trade-off study between four conceptual three-beam BC's, that are capable of performing both as a nuller and as an imager. A proposed breadboard design will demonstrate end-to-end Fringe-Tracking (FT) and Optical Path-Length (OPL) control. The BC concept is based on a pupil-plane (Michelson) beam combination scheme. Pupil-plane imaging BC's offer a large overlap in terms of optical layout with the nulling BC concept, making it possible to develop a combined nulling- and imaging BC. This means that a reduced number of optical components can be used compared to a scheme with separate BC's. The BC concept inherently compensates for unequal OPL's, which in ground-based interferometers is compensated for by long stroke Optical Delay Lines (ODL's).

The technique of wide field imaging for optical/IR interferometers for missions like Space Infrared Interferometric (SPIRIT), Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder (TPF-I)/DARWIN has been demonstrated through the Wide-field Imaging Interferometry Testbed (WIIT). In this paper, we present an optical model of the WIIT testbed using the commercially available optical modeling and analysis software FRED. Interferometric results for some simple source targets are presented for a model with ideal surfaces and compared with theoretical closed form solutions. Measured surface deformation data of all mirror surfaces in the form of Zernike coefficients are then added to the optical model compared with results of some simple source targets to laboratory test data. We discuss the sources of error and approximations in the current FRED optical model. Future plans to refine the optical model are also be discussed.

We present the first interferometric NIR observations of the LBV η Carinae with high spectral resolution. The observations were carried out with three 8.2 m VLTI Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1,500 (MR-K mode) and 12,000 (HR-K mode). The observations were performed in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of 4.0±0.2 mas (Gaussian FWHM) was measured for η Car's optically thick wind region, whereas the Brγ and He I emission line regions are larger. If we fit Hillier et al. model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.3, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγemission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of 128° ± 15° was found, consistent with previous VLTI/VINCI measurements. We find good agreement between the measured visibilities and the predictions of the radiative transfer model of Hillier et al. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.

We have obtained high resolution orbital data with the CHARA Array for the bright star 12 Persei, a resolved double-lined spectroscopic binary, an example of a Separated Fringe Packet Binary. We describe the data reduction process involved. By using a technique we have developed of 'side-lobe verniering', we can obtain an improved precision in separation of up to 25 micro-arcsec along a given baseline. For this object we find a semi-major axis 0.3 of Barlow, Scarfe, and Fekel (1998) [BSF], but with an increased inclination angle. The revised masses are therefore almost 6% greater than those of BSF. The overall accuracy in the masses is about 1.3%, now primarily limited by the spectroscopically determined radial velocities. The precision of the masses due to the interferometrically derived "visual" orbit alone is only about 0.2%. We expect that improved RVs and improved absolute calibration can bring down the mass errors to below 1%.

The use of high-resolution techniques for detecting binary and multiple star systems, such as speckle interferometry, and extensive spectroscopic survey efforts have led to the discovery of stellar systems over a broad range of orbital periods. However, there remains a gap between these two techniques, wherein neither is sensitive to detection of companions. Thus, it is possible that some nearby stars may have companions that have been overlooked. Using the longest baselines of the CHARA Array (~275-330 m), we are examining 158 nearby F and G dwarfs previously included in speckle interferometry and radial velocity surveys. Included in this sample are previously unresolved double- and single-lined spectroscopic binaries that show separated fringe packets measured on two nearly perpendicular baselines to determine true position angle and angular separation. Specifically, we are exploring the spectroscopic sample of Duquennoy and Mayor and include selected systems from the CHARA Catalog of Orbital Elements of Spectroscopic Binaries that have predicted separations that fall in this gap. In addition to the search for new companions, we will attempt to use astrometric data to determine orbital inclination in conjunction with previously determined spectroscopic orbits for accurate mass determination. We intend to utilize the Array to more fully explore the undersampled regime of approximately 5-50 mas to characterize the completeness of the multiplicity in the stellar neighborhood.

We present the results of N-band spectro-interferometric observations of the silicate carbon star Hen 38 (IRAS08002-3803) with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). Our observations of IRAS08002-3803 with baseline lengths of 39-47 m have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ~36 mas (72 R_*) between 8 and 10 μm, while it steeply increases longward of 10 μm to reach ~53 mas (106 R_*) at 13 μm. Neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly - though not entirely satisfactorily - reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.

We describe the results of laboratory experiments, using a mock-up stellar interferometer equipped with specialized hardware, undertaken to measure differential-phase to considerable precision (0.1 mrad) over an octave of bandwidth in the infrared. Differential-phase is a precision technique that can detect subtle temporal changes in the relative (color-dependent) photocenter of an astronomical target - making it useful for direct detection of some hot-Jupiter planets from the ground. The set up described herein was built as part of the Keck Interferometer project.

