SIM is the first of the new origins missions that combines major advances in technology to enable major advances in science. This paper is a brief overview of the project to serve as an introduction to the many other technical papers presented at this conference.

GAIA is a short-listed candidate for the ESA Cornerstone mission C5, meeting the ESA Survey Committee requirement for an observatory mission, dedicated to astrometry, providing 10 micro-arcsecond accuracy at 15th magnitude. The GAIA mission concept follows the dramatic success of the ESA HIPPARCOS mission, utilizing a continuously scanning spacecraft, accurately measuring 1D coordinates along great circles, in two simultaneous fields of view, separated by a known angle. These 1D relative coordinates are later converted to the five astrometric parameters of position and motions in a global analysis. GAIA will provide precise astrometry and multi-color photometry for all the one billion stars, quasars, and compact galaxies to I equals 20 on the sky. GAIA will additionally provide the sixth phase- space parameter, radial velocity, from a slitless spectroscopic survey of most stars brighter than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are spectacular, with greatest impact in the study of stellar populations and dynamical structure of the galaxies of our local group, and in providing the first complete census of the stars and massive planets in the solar neighborhood. GAIA will revolutionize our knowledge of the origin and evolution of our Milky Way Galaxy, and of the distribution of planetary system around other stars.

The Space Interferometry Mission (SIM) will be NASA's first space-based optical interferometer. SIM will produce a wealth of new astronomical data and serve as a technology pathfinder for future astrophysics missions. The SIM architecture uses a 10-m Michelson interferometer in Earth- trailing solar orbit to provide 4 microarcsecond precision absolute position measurements of stars down to 20 magnitude. The corresponding parallax accuracy allows distance measurements to 10 percent accuracy on the far side of the Galaxy. With high-precision proper motions derived during its 5-year lifetime, SIM will address a variety of science questions relating to the formation and dynamics of our Galaxy. Using aperture synthesis, SIM will image in the visible waveband to a resolution of 10 milliarcsec, and will demonstrate interferometric nulling with suppression of the on-axis starlight to a level of 10^-4. In this paper we present selected topics from the SIM science program focusing on some specific astronomical questions to be addressed.

The Space Interferometry Mission (SIM) promises to revolutionize optical astrometry with its extraordinary astrometric accuracy of 4μas. The fringe phase stability required to provide this accuracy (≈ 0.14°) will also enable a unique and unprecedented capability for high-dynamic-range synthesis imaging in space at optical wavelengths with an angular resolution of typically 10 milliarcseconds. We summarize the characteristics of the imaging mode of SIM and compare it to ground-based synthesis imaging instruments, which operate only at radio wavelengths. In some respects SIM is an optical version of the Westerbork Synthesis Radio Telescope in the Netherlands.

This paper describes the Micro-Precision Interferometer (MPI) testbed and its major achievements to date related to mitigating risk for future spaceborne optical interferometer missions. The MPI testbed is ground-based hardware model of a future spaceborne interferometer. The three primary objectives of the testbed are to: (1) demonstrate the 10 nm positional stability requirement in the ambient lab disturbance environment, (2) predict whether the 10 nm positional stability requirement can be achieved in the anticipated on-orbit disturbance environment, and (3) validate integrated modeling tools that will ultimately tools that will ultimately to be used to design the actual space missions. This paper describes the hardware testbed in its present configuration. The testbed simulation model, as it stands today, will be described elsewhere. The paper presents results concerning closed loop positional stabilities at or below the 10 nm requirement for both the ambient and on-orbit disturbance environments. These encouraging results confirm that the MPI testbed provides an essential link between the extensive ongoing ground-based interferometer technology development activities and the technology needs of future spaceborne optical interferometers.

The Space Interferometer Mission (SIM) will achieve important science objectives of NASA's Origins program by performing astrometric measurements of targets within and beyond our galaxy with a precision of the order of a few microarcseconds. This accuracy can only be attained by carefully balancing sources of astrometric error arising from the systematic accuracy of the interferometer, thermal distortions, vibration caused by spacecraft systems, and errors in knowledge of the spacecraft's altitude and velocity. A rigorous systems engineering approach must be applied to the conduct of trades between these different sources of error and the TRW SIM Study Team has developed a mathematical model of the performance of SIM to support these trades. This paper shows how the model is constructed using simple analytical relationships and then employed to determine the dependence of system performance on a wide range of configuration parameters, such as baseline length, aperture size, and attitude knowledge. These studies indicate where subsystem performance requirements are critical and where requirements may be relaxed with little degradation of overall astrometric accuracy.

The micro-arcsecond metrology testbed (MAM) is a high- precision long baseline interferometer inside a vibration- isolated vacuum tank. The instrument consists of an artificial star, a laser metrology system, and a single- baseline interferometer with a 1.8m baseline and a 5cm clear aperture. MAM's purpose is to demonstrate that the astrometric error budget specified for the Space Interferometry Mission can be met.

The DS3 mission will launch a space optical interferometer into heliocentric orbit, for observation of 50-100 sources on baselines up to 1000-2000 m. The detection threshold will be visual magnitude 12-13, and the angular resolution in the 500-900 nm passband will be approximately ≈100 microseconds. Interesting science targets which could be imaged include: Cyg X-1, Wolf-Rayet stars, and FU Ori stars. With a modest improvement in sensitivity, the structure of the broad line emission regions of a few bright AGNs could be measured.

A metrology subsystem on board the Deep Space 3, a separated spacecraft interferometer mission, is used to determine stellar fringe delay jitter, delay rate, and initial delay. The subsystem implements two capabilities: linear metrology for optical pathlength determination and angular metrology needed to determine the configuration and orientation of the spacecraft constellation. Frequency modulated metrology concept is used to implement high-precision (5nm) interferometric linear measurements over large target ranges (1km). System is made angle sensitive by using an articulated flat mirror at the target.

A separated spacecraft interferometer (SSI) demonstration mission with three spacecraft has been proposed for flight under the NASA New Millennium Technology Development Program. Both the rotation rate of the interferometer about the normal to the plane containing the three spacecraft and the orientation in the plane must be determined accurately in order to permit the detection of white light fringes from sources that have visual magnitudes as high as possible. It presently is planned to use signals from tracking the science object plus other auxiliary information to determine the interferometer rotation rate dθ/dt and the angular position θ . We have investigated a possible supplementary approach that makes use of a combined beacon tracker and narrow-field star tracker on one of the two collector spacecraft. A very small beacon mounted on the other collector spacecraft can be viewed with respect to a reference star nearly 180 degrees away to determine dθ/dt and θ for the interferometer. Beacon/star tracker observations over roughly an hour appear sufficient to determine the sweep rate for starlight fringes in the interferometer to adequate accuracy and to detect the fringes.

Spacecraft trajectories for providing adequate image plane (u-v)coverage for NASA's Deep Space 3 (DS3) mission have been proposed. Due to the high cost of time and fuel in synthesizing images using DS3, this paper addresses time and fuel optimal trajectories. Using the Point Spread Function (PSF) of a densely-filled coverage as the basis for comparison, the locations of a given number of u-v points are selected to best replicate this reference PSF. Subsequently, the minimum time and fuel in a 'Stop and Stare' mode are determined. Finally, these result are compared to several other trajectory options.

The interferometry program experiment (IPEX) I and II are a series of flight experiments designed to characterize microdynamics of structures in space. These technology demonstration flight experiments are precursors to the Space Interferometry Mission (SIM), Next Generation Space Telescope (NGST) and Terrestrial Planet Finder (TPF), and will address the missions' nanometer-level structural stability requirements. Of particular interest is potential thermal snapping when space structures undergo rapid thermal variations, such as a sun-to-shade transition. This information is needed to characterize uncontrollable high frequency disturbances, and to validate structural designs and modeling approaches for joint-dominated extruding structures. Another objective of the experiments is to characterize typical mechanical disturbances of spacecraft while on-orbit for the purpose of modeling and disturbance response prediction for future optical space missions. Both experiments are performed on the German DARA/DASA free- flying platform Astro-Spas, which is sorted out of the shuttle and retrieved after an independent 10-day mission. IPEX-I, performed during the STS-80 mission in December 1996, characterized the on-board dynamics of the spacecraft during normal operations and quiescent periods. IPEX-II, performed during the STS-85 mission in August 1997, monitored the microdynamic behavior of a representative 9- bay AEC-ABLE mast. The flight data demonstrates the existence of transient microdynamic events that are correlated with temperature transitions. However, the overall spacecraft flight disturbances and broadband boom stability meet the requirements of precision space optical systems.

Optical and IR interferometry will open new vistas for astronomy over the next decade. Space based interferometers, operating unfettered by the Earth's atmosphere, will offer the greatest scientific payoff. They also present the greatest technological challenge: 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 interferometry technology program at NASA's JPL is addressing these challenges with a development program that plans to establish technology readiness for the Space Interferometry Mission by early in the year 2001.

Darwin was proposed in 1993 to the European Space Agency as a mid-IR (5-30 micron) interferometry observatory with baselines greater than 50 meters. It would be a long-duration general purpose radiatively cooled observatory, to be launched in the 2009-2018 timeframe. Since then ESA has started a study of such a mission, called the Infrared Space Interferometer (IRSI), as one of its candidate Cornerstone missions in its Horizons 2000 plan. This paper describes some of the aspects of the Darwin concept as presently conceived by the members of the Darwin Informal Team. This team is comprised of the original proposal authors and a number of additional persons.

A major design trade for the Terrestrial Planet Finder is the choice between a structurally connected and a separated spacecraft interferometer architecture. Three metrics are applied to the comparison and serve to show which aspects of each architecture drive their design. The primary metric is the wet mass for which reference designs are discussed and the relevant mass drivers are examined. Capability and adaptability metrics are used to include more qualitative design comparisons, with the result being a more complete understanding of the trade-offs involved.

