In the two years since the last SPIE meeting on this topic there has been much activity in both ground and space based interferometry. I review those developments, I also summarize the Strawman Science Proposal prepared by the Space Interferometry Science Working Group as a gauge for evaluating the AIM instrument proposals. I then review the recent discovery of the disk structure in M106 using radio interferometry. As an example of where we want to go with optical interferometry, the M106 case argues for infrared capabilities, significant fields of view, and the availability of auxiliary instruments, e.g. spectrographs, in the imaging focal plane.

This paper discusses the vibration isolation problem as it applies to spaceborne interferometers, and presents evidence that vibration isolation will be a required technology for these instruments. A hardware solution to the spaceborne interferometer isolation problem is offered with experimental evidence of its effectiveness.

Lightwave Electronics manufactures diode-pumped single-frequency Nd:YAG lasers at 1064 nm and 1319 nm. Short term (msec) frequency fluctuations of these lasers are in the kHz range, while longer term (hour) fluctuations are many MHz. The rms intensity fluctuations of these lasers have been lowered to 0.0038% using a noise-reducing feedback circuit. A laser has been tested operationally from -54 degree(s)C to +54 degree(s)C. Lasers can be tuned from 1319.03 nm to 1319.44 nm or from 1064.43 nm to 1064.73 nm.

Temperature controlling is important for optoelectronic device's stable and reliable working, especially in poor circumstance or for long time. This article represents an active temperature control system used to control the temperature of a tunable narrow-linewidth external cavity semiconductor laser. A double deck cooling structure controlled by an intelligent driver is reported and the feature of the system is investigated.

The LAser-Stabilized Imaging Interferometer (LASII) concept is being developed as an astronomical telescope for the next generation of optical resolution beyond Hubble Space Telescope (HST). The essential ingredients are: a rigid and stable structure to minimize mechanical and thermal distortion, active control of the optical geometry by a laser metrology system, a self-deploying structure fitting into a single launch vehicle, and ultraviolet operation. We have modified earlier design concepts to fit the scale of an intermediate sized NASA mission. Our present design calls for 24 0.5 m apertures in a Mills Cross configuration, supported on four trusses. A fifth truss perpendicular to the primary surface would support the secondary mirror and the laser metrology control points. Either separate interferometers or two guide telescopes would track guide stars. This instrument would have about 6 times the resolution of HST in the visible and the same collecting area. The resolution would reach 2.5 mas at 150 nm. The primary trusses would fold along the secondary truss for launch, and automatically deploy on orbit. Possible orbits are sun-synchronous at 900 km altitude, high earth orbit or solar orbit. Infrared capability could be included, if desired.

This paper presents the results of an investigation of the problem of fringe acquisition (FA) as it applies to high-precision space-based optical interferometers. The POINTS (for Precision Optical INTerferometry in Space) instrument concept, being developed in a collaborative effort between the Smithsonian Astrophysical Observatory and the Jet Propulsion Laboratory as a wide-band space-based optical interferometer dedicated to astrometry, is used as a test baseline. In this study we analyze three candidate FA algorithms: linear correlation techniques (CT); nonlinear least squares (NLS); and, a discrete Bayes approach (DBA). Analytical methods of evaluating the probability of detection for the NLS are developed which enable a multi-parametric study of the FA problem. Among other parameters, the study examines the effects of varying the magnitude of the electronic readout noise of the focal plane detector cells (assumed to be CCD's), of increasing the bolometric magnitude of the target stars, of allowing constant drifts of the optical path difference, and of co-adding CCD cells to reduce readout noise. A signal-to-noise measure is selected that exhibits a high correlation with the FA performance and a study of the a posteriori density space reveals insight into the nonlinear nature of the interferometric measurement function. The study concludes with selected Monte Carlo simulations to confirm the analytical predictions and to compare the performance and robustness of the CT and NLS to those attainable with the DBA.

The purpose of the fringe tracking algorithms is to maintain lock on the target star after acquisition and to obtain the most accurate estimate possible of the scientific quantity (or quantities) of interest in the presence of dynamic disturbances to the spacecraft/interferometer ensemble. This study carries out an analysis of the performance and robustness achievable by four candidate estimation techniques when applied to an ultra-high-precision fringe tracking task (5 micro-arcsecond ultimate accuracy). The first class of fringe trackers studied include the Extended Kalman Filter. This class is followed by extensions to second and third order nonlinear filters developed by the authors. The higher order filters have expanded regions of convergence. Third, we consider the use of an invariant filter (IF) to estimate the angle between two target stars (using POINTS as a test case). The IF offers the advantage of improved robustness in the dynamical case, being in effect `invariant' to dynamics. Finally Discrete Bayes Algorithms make use of Bayes' decision rule to propagate the a posteriori distribution of the true parameter and take into account the discrete character of the Poisson photon arrival events. Variations of these algorithms, known as multiple hypotheses trackers, offer great promise for dim star tracking. An exploration of filter performance with respect to several parameters is carried out analytically and selected Monte Carlo simulations are carried out both to verify analytical predictions and to study performance.