One of the key components of the planned VLTI dual feed facility PRIMA is the Fringe Sensor Unit (FSU). Its basic function is the instantaneous measurement of the Optical Path Difference (OPD) between two beams. The FSU acts as the sensor for a complex control system involving optical delay lines and laser metrology with the aim of removing any OPD introduced by the atmosphere and the beam relay. We have initiated a cooperation between ESO and MPE with the purpose of systematically testing this Fringe Tracking Control System in a laboratory environment. This testbed facility is being built at MPE laboratories with the aim to simulate the VLTI and includes FSUs, OPD controller, metrology and in-house built delay lines. In this article we describe this testbed in detail, including the environmental conditions in the laboratory, and present the results of the testbed subsystem characterisation.

It is commonly accepted that highly challenging planet finding missions such as Darwin and TPF need precursors on the ground, for both technological demonstration and study of the exozodiacal clouds around potential targets. A first instrument, GENIE, designed to be implemented in the interferometric laboratory of the VLTI, was studied by ESA and scientific/industrial teams. In this paper we present a concept for ALADDIN, an operational nulling instrument to be implemented at Dome C in Antarctica, and discuss the comparison with GENIE from the instrumental point of view. Our preliminary design involves moderate ~1m size telescopes mounted on a 40m long rotating beam allowing baselines up to 30m and feeding a 2-arm nulling beam combiner. When compared to GENIE, the rotating beam design has the advantage of removing the need for both long-stroke delay line and dispersion control equipments. As a side effect, the instrumental arrangement of ALADDIN finds itself more representative of what Darwin will be. Furthermore, critical issues like phase control, photometric balance and instrumental background suppression are expected to be relaxed by the improved atmospheric conditions, lower temperature, and simpler optical trains. Calibration of geometrical stellar leakage will make advantage of the continuously adjustable baseline. As results, a simpler instrument showing improved performance is expected. In conclusion, we see our ALADDIN concept as a valuable alternative to GENIE, with a quite stronger scientific potential and a considerably simplified instrumental design.

The basic advantage of single-mode fibers for deep nulling applications resides in their spatial filtering ability, and has now long been known. However, and as suggested more recently, a single-mode fiber can also be used for direct coherent recombination of spatially separated beams, i.e. in a "multi-axial" nulling scheme. After the first successful demonstration of deep (<2e-6) visible LASER nulls using this technique (Haguenauer & Serabyn, Applied Optics 2006), we decided to work on an infrared extension for ground based astronomical observations, e.g. using two or more off-axis sub-apertures of a large ground based telescope. In preparation for such a system, we built and tested a laboratory infrared fiber nuller working in a wavelength regime where atmospheric turbulence can be efficiently corrected, over a pass band (~1.5 to 1.8 micron) broad enough to provide reasonable sensitivity. In addition, since no snapshot images are readily accessible with a (single) fiber nuller, we also tested baseline rotation as an approach to detect off-axis companions while keeping a central null. This modulation technique is identical to the baseline rotation envisioned for the TPF-I space mission. Within this context, we report here on early laboratory results showing deep stable broad-band dual polarization infrared nulls < 5e-4 (currently limited by detector noise), and visible LASER nulls better than 3e-4 over a 360 degree rotation of the baseline. While further work will take place in the laboratory to achieve deeper stable broad-band nulls and test off-axis sources detection through rotation, the emphasis will be put on bringing such a system to a telescope as soon as possible. Detection capability at the 500:1 contrast ratio in the K band (~2.2 microns) seem readily accessible within 50-100 mas of the optical axis, even with a first generation system mounted on a >5m AO equipped telescope such as the Palomar Hale 200 inch, the Keck, Subaru or Gemini telescopes.

Presented is the design of a nulling interferometer testbed which is capable of maintaining the suppression of a broadband, infrared source in the presence of external perturbations. Pathlength stability is accomplished by introducing a dispersive phase shift which allows light at a SWIR band to be used as a wavefront sensor to stabilize the nulled output of a broadband MWIR channel. Since both channels are common path, fluctuations in OPD observed with the wavefront sensor directly correlate to fluctuations of the nulling passband. Results obtained from the testbed will be useful to future nulling interferometers such as the Large Binocular Telescope Interferometer and the Terrestrial Planet Finder Interferometer which are currently being designed to aid in the search for earth-like planets outside our solar system.

We discuss the previously-reported measurements of a three-beam nulling interferometer without achromatic phase-shifters, using delay lines only. The theoretical rejection ratio of a few thousand has not been achieved experimentally. In order to explain the obtained results, some direct spectral and polarization measurements have been performed. We present here the latest results and discuss some asymmetries in the interference patterns.

Integrated optics can provide compact and robust solutions for ground and space-based interferometry by integrating optical devices with different functionalities, such as spatial filters, combiners/nullers, and phase modulators, on a single chip. Lithium niobate (LiNbO₃) has two distinct advantages over silica-based technologies, including good transparency further into the near-infrared (covering J, H, K, and L bands) and the ability to support electrically-controlled phase modulation through the linear electro-optic (EO) effect. The design, fabrication and preliminary tests of integrated optic components on LiNbO₃ substrates for astronomical beam combiners operating in the H and L bands is reported. The components include single-mode waveguides of sufficient length for spatial filtering, symmetric junctions for wavelength insensitive power splitters/combiners, and electro-optic waveguide modulators for path-length control.