We consider a possible precursor interferometer to Terrestrial Planet Finder. The precursor called Planet Discoverer Interferometer (PDI) would search for broadband 10 μm radiation from possible terrestrial planets orbiting stars out to a distance of 8-10pc and at an angular separation of at least 0.1 arcseconds. There are about 20 stars of types A,F,G and K around which an Earth-analog might be detected. PDI would be able to confirm such planets by seeing their orbital motion. PDI would also be able to observe 5 μm radiation from the more massive and younger gas-giant planets around stars up to distances ∼ 150 pc, separated from their star by more than 0.05 arc seconds. It would also see the re-radiated thermal radiation of Jupiter-like planets at temperatures above ∼130K. The device would be a 15m long truss with four SIRTF-like telescopes. It would need to be in a SIRTF-like Earth-trailing orbit, and would be radiatively cooled. A very preliminary design suggest that PDI could fit into the shroud of a Delta II rocket. Similar preliminary calculations suggest that the total lifetime cost of such a mission would be under $300M. Detailed studies of this concept are in process.

The goal of NASA's Terrestrial PLanet Finder program is to detect Earth-size planets orbiting other stars and evaluate their ability to sustain life. This will be accomplished through spaced-based infrared interferometry using baselines much longer than previously flown. This paper presents the technical trades being evaluated by TRW to implement this investigation. Two primary concepts are considered: a single monolithic deployed interferometer with a baseline of up to ∼100 meters and a free-flying constellation of interferometer components featuring precision station-keeping over baselines of up to several kilometers. Exo-planet detection is best performed at ∼ 10 micron wavelength requiring the instrument to operate at cryogenic temperatures to minimize the effects of telescope thermal emissions. Further improvements in sensitivity can be facilitated by flying on a deep space trajectory away from the Sun to reduce zodiacal background emissions. Numerous technical innovations are necessary to enable such a system; however, most of these technologies are being developed by existing programs and there should be no roadblocks to fielding such a terrestrial planet finder in the 2010-2020 time frame.

We propose a concept for a space mission designed to make a survey of potential zodiacal dust disks around nearby stars in the mid-IR. We show that a 10-meter baseline nulling interferometer with two 0.6-meter apertures located in a 1 X 1 AU heliocentric orbit would allow for the survey of about 400 stars in the solar neighborhood and permit a first order determination of the disk inclination and of the dust density and temperature radius dependence. The high dynamic range of the instrument may also be used to study an additional astrophysical phenomena. Beyond its own scientific merit, such a mission would also serve as a technological precursor to a larger interferometer of the type being considered for the detection of earth-like planets.

Astronomical interferometry at the JPL has grown rapidly in the last two years. JPL is now engaged in a number of interferometry projects and is also developing a number of internal testbeds to support those projects. While each of these projects and testbeds has its own unique properties, they do share a lot of common features, and JPL is striving, through its interferometer technology program (ITP), to develop common components, software, and hardware that can be reused by multiple projects. The discipline where this commonality is probably most apparent is in the area of realtime control systems, specifically the software and electronics that drive the instrument control loops and sequence the subsystems. To this end, within the ITP, JPL has developed the realtime interferometer control systems testbed (RICST) as a facility where a common software and electronics core, essentially a control system for a generic interferometer, can be developed. The realtime control (RTC) team in the ITP program consists of about 20 full-time equivalent engineers, technicians, quality assurance personnel, architects, and managers. The remainder of this paper will describe the interferometry landscape at JPL, the RTC effort, an overview of the RICST testbed, and the generic interferometer control system architecture that has been developed.

The Terrestrial Planet Finder (TPF) will detect and characterize Earth-like planets around nearby stars. NASA is currently funding a number of small studies to look at trade-offs in the design of TPF. The possible trade-offs include orbit location (1 to 5 AU), aperture size (4 to 2 m), and physically connected baselines vs. separated spacecraft flying in close formation. The performance of TPF depends critically on the brightness of the local zodiacal dust cloud at the observing site, and on the brightness and degree of structure in the zodiacal dust cloud around other stars. Sensitivity calculations indicate that TPF could accomplish its goals using 4-5 m telescopes operating at 1 AU. Such a mission would have many advantages relative to a mission operating smaller telescopes in lower background conditions at 5 AU.

The Sydney University Stellar Interferometer (SUSI) is a long baseline optical interferometer located at the Paul Wild Observatory in northern New South Wales, some 400km NNW of Sydney. SUSI has been designed to measure the angular sizes of stars of essentially all spectral types and luminosity classes and to measure the angular separations of close binary stars. In addition to the science programs planned for SUSI, the technical features of the instrument dictated by these programs are discussed. The current status of the instrument and science programs, and the plans for further development of the instrument are described.

The Magellan Project is building two 6.5-m telescopes, 60 m apart, at the Las Campanas Observatory in Chile. There are on-going plans to combine the beams of the two main telescopes, and of smaller auxiliary telescopes, for interferometric measurements. In this paper we consider the array of auxiliary telescopes as a stand-alone instrument, recognizing that it will operate as such for some large fraction of the time. Our interest is sharpened by the availability of six 1.8-m optical systems, retired from the Smithsonian-Arizona Multiple-Mirror Telescope in preparation for the installation of a single-mirror 6.5-m system. We have completed a design for a 1.8-m telescope, in which the MMT components are supported on a proven tripod mount. The optics-support uses steel for stiffness, and low-thermal- expansion rods for passive stability. This array will be a powerful tool for the investigation of stellar limb darkening, surface features, and changes of diameter in pulsations, as well as dust disks, shells, and binary companions. The 1.8-m telescopes on good sites such as Magellan's should be able to operate at full aperture for interferometry at 2.2 micrometers . They should therefore be able to reach to magnitude K equals 10 or so, and thus to cover substantial samples of both main-sequence and pre-main- sequence stars, and of fully evolved stars as well.

This paper reviews the current performance of the Cambridge Optical Aperture Synthesis Telescope as an imaging array. Tests of the hardware and methods of measuring fringe visibility and closure phase are described in the context of prospects for a Large Optical Array.

There are difficult problems involved in building a near-infrared interferometer which uses more than two elements simultaneously. These problems have been overcome at the Cambridge Optical Aperture Synthesis Telescope(COAST). This has allowed us to make the first closure phase measurements on an astronomical source in the infrared. We describe a scheme for fast, low noise readout of an infrared focal-plane array detector, capable of adequately sampling pupil plane fringes on three simultaneous baselines, as well as a procedure for aligning a many-component beam-combiner in the infrared. Finally, the performance of the working COAST infrared system is discussed.

A low-resolution CCD spectrometer has been installed at COAST to provide multi-wavelength fringe measurements across the band 650-950 nm. The measurements are based on the analysis of time-series of channeled spectra. Laboratory tests and stellar observations are presented. The advantages and limitations of the system are discussed.

Visibility measurements obtained with optical astronomical interferometers are corrupted by random wavefront distortions of atmospheric origin. In this paper we discus how spatial filtering using pinholes can lead to increased measured visibility, improved signal-to-noise ratio and reduced dependence on seeing fluctuations. The potential for calibrating visibility measurements without resorting to a separate calibrator target is also discussed. Results of preliminary pinhole experiments carried out with the Cambridge Optical Aperture Synthesis Telescope are presented.

The Keck Interferometer is being developed by JPL and CARA as one of the ground-based components of NASA's Origins Program. The interferometer will combine the two 10-m Keck telescopes with four proposed 1.8-m outrigger telescopes located at the periphery of the Keck site on Mauna Kea. Incorporation of adaptive optics on the Keck telescopes with cophasing using an isoplanatic reference provides high sensitivity. Back-end instrumentation will include two-way combiners for cophasing and single-baseline measurements, a nulling combiner for high-dynamic range measurements, and a multi-way imaging combiner. Science objectives include the characterization of zodiacal dust around other stars, detection of hot Jupiters and brown dwarfs through multi- color differential-phase measurements, astrometric searches for planets down to Uranus-mass, and a wide range of IR imaging.

We present and discuss the possibility to coherently couple several (up to seven) large telescopes located on the Mauna Kea summit, in order to obtain interferometric capabilities at visible and infrared wavelengths. The advent of adaptive optics and single mode optical fibers allows the use of telescopes which were not built for interferometric use. The diameter of the telescopes, the orientations and lengths of potential baselines can lead to impressive performances in terms of sensitivity and angular resolution, with a negligible site impact.

Laser guide stars have been used successfully as a reference source for adaptive optics systems. We present a possible method for utilizing laser beacons as sources for interferometric phasing. The technique would extend the sky coverage for wide baseline interferometers and allow interferometric measurement and imaging of dim objects.

The VLTI (Very Large Telescope Interferometer) is one of the operating modes of the VLT, presently being built on Cerro Paranal, Chile. It aims at providing access to an observing mode at very high angular resolution and very high sensitivity (with respect to the currently operating astronomical interferometers). After a long period of conceptual, then detailed, studies, ESO is starting to build and to procure the main components of the interferometer in order to open this unpaired observing facility by the turn of the century.

PRIMA (instrument for Phase-Referenced Imaging and Microarcsecond Astrometry) is a conceptual study for a single-baseline dual-feed instrument for the Very Large Telescope Interferometer (VLTI), which is under construction by the European Southern Observatory on Cerro Paranal in Chile. The goals of PRIMA include narrow-angle astrometry with a precision of 10 μas over an arc of 10", and imaging of faint sources with the full sensitivity of the 8 m telescopes in the VLT array. Key scientific programs that can be carried out with PRIMA in imaging mode include observations of active galactic nuclei, the Galactic Center, stars, and circumstellar matter. Scientific drivers for the astrometry are searches for planets and low-mass stellar companions, binary stars, dynamics of clusters, and (relative) parallaxes. We list the main performance requirements for PRIMA, present system architectures for the dual-beam system, and discuss limitations of the interferometric field-of-view.

By the middle of the year 2000 the VLT interferometer will become operational. The design of the telescopes, Coude trains and delay lines is finished and the different beam combination instruments for use in the near and mid-IR are under design. Our goal is to understand this complex machine in full detail and analyze the influence of the several subsystems on the data formation process. ESO has already started to model the interferometer from an optomechanical point of view which resulted in the VLTI end-to-end model. The end-to-end model includes the complete optical trains for the 4 unit telescopes and up to 4 auxiliary telescopes located on nay of the 30 telescopes stations. The optical trains consist of 16 mirrors from the primary mirror to the beam combination lab. The model also includes seismic noise, wind load, atmospheric effects, delay line control, fringe sensor unit, and fast image stabilization. The output of this model enters the scientific simulations, where the beam combination instruments are modeled. We will explain the underlying philosophy of our model and show first illustrations from the simulations.