At visible frequencies the comparative sensitivity achieved from space-borne or terrestrial interferometers depends on source brightness, source complexity and on the waveband considered. At short wavelengths a space-borne instrument is always superior and at longer `visible' wavelengths (up to K-Band) a space-borne instrument is superior for faint and/or complex targets. At thermal infrared wavelengths the sensitivity comparison depends on source brightness but not its complexity. A space-borne interferometer gives superior sensitivity at M-Band and between atmospheric transmission windows. To obtain adequate sensitivity at N-Band and longer wavelengths a space-borne interferometer cooled below 80 degree(s)K is required.

GAIA represents a preliminary concept for an astrometric mission being considered in the context of ESA's `Horizon 2000 Plus' long-term scientific program. It comprises three stacked Fizeau interferometers viewing different directions within an instantaneous scanning circle, each interferometer consisting of two 50 cm aperture mirrors with a baseline separation of 2.5 m. Equipped with a modulating grid, and using CCDs, at least as the baseline detector, repeated scanning of the celestial sphere over a period of five years is estimated to lead to positions, proper motions, and parallaxes of some 50 million objects, down to about V equals 15 mag, with an accuracy of better than 10 microarcsec, along with multi-color multi-epoch photometry of each object. The scientific case for such a mission is compelling: distances and kinematical motions for objects throughout our Galaxy would be obtained, along with valuable information on the space-time metric ((gamma) ), angular diameters of hundreds of stars, and a vast body of information on double and multiple systems. Screening of all 100,000 stars within 100 pc for periodic photocentric motions would provide the most powerful and systematic method of detecting possible planetary companions proposed to date.

This paper presents initial results that demonstrate the end-to-end operation of the Micro- Precision Interferometer (MPI) testbed. The testbed is a full-scale model of a future space- based interferometer, containing all the spacecraft and support systems necessary to perform an astrometric measurement. The primary objective of the testbed is to provide an end-to-end problem to evaluate and integrate new interferometer technologies, such as vibration isolation, structural quieting, active optics, and metrology systems. This paper shows initial testbed functionality in terms of the ultimate performance metric: stabilization of stellar fringes (from a pseudo star). The present incarnation of the evolving testbed uses a fringe tracker and pointing control subsystem to stabilize the fringe position to the 72 nm (RMS) level in the presence of the ambient laboratory seismic noise environment which is a factor of 10 higher than that expected on-orbit. These encouraging preliminary 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 interferometers.

DARWIN is a space mission concept proposed in the framework of the European Space Agency `Call for Ideas' for the Horizon 2000+ plan. It aims at detecting terrestrial exo- planets by interferometric coronography in the infrared, then searching for the presence of atmospheric ozone which traces the presence of an active biological photosynthesis. The concept itself is elaborated on ideas previously proposed and developed by Bracewell, Owens, Angel et al.,...Its main originality is that we believe that the goal of detecting extra-solar life can be achieved with a reasonably simple instrumental concept, i.e. passively cooled medium- size (approximately 1 m diameter) space telescopes located at more than 3.5 AU from the Sun in order to avoid the bulk of the zodiacal thermal emission.

We discuss an optical interferometer's beamsplitter from the points of view of visibility loss and phase, critical to astrometry. A beamsplitter having no symmetries is described by 16 parameters, all functions of optical frequency; if all symmetries are present this number is reduced to four, only two of which are relevant to astronomical interferometry. We have developed a novel multi-layer design which covers fully a factor of three in wavelength while contributing a minimum of visibility loss, light loss, and systematic error.

I present a spectrometer for a dispersed-fringe optical astrometric interferometer. The two primary requirements, that integration time and systematic error be minimized, do not interact, and both are met. The integration time at the minimum is only a weak function of the parameters, allowing adjustment for secondary criteria such as cross-dispersion resolution without significantly increasing integration time.

POINTS comprises a pair of independent Michelson stellar interferometers and a laser metrology system that measures both the critical starlight paths and the angle between the two baselines. The nominal design has baselines of 2 m, subapertures of 35 cm, and a single- measurement accuracy of 5 microarcseconds for targets separated by approximately equals 90 degree(s). In a five-year mission, POINTS could yield, e.g., a 1% Cepheid distance scale, galactic mass distribution, knowledge of cluster dynamics, and stellar masses and luminosities. In a ten-year mission, POINTS could perform a deep search for other planetary systems, using only 20% of the available observing time. POINTS does global astrometry, i.e., it measures widely separated targets, which yields closure calibration, numerous bright reference stars, and absolute parallax. The instrument has only three moving-part mechanisms, and only one of these must move with sub-milliradian accuracy. On each side of the interferometer, there are only three (interferometrically critical) optical surfaces preceding the beamsplitter or its fold flat. POINTS is small, agile, and mechanically simple. It would prove much of the technology for future imaging interferometers.