This paper wants to be a practical example in building a real-time data-acquisition and control system from scratch using relatively non-expensive PC hardware and open-source software. The practical example of building the control system for the Michigan Infrared Combiner (MIRC) at the CHARA interferometer will be used to give the reader a 'hands-on' experience in installing and configuring the RTAI-Fusion real-time operating system and developing a complete control system with it.

We present recent results from the Wide-Field Imaging Interferometry Testbed (WIIT). The data acquired with the WIIT is "double Fourier" data, including both spatial and spectral information within each data cube. We have been working with this data, and starting to develop algorithms, implementations, and techniques for reducing this data. Such algorithms and tools are of great import for a number of future missions, including the Space Infrared Interferometric Telescope (SPIRIT), the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), and the Terrestrial Planet Finder Interferometer (TPF-I)/Darwin. Recent results are discussed and future study directions are described.

For temporally modulated fringe patterns, stellar interferometric fringe acquisition rates must generally exceed 1kHz to avoid significant atmospheric related loss of contrast and cross-talk between fringe components. Furthermore, sufficient travel and high waveform stability in the temporal phase modulation are essential to clean fringe visibility extraction. The authors present a system utilising a piezoelectric actuator that takes advantage of a resonating stage to achieve an accurate and stable high amplitude motion. Nanometre accuracy in waveform optimisation and in continuous waveform stability is demonstrated.

Spatial filtering is necessary to achieve deep nulls in optical interferometer and single mode infrared fibers can serve as spatial filters. The filtering function is based on the ability of these devices to perform the mode-cleaning function: only the component of the input field that is coupled to the single bound (fundamental) mode of the device propagates to the output without substantial loss. In practical fiber devices, there are leakage channels that cause light not coupled into the fundamental mode to propagate to the output. These include propagation through the fiber cladding and by means of a leaky mode. We propose a technique for measuring the magnitude of this leakage and apply it to infrared fibers made at the Naval Research Laboratory and at Tel Aviv University. All measurements are performed at 10.5 μm wavelength.

This paper presents the experimental laboratory interferometer simulator implemented to prepare the VSI-Vitruv instrument. In its final phase, VSI-Vitruv will recombine up to 6 beams of the VLTI thanks to a set of planar Integrated Optics (IO) combiners. This simulator will be used to characterize the IO combiner concepts developed for VSI-Vitruv and will allow to optimize the system up to the data reduction and image reconstruction issues. The simulator consists of three sub-systems: a binary object simulator, a VLTI simulator presenting an array of 8 telescopes, the IO beam combiner and a low resolution spectro-imager. After a detailled presentation of the bench, we will present the first validation tests and the performances of the experiment. The characterizations of the combiners developped for VSI-Vitruv are presented in the accompanying paper (Benisty et al.,6268-89).

The Cophasing Sensor (CS), which measures the disturbances between the sub-apertures, is a key component of multiple-aperture telescopes. As multiple-aperture telescopes become more ambitious, requirements for the CS become more demanding: low flux (for stellar interferometers), sub-nanometric accuracy (for interferometric nullers), image with very small contrast (for wide-field telescopes such as spaceborne Earth imagers), larger number of beams (for all applications). Focal-plane sensing is a solution to cope with all these requirements, with a very simple opto-mechanical setup. Two implementations have been investigated at ONERA: phase retrieval, using the sole focal-plane image, and phase diversity, based on the joint analysis of a focal and an extra-focal images. Phase diversity can measure any mode on any source, while phase retrieval is more suited to real-time piston/tip/tilt measurements on an unresolved (or partially resolved) source. To evaluate accurately the performance of CS or other high-resolution devices, ONERA has built a multipurpose bench called BRISE (Banc Reconfigurable d'Interferometrie sur Sources Etendues). BRISE mainly includes an extended scene and a reference point source, a deformable mirror, a focal-plane CS, afocal input/output ports to interface with other instruments, and a general purpose code MASTIC (Multiple-Aperture Software for Telescope Imaging and Cophasing). BRISE has already been (or will be) used for several applications, such as the validation of CSs for Earth imaging or nulling interferometry, or the exploration of advanced nulling techniques. This paper describes the bench and the investigated CSs, the experiments performed on BRISE, and reports main results such as nanometric accuracy or three-beam nulling.

We present the results of differential phase experiments done with data from the Navy Prototype Optical Interferometer (NPOI). We take advantage of the fact that this instrument simultaneously records 16 spectral channels in the wavelength range 550-850nm, for multiple baselines. We discuss the corrections applied to the data, and show the results obtained for Vega and the Be star β Lyrae.