A variable curvature mirror is a powerful device that can increase the field of view of optical interferometers. Such a mirror has being developed for the coherent combined focus of the European Southern Observatory Very Large Telescope Interferometer. The variable focal length permits positioning of the pupil image of an individual telescope at a precise location after the delay-line. This property is necessary to exactly remap homothetically the output pupil configuration at the image beam combiner. Given the large zoom range that is needed in the delay line, when the mirror is not stressed the optical surface is a plane while it is convex with f/2.5 at maximum stress. The mirror itself is a very small stainless steel meniscus, with a 300 μm thickness, because only the high elasticity of such material allows to achieve the full domain of curvature. The thickness distribution of the meniscus is calculated using elasticity theory in the case of a large deformation. The realization of this micro-optic active device requires advanced techniques in optical fabrication and in particular high precision manufacturing with numerical command lathe. This article also presents the testing of this highly Variable Curvature Mirror (VCM) and the surface quality obtained within the full curvature range.

The IOTA interferometer has been in scheduled operation since 1995 with two 0.45 m telescopes, and baselines of up to 38 m; a third telescope will be installed in 1998 and three-beam combination will be implemented in 1999. We currently have three major detection systems, built around a CCD, a NICMOS3, and the FLUOR fiber beam combiner. A piezo- driven rapid scan delay is also available. Our experience in using these systems is reviewed here, and the corresponding magnitude limits of fringe measurements in the V, R, I, J, H, K bands are discussed. Recent result from these new detector systems include measurements of stellar diameters and temperatures, stellar surface features, and circumstellar dust emission.

The FLUOR project started in 1991 with a prototype fiber recombination unit that transformed a pair of independent 80 cm telescopes into a stellar interferometer. An improved version of this unit is now used as part of the instrumentation at the IOTA interferometer on Mt Hopkins (Arizona). The system is based on fluoride glass single-mode waveguides (non polarization-preserving) for observations at infrared wavelengths between 2 and 2.4 μm. A triple coupler performs the coherent recombination of the beams and extracts two calibration signals. A passive polarization control is sufficient to maintain the interferornetric efficiency above 80 %, with variations of the order of a few percents from one night to the next. The combination FLUOR/JOTA now routinely provides stellar interferograms on baselines ranging between 5 and 38 m, with an accuracy of 1 % or better in the fringe visibility measurements.

The Palomar Testbed Interferometer (PTI) is an infrared, phase-tracking interferometer in operation at Palomar Mountain since July 1995. It was funded by NASA for the purpose of developing techniques and methodologies for doing narrowangle astrometry for the purpose of detecting extrasolar planets. The instrument employs active fringe trackingin the infrared (2.0-2.4 μm) to monitor fringe phase. It is a dual-star interferometer; it is able to measure fringes on two separate stars simultaneously. An end-to-end heterodyne laser metrology system is used to monitor the optical path length of the starlight. Recently completed engineering upgrades have improved the initial instrument performance. These upgrades are:extended wavelength coverage, a single mode fiber for spatial filtering, vacuum pipes to relay the beams, accelerometers on the siderostat mirrors and a new baseline. Results of recent astrometry data indicate the instrument is approaching the astrometric limit as set by the atmosphere.

The Palomar Testbed Interferometer is a long-baseline, near-infrared astronomical interferometer capable of visibility measurement and narrow-angle differential astrometry. In this submission we consider the problem of fringe amplitude calibration as applied to the study of single and binary star systems with PTI. Methodologies for selecting appropriate calibration objects, and performing the calibrations to produce consistent multi-night datasets are considered. Applications of such calibrated datasets to binary orbit determination and stellar diameter measurements will be presented.

The Large Binocular Telescope (LBT) has been designed for optical/infrared interferometry that combines high sensitivity and resolution. Key scientific projects will be deep, wide field infrared images of the Hubble Deep Field, with nearly ten times the resolution of the Hubble telescope, and the study of planets and dust in extra-solar systems, from their formation onward. A basic requirement for interferometry of faint objects is that the aberrations across the two 8.4 m telescopes be corrected for atmospheric phase errors. This will be done at the telescopes' secondary mirrors, so as to preserve the very low emissivity of the direct beam combination optics. Sodium lasers projected co-axially from above each secondary will allow wavefront sensing for correction of even the faintest objects. The two telescopes are rigidly mounted close together on a single alt-azimuth mount, to cover a large fraction of the u-v plane in a single exposure, with baselines continuous from 0 to 23 m. Field rotation during the night completes the cover, to allow recovery of images with the full resolution of a diffraction limited 23 m telescope. The beam combining optics will be cryogenically cooled to maintain the very low thermal background from only 3 warm reflections in total (primary, adaptive secondary, tertiary). For wide field imaging, the beams will be combined and stabilized so that in a long exposure every source across a ∼ 1 arcminute field is crossed by nterference fringes. From a set of such exposures the resultant deep image will have a resolution 0.02 arcsec in the 2.2.μm K band. For high contrast studies of exo-planetary systems, a Bracewell nulling system will be used with superposition by division of amplitude, for 99.99% suppression of the stellar radiation.

A two multi-r_o telescope interferometer was built at Air Force Research Lab in Albuquerque New Mexico as a development testbed. The principal objective of this testbed is to develop existing techniques and to test novel low-cost technologies for applications in future interferometers. These technologies include a tip/tilt piston mirror that has a 500-Hz bandwidth with a 200-wave adjustable piston capability at 633nm. This type of mirror has been installed on both telescopes and is used to track objects and scan for fringes. The data obtained on these objects will be used to determine algorithms for measuring fringe visibility at low light level. Additional technologies include liquid crystal devices that have been used to correct static aberrations in the optical system and will be used with a new wavefront sensing technique to correct low order atmospheric aberrations. The new wavefront sensor currently being developed in-house uses a GEN III intensifier optically coupled to a Dalsa camera to provide atmospheric correction on faint extended objects. The testbed will also be utilized to test single mode fiber optics as a replacement to traditional recombining optics. This will potentially reduce the cost and simplify the alignment of multi telescope interferometers.

We present results on the application of integrated optics to singlemode near infrared astronomical interferometry. We review the laboratory optical properties of a two-telescope beamcombiner made by ion exchange on glass substrate. We report simultaneous detection of white-light interferograms with photometric calibration on the same detector. These encouraging results lead us into the design and the development of IONIC: an Integrated Optics Near-infrared Interferometric Camera. Finally we briefly discuss future developments of integrated optics applied to near-infrared interferometry carried out at LAOG.

The Infrared Spatial Interferometer (ISI) is an interferometer installed on Mt. Wilson and operating in the 10 μm wavelength region, using heterodyne detection and two movable 1.65 m telescopes. Its general technology and characteristics, recent changes, and observational results are broadly discussed. Some compensation for atmospheric path length fluctuations is demonstrated. Stellar observations show, among other characteristics, that many stars emit gas and dust episodically with times of 10-100 years between events, and that stellar diameters measured in the mid-infrared region are about 10 percent larger than those measured with interferometry using visible light.

The UC Berkeley Infrared Spatial Interferometer (ISI) is a heterodyne stellar interferometer which operates in the mid-infrared between 9 and 12 microns. This wavelength range makes the ISI particularly well-suited to high resolution study of late-type stars and other objects surrounded by relatively cool dust, the emission from which peaks in the mid-infrared. Unfortunately, this same dust tends to reduce the amount of visible light available from the sources, making many interesting infrared objects too faint for the original telescope guiding system, which used a silicon CCD camera. This system has been replaced by a guiding and tip-tilt correction system based on an InSb IR camera and a fast, controllable tilting mount for one of the mirrors in the signal path. The new system has improved the quality of ISI fringe visibility data on bright sources and allowed the study of previously inaccessible objects.

The Navy Prototype Optical Interferometer contains two major subarrays: one for imaging the surface of stars, the other for measuring the precise positions of bright stars. In order to image the surface of stars, six 50 cm elements which can be reconfigured to sample many spatial frequencies using 32 spectral channels equally spaced in wavenumber between 450 and 850 nanometers. The imaging array elements are distributed on a 'Y' with a maximum spacing for imaging of 437 m, yielding a spatial resolution of 0.2 mas. The positions of stars will be measured using four fixed 50 cm siderostats with element spacings from 19 to 38 meters. The delay line compensation and beam paths are in vacuum to eliminate the effects of atmospheric dispersion. The astrometric goal is an accuracy of 2 mas over wide angles.

The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University is building an interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The "CHARA Array" will initially consist offive 1-rn diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. The facility is being constructed on Mt. Wilson, near Pasadena, California, a site noted for stable atmospheric conditions that often gives rise to exceptional image quality. The Array will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared (2.2μm) spectral regions. This project is being funded in approximately 50/50% shares by Georgia State University and the National Science Foundation. The CHARA Array is expected to become operational during 1999. This paper presents a project status report. An extensive collection of project reports and images are available at our website (http://www.chara.gsu.edu).

The telescope requirements of optical interferometry are somewhat different from conventional astronomy. The need for multiple units (in the CHARA case initially five, eventually seven) accentuates the importance of cost control, and at the same time provides opportunity for cost savings by careful procurement and production practices. Modern ideas about telescope enclosures offer significantly reduced dome seeing, but it is difficult to capture these benefits at low cost. The CHARA group has followed a series of design and bid procedures intended to optimize the costperformance of the telescope+enclosures. These have led to a compact but massive telescope design, blending modern and classical features, an unusual mirror blank selection process (directly ompeting several mirror blank technologies) , and a novel telescope enclosure concept which allows a continuous trade between wind protection and natural ventilation. This contribution will review and motivate the design decisions and show the resulting equipment and facilities.