We describe a set of general purpose utilities for visualizing and manipulating optical interferometry data stored in the FITS-based OIFITS data format. This class of routines contains code like the OiPlot navigation/visualization tool which allows the user to extract visibility, closure phase and UV-coverage information from the OIFITS files and to display the information in various ways. OiPlot also has basic data model fitting capabilities which can be used for a rapid first analysis of the scientific data. More advanced image reconstruction techniques are part of a dedicated utility. In addition, these routines allow data from multiple interferometers to be combined and used together. Part of our work also aims at developing software specific to the Michigan InfraRed Combiner (MIRC). Our experience designing a flexible and robust graphical user interfaced based on sockets using python libraries has wide applicability and this paper will discuss practicalities.

We have recently shown the refractive index structure constant C²_n in the visible and near infrared to be a strong function of humidity in the absence of solar insolation effects, in stark contrast to the commonly held assumption that the humidity contribution can be ignored in that waveband. We expand our analysis of the effects of humidity on C²_n as measured across a 100-m long horizontal beam path to include temperature. Also we present a new technique for extracting information on changes in the parameter space of C²_n and local weather variables, which we term Hilbert Phase Analysis (HPA). This methodology, based on extracting the phase of the analytic signal via Hilbert transforms, reveals a wealth of detail that conventional analysis techniques cannot determine. The HPA provides additional confirmation that C²_n is strongly influenced by local humidity in the visible region. We have also found that HPA provides a clear demonstration that humidity competes with temperature in affecting the value of C²_n.

The Hilbert Huang Transform is a new technique for the analysis of non-stationary signals. It comprises two distinct parts: Empirical Mode Decomposition (EMD) and the Hilbert Transform of each of the modes found from the first step to produce a Hilbert Spectrum. The EMD is an adaptive decomposition of the data, which results in the extraction of Intrinsic Mode Functions (IMFs). We discuss the application of the EMD to the calibration of two optical scintillometers that have been used to measure C_n² over horizontal paths on a building rooftop, and discuss the advantage of using the Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.

An astronomical beam combiner combines the two beams of starlight to form white-light fringes. It is desirable that the dispersion of the beam combiner be minimized across the observation wavelength range. We present here an analysis of the phase dispersion from coatings for a symmetric beam combiner. The sensitivity of the dispersion to a slight mismatch in beamsplitter coatings is also studied.

A new observation mode, a survey of temporal coherence in large fields, is described. Longitudinal coherence can arise in many astrophysical scenarios and at different wave lengths, from plasma effects, scattering of electrons on periodic electromagnetic volumes, to Einstein rings and even extraterrestrial unintended signaling. There is a plurality of coherence seeking devices, developed for military purposes, which can be easily adopted for this task. Many are based on unbalanced interferometers, with a path difference that exceeds the white light envelope. They were shown to be able to discern very weak coherent sources in a heavily cluttered environment. For searches where the coherent wave length was unknown, a signal to background ratio of 1:10,000 was demonstrated. At a known wave length (e.g. molecular lines) one can even expect 1:1,000,000 detection ratio. Once such sources are found, they can be better monitored by most astronomical interferometers, whose field of view is usually rather narrow.

The regularized and space-variant Building Block method allow the reconstruction of diffraction-limited aperture-synthesis images from Large Binocular Telescope (LBT) LINC-NIRVANA data. Images with the diffraction-limited resolution of a 22.8 m single-dish telescope can be reconstructed if raw images are taken at several different hour angles. Computer-generated and laboratory LBT interferograms were simulated that are similar to the data which can be obtained with the LINC-NIRVANA beam combiner instrument. From the simulated interferograms, diffraction-limited images were reconstructed with the regularized Building Block method, which is an extension of the Building Block method. We compare the Building Block reconstructions to images obtained with the Richardson-Lucy (RL) method and the Ordered Subsets Expectation Maximization (OSEM) method. Our image reconstruction studies were performed with computer-simulated J-band and laboratory H-band raw data of a galaxy with simulated total magnitudes of J = 16 to 18 and H = 16 to 19, respectively. One of the faintest structures in the images has a brightness of J~25. The simulated reference stars within the isoplanatic patch have magnitudes of J = 20 - 21 and H = 19. All three methods are able to reconstruct diffraction-limited images of similar quality.

Aperture synthesis imaging provides a way to overcome the ambiguities which often exist in the interpretation of single-baseline interferometric visibility measurements. The mid-infrared imager MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment), which was proposed to ESO as a second-generation VLTI instrument, is designed to combine up to four 8.2 m VLTI UTs or 1.8 m ATs while simultaneously providing a high spectroscopic resolution. To demonstrate that MATISSE will allow high-quality interferometric imaging within realistic observation time constraints, we performed an image reconstruction study, for which we simulated the uv-coverage achievable in 3, 5, or 7 nights with 3 or 4 telescopes. As input image for our studies, a protostellar disk image was simulated with the radiative transfer code MC3D¹ . From the simulated visibilities and closure phases, we derived aperture synthesis images using the Building Block algorithm² . The main features of the disk image could be reconstructed in the presence of noise and assuming the sparse uv-coverage achievable within just 3 nights of observations.