The 'densified pupil' imaging mode, now developed for large multi-telescope interferometers, will provide images and spectro-images of compact objects, directly at the recombined focus. It requires telescopes of identical sizes and allows trading field for luminosity. The principle is applicable to dilute arrays of small, medium or large telescopes, 0.2m to beyond 25m in size, using similar recombination systems and cophasing methods. Design solutions are discussed for each case, and particularly for the medium-scale Optical Very Large Array of 27 telescopes, spanning one kilometer, studied at Haute Provence. We build a prototype 1.5m OVLA element. Solutions for the beam recombiner are discussed, and will be assessed with a testbed interferometer involving 27 small mobile heliostats forming a 100 or 300m ring. Larger versions of the OVLA, employing unit telescopes of 10 to 25m, are also considered, in connection with the large telescope study initiated by the Lund group. In space, arrays of free-flying telescopes can in principle resolve continental detail of exo-planets. Equipped with additional out-rigger mirrors providing baselines of 10,000 to 100,000 kilometers, such space arrays can in principle provide images of pulsars and naked neutron stars.

We describe the development, functional performance, and space-qualification status of a Metrology Source suitable for implementation of space-based metrology systems with picometer-level relative displacement measurement and micron-level absolute displacement measurement resolution. The Metrology Source consists of the following components: lasers, frequency stabilization system, frequency shifters, and frequency modulators. All components are interconnected by polarization maintaining fibers to facilitate integration into a lightweight space-qualifiable module.

Heterodyne interferometer laser gauges are used in space- based astronomical interferometers to very accurately measure and compensate for variations in starlight pathlength. Bragg cells have been traditionally used to generate the heterodyne signal by shifting the frequency of the laser light. This paper presents the development and qualification of an integrated optic frequency shifter (IOFS) which offers improved performance and reliability compared with Bragg cell technology. The most critical advantage of the IOFS for space applications is that it enables fiber optic metrology source integration, which facilitates the integration process and result in more reliable and compact heterodyne interferometer laser gauges.

The first imaging devices of optical interferometry are likely to be of weak phase, typically: a set of three- element arrays, coherent and stable, independently observing the same object. The study of their imaging capabilities essentially addresses the self-calibration problem and its stability. Like in VLBI, the principle of our self- calibration methods consists in preforming a series of alternate phase calibration operations and Fourier synthesis processes. Algebraic graph theory and algebraic number theory prove to be the key topics involved in the phase calibration operation. The latter can often be written in closed form. As expected, the relative expressions explicitly refer to a set of independent closure phases. To illustrate this essential point, we consider the special case of three-element arrays. The corresponding phase calibration formula, which is then particularly simple, provides all the elements for coping with the possible global instabilities. The Fourier synthesis process, which is also involved in the self-calibration cycles, is performed via WIPE, a methodology recently introduced in radio imaging and optical interferometry. The robustness of the image reconstruction process can then be well controlled.

One of the critical components of a separated element interferometer is the beam combiner. The initial alignment of the separate optical elements that make up this device and the maintenance of that alignment is usually problematic. Fiber optic devices provide an answer to the alignment difficulties but in single mode form have a restricted bandwidth. This paper discusses the design of a number of devices to overcome these short comings. These beam combiners can be small in size, their dimensions largely governed by the beam diameter. Large diameter beams are only necessary to reduce diffraction effects on the journey from the telescope to the beam combiner. On arrival the beam diameter can then be reduced to suit the beam combiner. Small, stable and low weight beam combiners are an advantage on the ground but even more so in space applications. Designs for combining the beams for large numbers of telescope are described.

In this paper we present a status report of our single mode (SM) fiber work for interferometry. In the past two years two experiments have been carried out by our group at the Maui Space Surveillance Site linking two 1.2 meter telescopes using single mode fibers and obtaining white light fringes on (alpha) Bootis. During this experiment we were able to measure coupling efficiency between atmospherically degraded images, D/r₀ approximately 10, and the fibers using a tip/tilt mirror on each of the two telescopes. We compared our results with theoretical calculations. Furthermore we will present some of the experimental result and 'lesson learned'. We will also present the progress on our multi-core single mode fibers program. We have a first four core prototype under test and preliminary results will be presented.

The angular resolution limit imposed by atmospheric seeing may be improved upon by the application of adaptive optics to short exposure images from a single telescope. However, it has not been possible to produce diffraction limited images in the invisible bandpass by this technique. We are developing a dilute aperture, single mode fiber interferometer with adaptive optics to reach the diffraction limit of a large telescope. The spatial filtering property of such fibers causes incident wavefront aberrations to be rejected, so that the visibility of the fringes is unaffected by atmospheric distortions. Using phase closure techniques, the multiple vector baselines of the dilute aperture will yield diffraction limited images.

This paper introduces a new method for tracking fringes in ground-based optical stellar interferometry at low light levels under conditions of variations in OPD due to atmospheric turbulence. Like the group delay tracking (GDT), the method involves processing of short-exposure frames of channeled spectra when operating in a fringe tracking mode. But while the GDT is based on the fast Fourier transform, the proposed method is base don the formalism of optical statistical decisions and Markov chain framework. The corresponding algorithm, doubled DBA makes it possible to utilize all available information about the problem's nonlinearities and statistics such as Poisson photon arrivals, Gaussian readout noise, the statistics of atmospheric turbulence. Simulation results show that the DBA tracks stars up to two bolometric magnitudes dimmer than is possible with the GDT.

Our goal is to improve fringe tracking in ground-based Michelson interferometry in order to reach fainter limiting magnitudes, and lower fringe visibility thresholds. The classical technique is the Fourier analysis of dispersed fringes (peak detection). It can be regarded as a maximum likelihood estimator. Although such an estimator is optimal by complying with Cramér-Rao bound rule, it does not use a priori information about the optical path difference (OPD) to be measured. We introduce a new signal analysis procedure based on the OPD drifts measured at the GI2T interferometer: if the signal is autocorrelated, then it would be possible to use a linear estimator giving a likelihood function from previous OPD values, reducing the noise in fringe Fourier analysis. Keywords: fringe tracking, photon counting, Fourier analysis, autoregressive modeling

The proposed Space Interferometry Mission (SIM) spacecraft designs include high resolution stellar interferometers for micro-arc-second accuracy astrometric measurements. The stellar interferometers require picometer accuracy 1D metrology gauges, surface metrology gauges and 3D metrology gauges to measure the required distances or to calibrate the fiducials that define the end points of the interferometric paths. The absolute metrology gauges required by these interferometers can be considerably less accurate due to the careful design of the astrometric interferometers and the fiducials on the spacecraft. An auto-aligning, 3D metrology gauge constructed using the sub-picosecond linear metrology gauges was described in earlier papers. The sub-nanometer, in-vacuum tracking result from this 3D metrology gauge are presented. The resulting jitter is analyzed and is shown to be caused by thermal drift in the alignment of the gauge heads, warpage of the base table and the time-dependent tilt of the experiment as a whole. The aberrations in the light beams of the laser distance gauges can result in errors in the distance measurements performed using these gauges. Simulations using spot shapes and aberrations present in a realistic measurement system used in a stellar interferometer is space are performed to quantify the amount of expected errors. The results of these simulations are presented.

This article is concerned with the discussion of a control law design for a brassboard optical delay line (ODL) developed for the interferometry technology program at the JPL to support the space-based optical interferometry missions. Variations on the ODL brassboard design will be flown on the space interferometry mission and new millennium separated spacecraft interferometer. The brassboard ODL was designed to meet both the performance and environmental requirements for space interferometry. A control experiment was contrived to evaluate how well the brassboard optical delay line can control optical pathlength jitter. Fringe visibility resolution requirements for space interferometry prescribe that the optical pathlength from the two collecting telescope apertures must be equal and stable to within a few nanometers RMS. This paper describes the classical frequency domain lop shaping techniques that were used to design a control law for the experiment. Included is a description of a methodology for managing the control authority for the three actuation stages of the ODL, as well as, an input shaping technique for handling the large dynamic range issues. Experimental performance results characterizing closed loop control of residual optical jitter in an ambient laboratory environment are reported.

This paper describes the design and performance of a brassboard astrometric beam combiner. The beam combiner was developed as part of the JPL Interferometry Technology Program . The purpose of this program is to test out design concepts in hardware that will eventually be used for the Space Interferometry Mission. The label brassboard implies that the beam combiner is flight-like in terms of fit and function. The beam combiner met its design performance except for fringe visibility. Although it has not been environmentally tested as an assembly, the beam combiner was designed to survive the appropriate thermal and vibration tests.

In an interferometer, an Optical Delay Line (ODL) must be able to inject a commanded pathlength change in incoming starlight as it proceeds from a collecting aperture to the beam combiner. Fringe visibility requirements for space interferometry prescribe that the optical path length difference between the two arms must be equal and stable to less than 5 nm RMS to a bandwidth of 1 kHz. For a space mission, an ODL must also operate in a vacuum for years, survive temperature extremes, and survive the launch environment. As part of the interferometer technology program (ITP) at JPL, a prototype ODL was designed and built to meet typical space mission requirements. It has survived environmental testing at flight qualification levels, and control design studies indicate the 5 nm RMS pathlength stability requirements can be met. The design philosophy for this ODL was to crete as many design concepts as possible which would allow a priori attainment of requirements, in order to minimize analysis, testing, and reliance on workmanship. Many of these concepts proved to be synergistic, and many attacked more than one requirement. This paper reviews the science and flight qualification requirements for the ITP ODL and details design concepts used to meet these requirements. Examples of hardware implementations are given, and general applicability to the field of optomechanics will be noted.

The Space Interferometry Mission (SIM) will be the first flight of an optical interferometer where the scientific return is co-equal with technology development objectives. The SIM Science Data Center will be designed to monitor, process and archive the instrument engineering the science data collected over the five year operational life of the mission. Furthermore, it will host the postprocessing to be carried out for three years after the end of mission operations. Since the instrument is a complex technological and operations challenge, and due to SIM's ambitious astrometry and imaging goals, a prototype Science Data Center will be built and operated in parallel with the development of the instrument. In particular, this prototype system will be tested with SIM System Testbed-3 (STB-3) to command and monitor that instrument. After launch the SIM Science Data Center will continue to have access to STB-3 for testing of sequences and instrument operations modes.

We present results from simulations of the imaging mode of the Space Interferometry Mission (SIM). In particular, we derive the SIM performance for imaging zodiacal disks around solar-type stars. We find that: Zodiacal disks like the one in our solar systems can not be imaged with SIM in a reasonable amount of time; However, SIM can detect and image systems with at least 100 times the solar dust content at distances between 100 pc and several kpc in about 5 hrs, provided that the nulling efficiency lives up to expectations; SIM performs better if the system is more distant because of the fading of the central star; The maximum distance for disk detection depends only on the size of the disk.