We present the design for a near-infrared (JHK) fringe tracker to be used at the CHARA Array, a long baseline optical interferometer located at Mount Wilson Observatory. The CHARA Michigan Phase-tracker (CHAMP) is being fabricated and tested at the University of Michigan and will be transported to the CHARA Array for general use. CHAMP is separate from the science combiners and can therefore be optimized for fringe tracking. It will modulate around fringe center by 1-2λ at up to 500 Hz and calculate phase offsets in real-time using a modified 'ABCD' method . Six pair-wise Mach-Zehnder combiners will phase the entire Array. We give an overview of the optical layout and discuss our design strategy. Components such as the path-length modulators, low-OH fiber transport system, 1024x1024 HAWAII-1 detector, and control computer are discussed.

The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in a real-time closed loop. Being a Fizeau-Interferometer, the LBT provides a large field of view (FoV). The FFTS can make use of the large FoV and increase the sky coverage of the overall instrument if it is able to acquire the light of a suitable fringe tracking reference star within the FoV. For this purpose, the FFTS detector needs to be moved to the position of the reference star PSF in the curved focal plane and needs to precisely follow its trajectory as the field rotates. Sub-pixel (1 pixel = 18.5 micron) positioning accuracy is required over a travel range of 200mm x 300mm x 70mm. Strong are the constraints imposed by the need of a cryogenic environment for the moving detector. We present a mechanical design, in which the Detector Positioning Unit (DPU) is realized with off-the-shelf micro-positioning stages, which can be kept at ambient temperature. A moving baffle will prevent the intrusion of radiation from the ambient temperature environment into the cryogenic interior of the camera. This baffle consists of two nested disks, which synchronously follow any derotation - or repositioning trajectory of the DPU. The detector, its fanout board and a filter wheel are integrated into a housing that is mounted on top of the DPU and that protects the FFTS detector from stray light. Long and flexible copper bands allow heat transfer from the housing to the LINC-NIRVANA heat exchanger.

A first generation of VLTI (Very Large Telescopes Interferometer) focal instruments, AMBER in the near-infrared and MIDI in the mid-infrared, has been already integrated and tested. New and important science results have been obtained. These instruments combine two (for MIDI) or three (for AMBER) beams coming from the eight telescopes installed at Cerro Paranal (four 8-meters and four 1.8-meters telescopes). In order to improve the capabilities of the interferometer and to engage a new scientific prospective, the second generation of VLTI instruments is currently under study. MATISSE belongs to this second generation. MATISSE objective is the image reconstruction. It will extend the astrophysical potential of the VLTI by overcoming the ambiguities existing in the interpretation of simple visibility measurements. It is a spectro-interferometer combining up to four beams with a large spectral coverage ranging from 3 to 25 μm (L, M, N and Q bands). Different spectral resolutions (between 30 and 1500) are foreseen. MATISSE will measure closure phase relations thus offering an efficient capability for image reconstruction. The concept of MATISSE is presented in this paper. The recombination mode of MATISSE is similar to the AMBER beam combination, but has been adapted to the constraints specific to the mid-infrared domain.

Magdalena Ridge Observatory (MRO) Interferometer is a ten telescope optical interferometer array being built on the Magdalena Mountains 20 miles west of Socorro, New Mexico. The interferometer is being designed by collaboration between New Mexico Institute of Mining and Technology and the University of Cambridge. The science mission and requirements have been finalized which has helped to begin engineering design and development culminating in detailed conceptual designs. Some of the proposed hardware and software implementations are currently being tested in the lab. We present an engineering overview of the conceptual design and the proposed hardware and software implementations.

The correction of atmospheric differential piston and instrumental flexure effects is mandatory for interferometric operation of the LBT NIR interferometric imaging camera LINC-NIRVANA. The task of the Fringe and Flexure Tracking System (FFTS) is to detect and correct these effects in real-time. In the fringe tracking concept that we present, differential piston information is gathered in the image plane by analyzing the PSF of a reference star anywhere in the large field of view of the LBT. We have developed and tested a fast PSF analysis algorithm that allows to clearly identify differential piston even in the case of low S/N. We present performance estimates of the algorithm. Since the performance of the FFTS algorithm has a strong impact on the overall sky coverage of LINC-NIRVANA, we studied the required limiting magnitudes of the fringe tracking reference star for different scenarios. As the FFTS may not necessarily operate on the science target, but rather uses a suitable reference star at a certain angular distance to the science target, differences between piston values at the two positions add to the residual piston of the FFTS. We have dealt with the question of differential piston angular anisoplanatism and studied a possible improvement of the isopistonic patch size by the use of multi-conjugate adaptive optics (MCAO). In its final stage, LINC-NIRVANA will be equipped with such a system.

We describe a project for the installation of a visible focal instrument at the CHARA Array, named VEGA for Visible spEctroGraph and polArimeter. This new instrument will further open the visible domain and offer both spectral and polarimetric capabilities at the CHARA Array. It will create a new and unique scientific niche for the CHARA Array, especially in the context of international competition. The combination of the visible domain and high spectral resolution mode combined with a good sensitivity will allow VEGA/CHARA to carve out a new piece of observational phase space and compliment many existing or planned near-infrared interferometers. VEGA will help make CHARA the interferometer with the largest spectral and spatial resolution worldwide.

Interferometry has been intensively done at long wavelengths, starting with the radio interferometers in the years 50 since it was easier to guide radio wavelengths in cable while keeping the phase information or using a local oscillator and a correlator to recombine "a posteriori" the beams over intercontinental distances. In the optical a lot of work as been done at IR and near-IR wavelengths since it was technically easier, or we must say, less difficult to recombine directly the optical beams since the coherence length is larger and the turbulence slower than at shorter wavelengths. Therefore, the visible domain of the electromagnetic spectrum is not covered at the same level than near or mid infrared. Some very nice and important results have been however obtained with the GI2T interferometer in south of France, the Mark III interferometer on the Mount Wilson, USA, the NPOI array in Flagstaff, USA or the SUSI interferometer in Australia. We will present in this paper the science cases of a new but already existing and tested instrument: the REGAIN focal instrument which was designed and built for the GI2T. This instrument, in his CHARA adaptation, called VEGA will open new fields in a wide range of Astrophysical topics only addressable in the visible domain. It will provide a spectral resolution up to 30000 within the spectral range 0.4-0.9 micron and a spatial resolution of less than 1mas for up to 4 telescopes in its X-lambda special configuration. A polarimetric device (SPIN) measuring simultaneously the polarization in 2 directions either circular or linear is also implemented in this instrument. Since VEGA was already tested on the sky on 1.5 m telescopes it is also very well suited for the 1m CHARA array and will only need minor adaptations for the injection of the CHARA beams. This paper will focus on some of the most promising science drivers only possible with this visible instrument.

Stellar amplitude interferometry is limited by the need to have optical distances known to a fraction of the wavelength. We suggest reviving intensity interferometry, which requires far less accurate hardware (~1cm mechanical precision) at the cost of more limited sensitivity. We present an algorithm that uses the very high redundancy of a uniform linear array to increase the sensitivity of the instrument by more than a hundredfold. An array of a hundred ~100m diameter elements can achieve a limiting magnitude of m_b=14.4. Off-line processing of the data will enable such a ground-based facility to transform a two-dimensional field of point-like sources to a three-dimensional distribution of micro-arcsec resolved systems, each imaged in several optical bands. Each system will also have its high resolution residual timing, high quality (inside each band) spectra and light curve, emergent flux, effective temperature, polarization effects and perhaps some thermodynamic properties, all directly measured in a single observation run of such a dedicated facility. Coronagraphy, selectively suppressing large scale structures of the sources, can also be achieved by specific aperture shapes. We conclude that after three decades of abandonment optical intensity interferometry deserves another review, also as a ground-based alternative to the science goals of space interferometers.

A new CCD based tip/tilt detection system was installed in the CHARA array on August 21, 2005. The new system can serve six telescopes simultaneously and is sensitive to a wavelength as long as 1 μm. The tip/tilt camera is based on an E2V CCD39-01, a small (80×80) back illuminated frame transfer device with a pixel size of 24×24 μm². The measured read-out noise and conversion gain of the camera is 6.4 e^- at 384 kpx s^-1 and 1.1 e^-/ADU, respectively at a temperature of -30 C°. Nine quad-pixel channels have been created on the CCD in a 10×10 pixel sub-array close to one of the read out amplifiers. Vignetting on the quad-pixel channels is negligible. Crosstalk between adjacent channels has been eliminated. The image scale on the CCD is 3.46 arcsecs/pixel. The limiting magnitude is expected to be V=12 at 20 ms integration time under good seeing conditions.

The Keck interferometer has its V² science mode open to its astronomical community and the Nuller science mode is maturing in its development. In order to push on and improve the limits of the instrument a program of analyzing the characteristics of its beamlines has begun. The purpose of this endeavor is to understand beamtrain characteristics for assessing and improving overall system performance. In this paper we present some of the initial results from measurements as well as preliminary analyses of polarization and wavefront quality. Polarization measurements were made on the internal beamtrains for two orthogonal telescope azimuth positions. The wavefront test is a static beamline measurement using a Shack-Hartmann sensor to sample the wavefront quality of the beamtrain. Also results from a dynamic beamtrain monitoring scheme is presented that involves measurements from the angle tracking system during on-sky operation.

We have developed an approach for systematically investigating the optical throughput performance of the different segments of a Michelson stellar interferometer, and applied it to the characterization of the Navy Prototype Optical Interferometer (NPOI). We report the results of the first phase of throughput measurements on NPOI, as well as some of the lessons learned. Since the current generation of ground-based optical interferometers all suffers from varying degree of throughput degradation while the dominant causes for throughput loss are expected to vary for each individual instrument, the methodologies and approaches developed here could be of general use for the quantitative characterization of the throughput performance of the different optical interferometers, a prerequisite for its ultimate improvement.