This study was undertaken at the JPL to identify salient features of two competing instrument designs and to select the design that best meets the goals of the Space Interferometry Mission. Features were examined in terms of meeting performance, cost, schedule and risk requirements. The study included the spacecraft, the space environment, metrology considerations, stabilization of optics with temperature, spacecraft structure, complexity, and end-to- end testing among other items. The most significant determinant was the fundamental implementation of the instrument's metrology system. The impact on the testbed program associated with the mission was considered the second most important issue. An error propagation formalism was developed to address various instrument geometries examined as part of this study. The formalism propagates metrology errors from the gauge readings through to the angle on the sky. An introduction to the formalism is presented.

A new type of laser retroreflector has been developed for JPL's future Space Interferometry Mission. The retroreflector consists of an assembly of prisms of form multiple hollow cornercubes. This allows the limited field of view of about 60 degrees of a single corner can be overcome, to comply with the geometry of an optical truss. In addition, an innovative feature is that the retroreflector has common vertices, in order to define a single point optical fiducial necessary for point-to-point 3D laser metrology. The multiple cornercube provides better thermal stability and optical performance than spherical and hemispherical type retroreflectors. In manufacturing the prototype, the key technology of assembling prisms to the interferometric accuracy has been demonstrated. A non common vertex error of a few micrometers has been achieved.

The Micro Arcsecond Metrology Testbed (MAM) is a laboratory- based, long baseline, white-light interferometer inside a vibration-isolated vacuum tank. The single baseline, high precision interferometer will be able to observe a translating, artificial star at a distance of 10.74 meters with 5 μ as accuracy. The MAM testbed consists of an artificial star, laser metrology and a high precision interferometer. This paper addresses the design and characteristics of the interferometer. The interferometer functions include both angle- and optical-path tracking. The optics are arranged to from dispersed fringes in a channeled spectrum on a charge coupled device and a true white-light fringe on an avalanche photodiode, while at the same time producing guide spots for angle tracking.

The Space Interferometry Mission (SIM) relies on the combination of interferometry with a metrology system capable of measuring picometer relative length changes and micrometer absolute lengths. We are designing the Micro-Arcsec Metrology Test-Bed (MAM) to put these two systems together in a large vacuum tank (12m long, 2.4m in diameter). The interferometer has a 1.8 m baseline and is looking at an artificial star 10 m away. The metrology system is measuring the distances between the interferometer mirrors, the interferometer mirrors and the 'star' (external metrology), and the interferometer arm lengths(internal metrology). We are using two common path laser heterodyne interferometers to monitor each of these distances. The light sources used are two Nd:YAG lasers with different frequencies, f₀ and F₀ + 30 GHz. This allows measurement of relative lengths changes as well as absolute lengths. The design for the heterodyne interferometers is in progress using our experience from 1-D and 3-D metrology experiments performed in the past. Modifications include reducing the cross-talk in the internal metrology and adding a polarizing beamsplitter to the laser light path to compensate for path lengths changes caused by temperature changes.

The SIM metrology subsystem utilizes cornercube retroreflectors as fiducials. These components will introduce errors in the metrology output that must be quantified. Eventually, a complete modeling of the metrology subsystem will be needed. For that purpose, we are developing an optical model for a cornercube retroreflector, taking into account most of the defects present in such an optical part. Our goal is to given a phase map of the wavefront produced by the interference of the reference beam and the metrology beam. Our first step towards this goal is the construction of an optical model and its validation, using the MACOS and VSIM packages.

We review the formalism for white light fringes in Michelson interferometers. For astronomical use, these instruments are typically limited in sensitivity by photon statistics. Thus we investigate by simulation an alternate approach to fringe detection which utilizes an optimal filter approach. This is compared to a highly successful method presently used in many such systems, based on quadrature fringe amplitude. It is found that the optical filter approach yields an improved sensitivity of between 0.3 and 0.8 visual stellar magnitudes. We also consider the possibility of using such an approach to search over trial values of fringe drift rate, corresponding to linear motion of the fringe. We find that a very significant improvement in performance is possible for system that utilize searches over both delay and delay rate.

The MicroPrecision Interferometer Testbed (MPI), at JPL is a dynamically and dimensionally representative hardware model of a future spaceborne optical interferometry. Over the past few years, several models of MPI have been created. These include detailed, high-fidelity models of MPI and several lower-fidelity models. These models were meant to answer two basic questions: (1) Does current modeling methodology allow accurate models of highly complex opto-mechanical systems such as the MPI testbed, and (2) given a valid modeling methodology, how much model fidelity is needed in models to accurately predict performance. In order to answer these questions, four models of the MPI testbed were created; each with a unique optical and structural model fidelity. This paper reviews results obtained for these models. It compares disturbance transfer function predictions from three of the models with measured disturbance transfer functions from the hardware testbed. Results suggest that it is possible to build a highly accurate high-fidelity model, thus validating the modeling methodology. With lower fidelity models, meaningful model prediction errors exist when simple models are used to represent the complex opto-mechanical system. However, modest increase in model fidelity can lead to significant improvement.

The last decade of the 20th century has seen the development of extremely sensitive instruments based on new emerging technologies. These instruments, which are widely distributed on Earth and currently being deployed in space, have provided the means necessary to prove more deeply into the nature and evolutionary history of the universe. Together with a noticeable progress in providing a less expensive launch services, this made a significant impact on a number of areas of fundamental research. One of such areas is the research in relativistic and gravitational physics. Motivated by the opportunities offered by the future Space Interferometry Mission, we discuss high-precision astrometric measurements and their significance for the relativistic gravitational theory.

Since modern astronomical interferometers require a large number of sensors and actuators for closed-loop control of opto-mechanical components, reconfigurability of the hardware is a strongly desired feature of interferometer control software. In order for software control systems to access hardware with a standard interface and be easily reconfigurable, a layer of software is needed to communicate with the hardware driver software that is modular. If the details of the hardware configuration can be abstracted from the controller software, moving a device to a different I/O board or channel becomes a much easier task. Device mobility is important when testing the performance of computer and instrument hardware, and controller software; it also makes the software much easier to reuse in different instruments. Object-oriented programming provides a model that permits the abstraction of this hardware driver layer. The JPL Realtime Interferometer Control System Testbed (RICST) has developed hardware driver software that employs an object- oriented paradigm and allows relatively simple reconfigurability of devices and I/O boards. This scheme is especially useful since RICST is developing software for use by multiple projects. The top level of the class hierarchy includes Boards, Channels, Channel Banks, Clocks, and Interrupts; these abstract objects provide a common interface for control software to communicate with the hardware.

RICST, the Realtime Interferometer Control System Testbed, is a testbed at NASA's JPL where teams are developing a common electronics and software foundation that will form the core of the control system for JPL's various interferometer projects. Important technologies being developed and demonstrated on the RICST testbed include hard real-time multiprocessing, generic interferometer software and hardware architecture, easy reconfigurability, abstract constructs for easy implementation of control loops and servos, and highly configurable telemetry streams. The basic hardware and software architecture developed by RICST can be applied to virtually any interferometer based on the combiner-collector architecture. The RICST project began in May 1996 and has been delivering control system 'increments' every few months since that time. Each increment adds new end-to-end functionality. White-light fringe tracking has been demonstrated in the lab already; the delivery of a fully operational testbed is scheduled for the end of April. Future enhancements will add support for a wide array of calibration, sequencing, and other support functions. RICST deliveries include support software, such as a graphical user interface and a prototype spacecraft bus interface, as well as the embedded software needed to run the instrument. The RICST program will make final deliveries to its various customers in 2000, after which the RICST testbed facility will continue on in support of the SIM mission.

This paper describes some of the software engineering practices that are being used by the Realtime Interferometer Control Systems Testbed (RICST) project at JPL to address integration and integratability issues.New documentation and review techniques based on formal methods permit early identification of potential interface problems. An incremental life cycle improves the manageability of the software development process. A 'cleanroom mindset' reduces the number of defects that have to be removed during integration and test. And team ownership of work products permits the project to grow while providing a variety of opportunities to team members. This paper presents data, including software metrics and analysis, from the first several incremental deliveries developed by the RICST project.

Deep Space 3 will fly a stellar optical interferometer on three separate spacecraft in heliocentric orbits: one spacecraft for the Michelson beam combining optics, and two spacecraft for each of the starlight apertures. The spacecraft will formation fly to relative spacecraft distances from 100 meters to 1 kilometer, enabling an instrument resolution of 1 to 0.1 milliarcsecond. At each baseline length and orientation - up to 100 points in the synthetic aperture plane for a given astrophysical target - the instrument will measure source visibility amplitude form which the source brightness distribution can be determined. An infrared metrology system performs both linear and angular metrology between spacecraft and is sued to estimate delay jitter, interferometer delay and delay rate. Pointing and control mechanisms use the metrology error signals to stabilize delay jitter and to null delay and delay rate to enable detection and tracking of a white light fringe on a photon-counting detector. Once stabilized, fringes can be dispersed on a CCD in up to 80 spectral channels to attain high-accuracy measurements of visibility amplitude as a function of wavelength.

Long-baseline optical and IR interferometers are being considered as future astronomical instrument plans in Japan since 1994. They are called MIRA projects, indicating Mitaka or Mauna Kea IR array. The next Mitaka optical and IR array proposal is called MIRA-II. It consists of four fixed telescopes as an array for 1mas astrometry and three movable ones for 0.2mas imaging. They are placed in a sideways T- configuration with three 128m arms and extended lines getting the longest baseline of 680m. Each of the telescopes is a 30cm siderostat added with a 20cm beam compressing telescope. MIRA-III is a proposal of Mauna Kea optical/IR array including a 1.4km baseline with 1.5m telescopes. Its shape is a modified Y-configuration. It also aims at precise astrometry including many quasars as well as high resolution imaging of fainter stellar objects than MIRA-II. MIRA-SG, a future proposal of Mauna Kea optical/IR array connecting Subaru with GEMINI, is one of the largest interferometer with an 800m baseline by 8m telescopes. It became possible by using optical fibers fed from each Cassegrain focus with an adaptive optics system. Keck telescopes and other large telescopes on Mauna Kea are also candidates to connect with Subaru.

A fine delay lien for long-baseline optical interferometers is under development in MIRA project. The test structure of the fine delay line consists of a micro-stepping motor cart and a cat's eye cart. It is most important for the fine delay line to be driven as smoothly as possible at a steady speed of theoretical delay rate. Particularly, high frequency jitter in driving speed must be suppressed so that fringe position dose not move larger than +/- (lambda) /10 in the integration time. To accomplish this capability, we improved mainly the structure of the motor cart. As a consequence, our motor cart realized a good performance without any special servo-control mechanism. We also made several test of soft connection between motor cart and cat's eye cart as well as between cat's eye cart and cat's eye assembly with rubbers. It is found that by using appropriate rubbers we can reduce high frequency component of delay errors to a sufficiently low level at the cat's eye assembly. With positive results in these experiences, we have started designing a prototype of the vacuum fine delay line for MIRA-II having a 680 m baseline.

The Mitaka optical-IR array (MIRA) project is a series of optical and IR interferometers by the National Astronomical Observatory of Japan. We call the first step the MIRA-I project, and the following projects are the MIRA-1.2, MIRA- II, MIRA-SG, and the MIRA-III. The MIRA-I is a prototype for demonstration of stellar interferometry, and the other projects, which are described by Sato et al. and Nishikawa et al., are practical instruments for science. The main purpose of the MIRA-I is to get stellar fringes and skill of fringe tracking. The MIRA-I instrument is located at the National Astronomical Observatory in Mitaka, Tokyo.It consists of two element telescope with 25 cm aperture placed on four meters N-S baseline. The fringe detector operates at visible or around 800 nm with high-speed sampling. In laboratory, we obtained artificial white light fringes in December 1995, and we moved the instruments to the telescope dome at January 1996. At present, we have been trying to get stellar fringes. This paper describe the current status and the progress of the MIRA-I.

The MIRA-1.2 system consists of two siderostats, beam reducers, vacuum delay liens, baseline metrology system, tip-tilt mirrors, beam combine optics, and fringe detector. Two siderostats, of which aperture of the flat mirror is 300mm, are placed apart by 4 meters in the north-south direction. Beam reducer is a Cassegrain optics with the paraboloidal primary and secondary mirrors of the diameter D₁ equals 200mm and D₂ equals 30mm, respectively. The metrology system with laser interferometers is set up to stabilize the baseline length for astrometry. Piezo-driven tip-tilt mirrors are equipped for the correction of image motions due to the air turbulence and other errors. Delay lines are placed in a vacuum tube. Experiments of the fiber optics is carried out as a part of MIRA-1.2. Developing MIRA-1.2 system, it is aimed to establish the basic techniques of astrometry and future projects, especially of MIRA-II.

Simulated interferometric patterns of the tilted wave front, generated by an extra-solar planet, and detected with a rotationally-shearing interferometer are presented. The shearing angle is seen to control the number of fringes and their orientation. For small shearing angles, the phase difference in the argument of the cosine function reduces to the derivative of the wave front multiplied by the shearing angle.

Space-based interferometry already exists. We describe our experiences with on-orbit calibration and scientific observations with Fine Guidance Sensor 3 (FGS 3), a white- light interferometer aboard Hubble Space Telescope. Our goal, 1 millisecond of arc precision small-field astrometry, has been achieved, but not without significant challenges. These included a mechanically noisy on-orbit environment, the self-calibration of FGS 3, and significant temporal changes in our instrument. Solutions included a denser set of drift check stars for each science observation, fine- tuning exposure times, overlapping field observations and analyses for calibration, and a continuing series of trend- monitoring observations. HST FGS 3 will remain a competitive astrometric tool for faint targets in crowded fields and for faint small-separation binaries until the advent of large- aperture, ground-based and longer-baseline space-based interferometers.

We plan to build a photon counting camera able to yield photo-event coordinates (x,y, and t) at maximum rates superior to a million per second with a high temporal resolution (2.6 μs) and a 512 X 592 field. Ground-based interferometric techniques (single or multi-aperture) require detectors providing short frames, to deal with atmospheric turbulence. In order to reach the high signal-to-noise ratios required for imaging capabilities with aperture synthesis, such photon counting detectors are required.

Small and simple ultra-violet interferometer is designed. The interferometer consists of very small ball and concave mirrors. The interferometer can be tested in a vacuum chamber or in space for ultra-violet region for better precision. Interferometer can be used in a very large spectral region as it consists of mirror surfaces only. It is possible to use the interferometer to test optics of a space interferometer or a space telescope. These interferometers are very cheap and easily can be manufactured in quantity. Interferometers are very small that is why they are especially recommended for testing optics in space.

Datasets obtained from lunar occultation in the thermal infrared with a classical direct imaging system are compared to those obtained with a two telescope interferometer. Angular resolution and sensitivity achievable with ESO's VLT- Interferometer as λ≈10 μm are roughly identical to the precision with which 1-dimensional cuts can be reconstructed from lunar occultations. Prototype results obtained at a 4m class telescope with lunar occultation only are presented. The concepts of an algorithm to reconstruct interferometric data under the constrain of compatibility with a light curve derived from a lunar occultation event is given. The improvement of image reconstruction of the combined data set relative to the single data sets is demonstrated by calculating theoretical point spread functions. The estimated 'PSF' width for this technique at λ≈10 μm is of order of 0.03 inches.

On the basis of comparative analysis of the existing concepts of aperture synthesis optical system construction, that is: segmented telescope; multimirror telescope and an array of individual telescopes, it is shown that today, due to technological, exploitation and financial problems, only an array of individual telescopes with bases up to 100m and higher, is usable for the effective observation of the far space objects. As a result, in consideration of the theoretical fundamentals of the aperture synthesis, an array is proposed, including N-movable telescopes with diameter D acquiring light radiation from a controlled object in the process of a spatially-temporal aperture synthesis, and one fixed array element, combining subbeams from moving elements and forming a synthesized image. The analytical expressions are derived for the tolerances for coherent interaction of array elements in pointing, phasing and figurization of subapertures and subbeams in the process of image forming, and the qualitative estimations are given.

This article presents a new approach to optical modeling of astronomical interferometers as part of a dynamic 'end-to- end' instrument simulation. In a specifically developed modeling tool the optical signal flow from the observed source to the detector is computed.A ray tracing algorithm forms the basis of the program. Extended by program modules for polarization and radiometry this allows to model geometrical optical wavefront propagations and to calculate the calibrated power flux through the system. When superposing electric field distributions originating from different interferometer arms the program takes temporal coherence effects into account. These can be modeled by specifying the frequency spectrum of the observed source. A diffraction model simulates optical imaging and diffraction losses. The optical modeling tool provides interfaces for integration into an end-to-end simulation environment. In addition to the simulation of ground- and space-based interferometers the software can be applied to a great variety of other optical instruments.

Coherent integration of fringe visibility in an optical stellar interferometer yields much higher signal-to-noise ratios is shorter integration periods for dim objects. Furthermore coherent integration, if performed simultaneously over multiple spectral channels, can yield direct determination of the phase of the visibility function, a quantity that is lost in incoherent integration. To perform coherent integration it is necessary to estimate the random atmospheric delay to well within a wavelength. That determination can be used either to compensate for the delay in hardware, or as a 'phase reference' for integration of fringe visibility with the correction applied in software. This paper primarily addresses the latter method, presenting algorithms for the proper estimation of optical correlation given the interaction between the statistics of the incoming light and the hardware. Of particular importance is the effect of error in the estimation of the atmospheric delay used as a reference phase. The author's previously published method for estimation of the atmospheric delay achieves well modeled error levels. In the case of very weak signal, even 'ambiguous' determinations of atmospheric delay can be used for phase referencing. The delay estimator may employ the same raw data used for estimation of fringe visibility.

We present a maximum entropy imaging algorithm using the bispectrum observed with interferometers. It is iterative and based on the steepest ascent method. Imaging simulation test show that this algorithm can reconstruct more reliable images of weak, extended sources than the conventional self- calibration method.

Because they have demonstrated very high visibility accuracies and have greatly simplified conventional interferometric recombination devices, single-mode fibers are being seriously considered in several optical interferometry projects. This paper deals with light coupling into single-mode fibers. An analytical expression of the coupling efficiency is derived for the monochromatic case. Then, the effect of purely static aberrations is considered. Finally, coupling in the presence of atmospheric turbulence is investigated for long exposure times. Using temporal sequencies of turbulent wavefronts, simulations are performed for a wide range of seeing conditions with both uncorrected turbulence and various levels of correction by an adaptive optics system.