The Keck Angle Tracker (KAT) is a key subsystem in the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. KAT, which has been in operation since the achievement of first fringes in March 2001, senses the tilt of the stellar wavefront for each of the beams from the interferometer telescopes and provides tilt error signals to fast tip/tilt mirrors for high-bandwidth, wavefront tilt correction. In addition, KAT passes low-bandwidth, desaturation offsets to the adaptive optics system of the Keck telescopes to correct for slow pointing drifts. We present an overview of the instrument design and recent performance of KAT in support of the V² science and nulling observing modes of the Keck Interferometer.

Since April of 2004, the MIDI mid-infrared beam combiner has been used on the VLTI of the European Southern Observatory on Cerro Paranal for service and visitor mode observations. All calibrator data taken are in the public archive, and tools are being developed and used by Paranal Science Operations and Garching Data Flow Operations and Quality Control to study instrument performance and provide users with nightly performance parameters. These tools are also available to the visitors of Paranal. We report on strategies and results for the first interferometric instrument which had been offered to the astronomical community in service mode. That operation model, with users not necessarily experts in interferometry, deserves special attention to the issues of successful use and reliability of the data.

We present an outline of the automated alignment system for the 350m baseline Magdalena Ridge Observatory Interferometer (MROI) which will manage the simultaneous alignment of its six principal optical subsystems (telescopes, beam relay trains, delay lines, beam reducing telescopes, switchyards, and beam combiners). Many of these components will be held under vacuum, will be subject to varying thermal loads and will use different coatings (optimized for either optical or near-IR wavelengths). We review the proposed architecture of our scheme and discuss the procedures, tools, and optical analyses we have used to design it.

Knowledge of the environmental conditions in the Very Large Telescope Interferometer (VLTI) is fundamental for assessing the performance of the scientific instruments and sub-systems of the VLTI, as well as for the calibration of measurement biases (e.g. in astrometry). Therefore, four temperature and humidity sensors were installed in the VLTI delay line tunnel and in the VLTI laboratory in September 2004. First results from the analysis of the long-term (18 months) humidity and temperature data measured by this network will be presented, along with a comparison of the temperatures monitored by the VLTI temperature sensors network and correlations with the external data of the Paranal weather station.

The Very Large Telescope Interferometer (VLTI)¹ that coherently combines the four VLT 8.2-m Unit Telescopes (UT's) is on the point to be fully equipped with its dedicated array of Auxiliary Telescopes (AT's). This array includes four 1.8-m telescopes which can be relocated on thirty observing stations distributed on the top of the Paranal Observatory. This array, albeit less sensitive than the array of UT's, is a key element for the scientific operation of the VLTI. Indeed, it will provide the best imaging capability thanks to the many possible baselines (up to 200m), it will be used for the Narrow Angle Astrometry mode which requires long term monitoring and the longest baselines not accessible with the UT's, and it will enable full-time use of the VLTI facilities even when the UT's are used for stand-alone observation. The Auxiliary Telescopes have been designed, manufactured and tested in Europe by the company AMOS (Belgium) under ESO contract. After acceptance in Europe, ESO takes over the responsibility for the transport to Paranal, reassembly and final commissioning. Currently the first three AT's have been put into operation on Paranal while the fourth one is scheduled to arrive at the observatory in August 2006. This paper presents the actual performances of the Auxiliary Telescopes, as measured during the commissioning of the first three AT's. An emphasis is given to the requirements dictated by the interferometer needs, including the ease and accuracy with which the telescopes can be relocated, the excellent image quality, and the nanometer-level stability for Optical Path Length.

The ESO VLT Interferometer (VLTI) is a general-user facility and is operated in service mode (SM) for a large part of the available time. An important aspect of this SM observing mode is the definition of a set of critical observing conditions that must be met at the time of executing the requested observation. There are a number of observing constraints that are specific to interferometric observations, such as the choice of the array configuration and the hour angle at time of observation, which is processed during the scheduling. On the other hand, classical constraints such as the regular seeing or the lunar illumination are less critical for observations using VLTI instruments than for those using classical VLT instruments. In particular, the use of the adaptive optics system MACAO for VLTI observations employing the Unit Telescopes (UTs) ensures a very good image quality even for moderate environmental conditions. However, the exact dependence between environmental conditions, the performance of the MACAO systems, the wavefront quality at the interferometric instruments, and the accuracy of the final visibility, are not yet known in much detail. In order to investigate this dependence we have started to monitor routinely the environmental conditions, the quality of the MACAO systems, the quality of the acquisition images, and the final data product for all VLTI observations since June 2005. Here, we present the details of this study, as well as first statistics and results.

Magdalena Ridge Observatory Interferometer (MROI) is a ten telescope optical interferometer array being built on the Magdalena Mountains 20 miles west of Socorro, New Mexico. A Radio Frequency (RF) survey in the 100 MHz to 3 GHz RF band has been conducted at the site of the interferometer array on the ridge. The RF site characterization plan is to conduct a pre-construction RF survey and document the existing RF background. A post-construction RF survey will also be conducted after installation and commissioning of the MROI to understand quantitatively any changes to the RF environment at the site. This paper describes the instrumentation and methods used for the RF survey and the results obtained to date. With Langmuir Laboratory for lightning research, MRO 2.4m Telescope co-located on the mountain and with the Very Large Array and the White Sands Missile Range facilities also near by; these data are presented as they may be useful for them and other facilities in future. The RF survey is also proposed as a useful tool to better design MROI facility with knowledge and understanding of the environment for RFI/EMC control and mitigation.

Extrasolar planetary systems are assumed as a sample to exhibit random orbital inclinations. The chance exists that a few of the 152 extrasolar planetary systems known to date may have face-on orbits for which the sin i factor will make a stellar-mass companion mimic a planetary-mass object. Such systems may thus harbor a late spectral type stellar companion instead of planets. Using Georgia State University's CHARA Array, we are undertaking an observing program on accessible extrasolar planetary systems that is expected to be completed in 2007. This effort will assist in culling the exoplanet list of some very low-inclination stellar interlopers that may be present. We will also determine the diameters of the central stars in an effort to refine our knowledge of the evolutionary status of the host stars.

ESO's PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at the VLT Interferometer on Cerro Paranal in Chile is expected to be fully operational in only a couple of years from now. With PRIMA/VLTI, it will then be possible to perform relative astrometry with an accuracy of the order of 10 μas over angles of about 10". The main science driver for this astrometric capability is the detection and characterization of extrasolar planets, including (1) the observation of known radial velocity planets and planetary systems to fully constrain their orbital geometry and accurately determine the mass of the planet(s), (2) a search for extrasolar planets around stars which are less suitable for the radial velocity method (for example young and active stars as well as early type stars), and (3) a systematic search around the most nearby stars to detect low mass planets (Uranus or Neptune masses). Preparatory observations of possible target stars, with the aim of identifying nearby suitable astrometric reference stars, have already started with SOFI at the NTT and are described. Furthermore, we compare the goals and prospects of the PRIMA Astrometric Planet Search to other projects aiming at detecting planets astrometrically, mostly from space.

SIM PlanetQuest is a space-borne Michelson interferometer with a nine meter baseline that will survey ~200 stars within 30 parsecs for terrestrial mass planets. Ultra-precise astrometric observations will reveal the gravitational wobble of the target star (due to a planetary companion) against an inertial frame of reference stars located within a 1.5 degree radius. Here, we report the results of multiple Monte Carlo simulations which have modeled SIM's ability to detect and determine the orbital parameters and masses of the terrestrial mass planets around its potential sample of target stars. We find that SIM will detect 80% of the planets in the 60 star sample. Out of those planets SIM detects, we will be able to estimate the masses of at least 50% of the planets to 30%. Whether SIM should observer 60 or 240 stars, will be aided by the results of the Kepler mission which will provide statistics on the frequency of terrestrial-mass planets around solar type stars. By determining the orbital phase of the planet, SIM will be able to assist TPF-C by telling it when to look to ensure that the planet will be outside the TPF-C 62 mas inner working angle. Furthermore, the masses determined by SIM will not suffer from the msini ambiguity inherent in radial velocity surveys.

Visibility measurements with Michelson interferometers, particularly the measurement of fringe contrast, are affected by various atmospheric and instrumental effects, all of which reduce the measured contrast. To compensate for this, stars with known or predictable diameters (calibrators) are observed so that the overall reduction in the visibility can be measured. Objects with the smallest possible diameters are preferred as calibrators, since the predicted visibilities become less sensitive to any uncertainties. Therefore, unreddened, early type stars are usually chosen if they are available because they are relatively bright for a given angular diameter. However, early type stars bring additional complications. Rapid rotation, common with these stars can cause variations in the visibility amplitudes due to oblateness and surface brightness asymmetries that are larger than implied by the usual error estimates. In addition, rotation can introduce significant phase offsets. Using Roche models, von Zeipel theory, and the observed constraints of V, B-V, and v sin i, it is possible to put limits on the size of these effects and even estimate the distribution of possible visibilities. To make this easily available to the community, we are in the process of creating a catalog of possible calibrators, including histograms of the visibilities, calculated for configurations used at a number of observatories. We show the examples of several early type stars which are potential calibrators using parameters appropriate for the Navy Prototype Optical Interferometer.

We demonstrate a new calibration technique that can be applied to multi-spectral interferometric observations. The technique measures a fixed-pattern in squared visibility measurements across the spectral channels of each baseline. Because the fixed-pattern appears to be stable on time scales longer than one night, nightly or weekly averages can be calculated based on observations of calibrator stars. The averaged fixed-pattern values can then be removed from data of target stars. We demonstrate the performance of the calibration technique on actual observations obtained with the Navy Prototype Optical Interferometer and show that the fixed-pattern effects can be suppressed by up to an order of magnitude.