The University ofPuerto Rico, Mayaguez, in conjunction with the Deep Space Surveillance Branch (DEBS) ofthe USAF Research Laboratory (AFRL) Phillips Site (PL) in Albuquerque, NM is initiating an Adaptive Optics (AO) Interferometry program. The program will begin with four projects. We currently have finding for a three element optical interferometer, described in this paper, using Technology developed at DEBS, for a new wavefront sensor and a Liquid Crystal (LC) wavefront compensator being presented at this meeting'9.and a Low Light Level Fringe Tracker (LLLFT)"6'1"24 Michelson: Interferometer. We are also developing a program to put a similarly configured inexpensive two-element interferometer test-bed in orbit. The interferometer would have optical elements on a 10-meter boom. It will use Aperture Synthesis by rotation and motion ofthe elements along the booms. The third project under development would incorporate the initial 3-element interferometer into a larger array with the additional collaboration ofNew Mexico Tech and New Mexico State University at a 10,600' site near Socorro, NM. As part ofthe ground based interferometry effort we are trying to develop inexpensive meter class telescopes. The 0.75meter telescopes we are building for our small interferometer will serve as prototypes and system test-beds. The telescopes will be robotic, remotely operable, essentially self-orienting, and portable. We hope to produce such systems for commercial distribution for approximately $250K each. All ofthe ground-based interferometric systems will be configured for remote operation and independent use ofsub-arrays while upgrades and repairs are underway. The major thrust ofthe UPR effort will be to develop inexpensive interferometers for diverse applications with the low light level capabilities and the LC adaptive optics developed at the Phillips Site. Particular applications will be for high-resolution astronomy and satellite imaging. The adaptive optics will be such that they can be placed on the individual telescopes and are not part ofthe interferometer. They will then serve as templates fbr AO systems ofgeneral interest. As an additional part ofall ofthese projects we will try to develop the use ofoptical fibers for several applications. We would like to couple the telescopes with fiber if we can develop an efficient way to couple the output signal from the telescope to the fibers. in addition we hope to use fiber stretchers for optical path compensation to replace expensive conventional optical delay lines. Key words; adaptive optics, interferometer, Liquid Crystal wavefront compensation

We present results of extensive simulation of the performance of an optical interferometer array based on a new fiber concept with closely spaced multiple cores symmetrically arranged inside a common cladding. While sharing the principal advantages of single-mode (SM) fibers, including spatial filtering of phase-corrupted light and lossless propagation, the multiple core (MC) fibers are predicted to have an enhanced coupling efficiency and comparable noise sensitivity for typical observing conditions of low light levels and moderate to strong turbulence. Moreover, MC fibers have unique practical advantages: by presenting a larger face, they permit relatively easy focusing of light into the fiber, and they are well studied to purely fiber-based beam splitting and beam recombination, making it possible to construct an all- fiber interferometer with reduced complexity and cost. Our simulations of the MC fiber-linked interferometer array encompass a variety of conditions of atmospheric turbulence and photon-counting noise, for observations of a monochromatic point source. We compare the result from simulation to predictions from previous detailed theoretical calculations. Comparisons are made to an interferometer linked with standard SM fibers. We find that the coupling efficiency and sensitivity of the simulated interferometer using MC fibers generally agrees with theoretical predictions. Efforts to manufacture mC fibers are also reviewed.

JPL and CARA are building a multi-element, IR interferometer for NASA to be situated at the twin Keck Observatories on Mauna Kea, Hawaii. Initially, the 10-m diameter Keck telescopes will be augmented with four fixed-location 2-m class outrigger telescopes resulting in 15 non-redundant baselines, the longest being approximately equals 110 m or nearly 5 X 10⁷ ((lambda) /2.2micrometers )^-1 wavelengths. Fast adaptive optics and tip-tilt corrections will be used to phase up the Keck and outrigger apertures, respectively. The entire array will be co-phased by observing a relatively bright target on the photon rich Keck-Keck (K-K) and Keck- outrigger (K-O) baselines. When fully phased, the projected fringe phaser sensitivity for unresolved targets will be K- 22.0, 20.0 and 17.9 on the K-K, K-O and O-O baselines, respectively. Synthetic imaging capability will be available in the 1.6-10.0 micrometers atmospheric transmission bands at angular resolutions of 4.0 milli-arcseconds. In this article, we briefly outline the adopted methodology, imaging hardware, projected sensitivities and summarize the scientific potential of the instrument as an imaging interferometer.

A key thrust of NASA's Origins program is the search for and detection of planetary systems about other stars. Pursuing this goal in a cost-effective and expedient manner from the ground has led NASA to begin work on the Keck Interferometer, which will add 4 1.8m 'outrigger' telescopes at the Keck Observatory on Mauna Kea. In addition to the imaging science to be performed by the Keck 10m telescopes with the outriggers, another one of the principal capabilities of the instrument will be the ability for the outriggers to conduct relative astrometry at the 25 microarcsecond level per root hour. Astrometry of this accuracy will enable the array to detect planetary systems composed of Uranus-mass or larger bodies orbiting at 5 AU solar mass stars at a distance of 20 pc; over 300 stars are to be surveyed by the outriggers annually. The astrometric capabilities of the Keck array can also be utilized other astrophysical investigations, such as characterization of spectroscopic binary orbits, and the measurement of the center-of-light shift of MACHO microlensing events, which will allow for a model-independent determinations of lens masses.

Active Optics methods are now capable to provide variable curvature mirrors (VCMs) having controlled sags in the focal range from f/∞to f/2.5. Those development have been carried out by the authors for the optical path equalizer dedicated to each Mersenne focus of the VLTI. The basic principle is to use VCMs as cat's eye mirrors in each delay line in order to achieve field compensations at the recombined Mersenne focii. During the VLTI development phase, cycloid form VCMs controlled by air pressure have been performed with a 10^-4 mirror sag resolution. The cycloid form has been selected for the VLTi delay lines. However, other analytical solutions from circular plates elasticity theory have been found. Two thickness distributions lead to tulip form VCMs controlled by a central force. One of them, using a lineic reaction at the edge is the object of this paper. Active optics design, construction features, test and experimental He-Ne interferograms obtained with 16mm boundary aperture and 10mm clear aperture are presented. The mean aspect-ratio of the tulip from VCM is d/t_0.5 approximately equals 60, providing a focal zoom range from f/∞ to f/2.5. The experiment is carried out form f/∞ to f/5.

In stellar interferometry, the precise knowledge of systematic optical path differences is essential because it directly influences the ability to rapidly acquire fringes. Besides, when an object is too faint to provide a direct measurement of the fringe position error, the VLTI will provide the possibility to blindly track the fringes in open loop, according to the precalculated evolution of the OPD over time. In this case, a poor estimation of the OPD implies that the delay lines follow a trajectory, which progressively introduces residual OPD eros larger than the coherence length, and thus prevents further fringe observation.

We describe the mid-IR interferometric instrument MIDI which is planned to come into operation at the ESO very large telescope interferometer early in 2001.

During an interferometric observation the successful coordination of all the complex and heterogeneous devices of the VLT interferometer (VLTI) depends on the effectiveness and reliability of the control system, which carries out the ultimate system integration. The VLTI control system (VLTICS) is designed to satisfy both specific technical requirements and general operational constraints. It profits by the valuable experience gained during the development of the control system for the VLT 8-m Unit Telescopes, inheriting the same principles and standards. Since the VLTICS is a direct client of the TCS, the homogeneous architecture simplifies the interface definition and the data communication protocol. Therefore we can focus on the identification of those functions that are peculiar to VLTI and work out the most challenging requirements. The VLTICS architecture is implemented and integrated within the framework of the VLT Observatory control environment. It relies on two hardware platforms; real-time VME control nodes and UNIX-based workstations. The VME nodes provide the interface to the controlled devices and run the data acquisition and digital control algorithms, while the workstations are used for coordinating and monitoring tasks, on-line database facility and graphical user interface. The standard control network is optical fiber Ethernet. ATM LAN links support image data transfer between the detectors and the workstation. In order to achieve the performance required by the fringe control loop, we have adopted the reflective memory technology, which offers a zero-latency deterministic data link with no software protocol overhead. This paper describes the basic requirements and the overall architecture of the VLTICS, the software design method and the interfaces towards external system and software clients. It draws also a parallel between VLTICS and TCS with respect to the main functions provided.

The very large telescope (VLT) project includes an interferometric mode which consists in combining coherently the four 8-m telescopes as well as a number of 1.8-m Auxiliary Telescopes to achieve milli-arcsecond angular resolution. This mode, referred to as the VLT Interferometer (VLTI), imposes specific requirements on the whole observatory and, in particular, on the telescopes. One of these specific requirements is the very high stability required on the optical path length (OPL) at the level of a few tens of nanometers. To verify the feasibility of achieving this requirement, a number of studies have been preformed in the past years with encouraging results. In the last months, the readiness of major VLT sub-systems permitted to start the verification of these conclusions by test. This paper presents these first results on OPL stability obtained during acceptance testing on several subsystems of the VLT 8-m Unit Telescopes.

The visibility performance of a stellar interferometer can be expressed by means of the instrumental visibility factor, defined as the ratio of measured to intrinsic object visibility, in the absence of atmospheric distortion. The instrumental visibility factor can be approximated by a product of Strehl ratios, each related to one of the main components of the instrument. For IOTA, which uses telescopes to collect starlight, the Strehl factor related to telescope alignment is a potentially significant term. This paper focuses on estimating instrumental visibility due to telescope misalignment. In practice we align each telescope using a collimated optics-filling laser beam projected out from the laboratory to the telescope and siderostat, adjusted so that the beam is returned to the laboratory. The return beam is viewed in either of two ways: (a) intensity mode, where the beam is simply;y displayed on a white card; (b) interference mode, where the beam is interfered at a beamsplitter with either a locally retroreflected plane wave, or a return beam from a second telescope.Both modes were simulated with a computer program that displays the expected fringe pattern. Many misalignment combinations were calculated. In either mode, the collection of resulting beam patterns is useful in guiding an iterative alignment procedure, and also in estimating any remaining residual error.

We present the result of the first stellar diameter measurements done in visible light(&lgr; = 665 nm) at the Infrared Optical Telescope Array (IOTA) located on Mt. Hopkins in southern Arizona. The data for these measurements were acquired using IOTA's 'rapid scanning piezo mirror' which allows us to acquire complete interferograms at a rate of about two per second. The resulting frequency distribution of visibilities matches the theoretical statistical distribution from radio astronomy when on a priori criterion is used to select 'good' scans. The interpretation of this result is difficult, however, due to the differences between the distribution of radio and optical fringe visibilities. Therefore, a very simple-minded selection criterion for 'good' fringes is used in the final analysis, and the resulting diameter measurements are in close agreement with other published results.

We summarize the characteristics and performance of a recent instrument for the detection of interference fringes in the near IR at the IR Optical Telescope Array (IOTA). The instrument had its first test run and saw 'first fringes' in Spring 1997, and has been regularly used since by several groups at the IOTA for a variety of high resolution investigations in the near IR. We present preliminary results from our group's campaign to study Mira variables and the circumstellar environments of Herbig AeBe stars.

We have successfully used nulling interferometry at 10 μm wavelength to interferometrically suppress a star's radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star α Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects.

A brief description of the problems found and methods used to calculate the exact position of each mirror of the CHARA Array on Mount Wilson. The problem is highly constrained due to polarization and logistic considerations, yet many unusable solutions are still possible.

The Navy Prototype Optical Interferometer (NPOI) has been routinely used since 1996 for observations of stellar diameters and limb darkening, and for the imaging of binary stars. Here, we present the first results of, and future goals for, wide-angle astrometric observations from the NPOI.

The high-spatial-frequency fringes that contain detailed information about a stellar image are generally too weak to track. The Navy Prototype Optical Interferometer (NPOI) is the first interferometer to measure these fringes with the techniques of phase bootstrapping - the use of fringe tracking on two short baselines AB and BC to keep a longer baseline AC phased up - and wavelength bootstrapping - the use of fringe tracking at long wavelengths to keep the long baseline phased up at short wavelengths. We demonstrate the utility of phase and wavelength bootstrapping with the NPOI observations of stellar limb darkening of (beta) Cancri of Pauls et al. in these proceedings. The NPOI baselines and wavelength coverage were 19, 22, and 38 m and (lambda) 450 to (lambda) 850 nm. For (lambda) < 675 nm, the 37 m baseline samples the uv plane beyond the first null of the Airy disk.

We present results of an on-going program to measure limb- darkened angular diameters of late-type giant stars using the Navy Prototype Optical Interferometer (NPOI). Observing with three elements of the NPOI and using twenty spectral channels covering the wavelength range from 520 nm to 850 nm we are able to extend the spatial frequency coverage beyond the first null in the stellar visibility function for a number of K giants. These observations make use of the technique of phase bootstrapping in which the shorter baselines with high visibilities are used to keep the longer baselines in phase. The data are inconsistent with a uniform brightness stellar disk. Adopting a particular limb- darkening model enables us to derive an angular diameter with high precision. The total uncertainty is dominated by our knowledge of the wavelength scale. These observations also include measurements of the closure phase, which shows a jump of 180 degrees at the position of the first null in the visibility amplitude. This is the first time a non-zero closure phase has been measured on a single star with a separate element optical interferometer.

The construction, infrastructure, and layout of the Navy Prototype Optical Interferometer is described in detail.

As a fringe tracking stellar interferometer, the NPOI routinely determines time series of fringe motion and provides the only currently available simultaneous multi- baseline atmospheric turbulence data. We have used these time series to determine properties of atmospheric turbulence at the Anderson Mesa astronomical site near Flagstaff AZ. The shapes of the temporal power spectra imply an infinite outer scale. However, the total atmospheric power is significantly less than expected. This implies a very short outer scale, despite the shape of the spectra. The data are presented and discussed.

We discuss reduction, calibration, and imaging of data from the multi-baseline, multi-channel Navy Prototype Optical Interferometer (NPOI). Images utilizing closure phase are presented for Mizar A and several other binaries on scales from a few milliarcseconds (mas) to several tens of mas. A complete orbital solution has been obtained for Mizar A with a median residual of 0.08 mas. A correlation between the visibility amplitude and a temporal seeing indicator is used to calibrate the amplitudes with an error of about 15 percent; closure phase offsets vary slowly with time and can be calibrated with an error of a few degrees. However, unmodeled visibility variations remain at a level above the design goals. Earth rotation aperture synthesis is very fast with the 520 nm to 850 nm wavelength coverage of the NPOI, so that as few as four scans result in images with dynamic range of 100:1 of simple binaries with negligible color difference between the components. Mapping was done in AIPS, a package developed for radio interferometry with the VLA and VLBI, but fundamental differences between radio and optical data are the availability from NPOI of an independently computed triple product of the complex visibilities, and the availability of the square of the visibility amplitude on single baselines. The latter quantity needs bias correction, whereas the former is practically unbiased if independent detectors contribute to the signal. Broad band optical interferometry mapping algorithms therefore present a new development challenge.

The Navy Prototype Optical Lnterferometer, NPOI, is routinely used to measure visibility amplitudes and closure phase for stellar objects at optical wavelengths (e.g. , Benson et al. ,' Hajian et al.2) . In this poster we describe the fringe data collection aspects and the real time algorithm that enables us to actively track fringes with the instrument. For a detailed description of the overall instrument see Armstrong et al. .

We have designed a method for introducing the large delays needed for the full 437 meter baseline imaging subarray of the Navy Prototype Optical Interferometer (NPOI). Long delay liens (LDLs) will introduce delay in discrete 29 meter increments for each of the six array elements. In conjunction with the 35 meters of delay from the continuously-variable fast delay lines, the LDLs will allow fringe tracking for all baselines at any position on the sky. We present the mechanical layout, alignment and vacuum design of the LDLs.

During the second servicing mission of the Hubble Space Telescope (HST), a newly refurbished fine guidance sensor (FGS-1R) was installed into the telescope's Radial Bay No. 1. The successful replacement of the existing FGS-1, whose degraded Star Selector Servo bearings were affecting the scheduling and acquisition of science data, was critical to continued success of the observatory. In addition to solving the bearing problem, the refurbished FGS-1R also provided an innovative approach to minimize the effects of spherical aberration on the interferometric signal generate by the FGS, hereafter referred to as an s-curve. Rather than try to remove the aberration from the wavefront over a very large field of view, FGS-1R was given the capability to realign the beam to the Koester's prism. A symmetric error, such as spherical aberration, which is divided perfectly at the beam center and folded onto itself, will have the effects of the aberration canceled. To this end, FGS-1R's FOld Flat No. 3 was retrofit with an Actuated Mechanisms Assembly (AMA), which allows on orbit correction of the beam alignment. This paper gives an overview of the theory of operation of the FGS, and characterizes the effects of spherical aberration on s-curve modulation. It discusses the theory of operation of the AMA, and how it is used to optimize the optical alignment. It describes the analysis tools and methods used to transform on orbit data into required adjustments of the AMA. Finally, it presents the result of the on orbit optimization of s-curve modulation, and briefly discusses some of the challenges faced in refurbishing the next FGS.

We have used the technique of aperture masking to transform the 10m Keck telescope into a separate-element, multiple aperture Michelson interferometer. This has allowed a dramatic gain in signal-to-noise to be achieved as compared to conventional full-pupil interferometry for bright targets such as evolved giant and supergiant stars. Preliminary results from a program of near-IR diffraction-limited imaging of such stars are presented. Multi-wavelength images in the IR JHK and L bands have revealed complex and asymmetric morphologies in the inner dust shells surrounding a number of proto-typical dust-enshrouded IR stars. In addition, we have imaged the stellar photospheres of some of our largest target stars, allowing us to measure diameters and search for structure, such as giant convective cells, on the stellar surface.

The fine guidance sensors (FGS) aboard the Hubble Space Telescope (HST) are optical white light shearing interferometers that offer a unique capability to astronomers. The FGS's photometric dynamic range, fringe visibility, and fringe tracking ability allow the instrument to exploit the benefits of performing interferometry form a space-based platform. The FGSs routinely provide HST with 2 milli-seconds of arc pointing stability. The FGS designated as the Astronomer, FGS3, has also been used to (1) perform 2 mas relative astrometry over the central 4 arc minutes of its field of view, (2) determine the true relative orbits of close faint binary systems, (3) measure the angular diameter of a giant star, (4) search for extra-solar planets, (5) observe occultations of stars by solar system objects, as well as (6) photometrically monitor stellar flares on a low mas M dwarf. In this paper we discuss this unique instrument, its design, performance, and the areas of science for which it is the only device able to successfully observe objects of interest.

The Space Interferometry Mission (SIM) will perform astrometry to a resolution of a few micro arc seconds. The development of this mission is being led by the California Institute of Technology, JPL for the National Aeronautics and Space Administration. A recent trade study was performed to compare two significantly different architectures. This paper will describe the two configurations and contrast some of their differences and similarities.

This paper reviews the scientific results obtained with the Grand Interferometre a 2 Telescopes (the GI2T interferometer) from 1990 to 1996. During this epoch, accurate spectroscopy coupled to interferometry were achieved on luminous and multiple stars. Subtle structures in circumstellar environments such as: jets in the binary system β Lyrae, dumpiness in the wind of P Cygni, a rotating arm in ζTau have been discovered. Measurements of angular diameter variability versus time and wavelength provide fundamental parameters which constraint δ Cephei and γCas models. In addition to GI2T results, we develop in our group hydrodynamic and radiative transfert models dedicated to the interpretation of interferometry results. These models can directly constrain luminous star physics through their observable parameters.

This paper presents the optical layout of the REGAIN beam combiner including the optical delay line LAROCA with its variable curvature mirror, the field rotator devices, the image and pupil tracking systems and the dedicated visible spectrography. Preliminary studies of foreseen improvements, such as adaptive optics, IR spectrograph and addition of a third telescope, will be discussed.

Several proposed spacecraft missions require positional knowledge of their optical elements to very high precision. This knowledge can be provided by a metrology system based on a laser interferometer incorporating the spacecraft optics. We present results from fabrication and testing of a lab-based frequency-modulated (FM) Michelson interferometer intended to maintain length stability to a few picometers. The instrument can be used to make precise relative distance measurements or it can be used to characterize orientation and polarization effects of system components commonly used in metrology gauges. External frequency modulation of a frequency-stabilized laser source and phase-sensitive detection are used to detect changes in the arm length difference of the interferometer. Arm length adjustments are made via a closed loop feedback system. A second system having a shared beampath with the primary system monitors the performance of the primary system. Preliminary data, operating in an ambient lab environment, demonstrate control to roughly 20 picometers rms for measurement times around 100 seconds.

Traditional methods of data collection in active fringe tracking Michelson stellar interferometers involve logging and analyzing the signal within the fringe tracking system for the scientific information about the object being observed. While these methods are robust and have produced excellent scientific results, they become more problematic as next-generation Michelson stellar interferometers are built with more telescopes and the aim of performing routine imaging. The Center for High Angular Resolution Astronomy (CHARA) Array is one such next-generation instrument presently under construction on Mt. Wilson, north of Los Angeles, California. The CHARA array will feature a separation of the tasks of active fringe tracking and imaging. In anticipation of the advantages afforded by the task separation, a prototype imager was developed. The prototype imager employs single-mode fiber optic strands to convey the light form simulated telescopes to a smaller, non-redundant, remapped pupil plane, which in turn feeds a low resolution prism spectrograph. The spectrograph features two cylindrical optical elements whose net effect is to focus the light to a smaller plate scale in the spectral dimension than in the orthogonal spatial dimension.