This paper lists several of the systematic error sources which we have identified in our studies of POINTS, concentrating on those which are expected to affect other spaceborne astrometric interferometers as well. Many of them also are expected to affect ground-based optical interferometers. Among the errors is a newly discovered systematic contribution which only occurs in the simultaneous presence of diffraction, aberration, spectrometer dispersion, and two-beam (Michelson) interference. This error can be as large as several nanometers of optical path difference, and thus may constitute an important source of systematic uncertainty in ground-based astrometric measurements as well.

Newcomb is a design concept for a low-cost astrometric optical interferometer with nominal single-measurement accuracy of 100 microseconds of arc ((mu) as). In a 30 month mission, it will make scientifically interesting measurements of O-star, RR Lyrae, and Cepheid distances, probe the dark matter in our Galaxy via parallax measurements of K giants in the disk, establish a reference grid with internal consistency better than 50 microsecond(s) , and lay groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century. With an extended mission life, Newcomb could do a useful preliminary search for other planetary systems.

The Orbiting Stellar Interferometer (OSI) is a concept for a first-generation space interferometer with astrometric and imaging capabilities and its responsive to the recommendations of the Astronomy and Astrophysics Survey Committee for an astrometric interferometry mission. OSI is a triple Michelson interferometer with articulating siderostats and optical delay lines. The design uses a 7 m maximum baseline and aperture diameters of 33 cm; the targeted astrometric performance is a wide-field accuracy of 5 (mu) as for 20-mag objects. The instrument would also be capable of synthesis imaging with a resolution of 10 mas. Laser metrology is used to relax structural requirements thereby reducing cost. The currently envisaged flight system fits into an Atlas II shroud, for insertion into a 900 km sun- synchronous orbit.

Very high resolution spatial interferometry requires picometer level 1D metrology, surface metrology and 3D metrology. Micron level accuracy is required for absolute metrology systems for spacecraft like the proposed Orbiting Stellar Interferometer carrying high resolution spatial interferometers. A surface metrology system with a repeatability of less than 0.1 nm over an aperture of several inches in vacuum has been demonstrated. An absolute calibration system for this gauge is in development. An absolute metrology system with an accuracy of 10 microns over a distance of 10 meters is also under construction. This system uses a 1319 nm, solid-state, infrared laser locked to an Ultra-Low-Expansion glass cavity to an accuracy exceeding 1 part in 10¹⁰. The length of the cavity is controlled by a thermal vacuum oven. 1 millidegree Centigrade root-mean-squared (rms) cavity temperature stability with the oven in vacuum has been achieved for time scales of days. The digital laser servo is capable of following the length of the cavity with an Allen deviation of few hundred Hertz for time scales of a day. Two lasers locked to the same cavity are used to supply a simultaneous cavity length measurement as well as the absolute distance measurement. The absolute distance measuring part of the gauge is under construction. An auto alignment system is being developed for our linear relative metrology gauge which had achieved an accuracy of 0.1 picometers. This gauge will be used to construct a 3D metrology gauge with an accuracy of less than 10 pm rms for time scales of minutes initially.

We present a method for performing global astrometry with the proposed Orbiting Stellar Interferometer. Because it is dedicated to wide-angle astrometry, OSI has the intrinsic capabilities to achieve global astrometry, even though it doesn't measure directly relative angles between pairs of stars, such as HIPPARCOS. In this paper, a time-independent model is shown, leading to a coherent solution for the positions of reference stars on the whole sky. With an initial measurement accuracy of 10 micro-arcseconds, corresponding to an accuracy of 340 picometers in the knowledge of the delay-line position of the observing interferometer, the consistent least-squares solution gives an accuracy by which the astrometric parameters can be obtained around 2 - 3 micro-arcseconds.

The Stellar Interferometer Technology Experiment (SITE) is a near-term precursor mission for spaceborne optical interferometry. Proposed by the MIT Space Engineering Research Center and NASA's Jet Propulsion Laboratory, SITE is a two-aperture stellar interferometer located in the payload bay of the Space Shuttle. It has a baseline of four meters, operates with a detection bandwidth of 300 nanometers in the visible spectrum, and consists of three optical benches kinematically mounted inside a precision truss structure. The objective of SITE is to demonstrate system-level functionality of a space-based stellar interferometer through the use of enabling and enhancing Controlled Structures Technologies such as vibration isolation and suppression. Moreover, SITE will validate, in the space environment, technologies such as optical delay lines, laser metrology systems, fringe detectors, active fringe trackers, and high- bandwidth pointing control systems which are critical for realizing future space-based astrometric and imaging interferometers.

This paper addresses the optimization of the relative arrangement (pupil configuration) of a phased array of optical telescopes, coherently combined to form images of extended objects in a common focal plane. A new optimality criterion, which is directly linked to the restoration error of the original object from the recorded image, is derived. The optimal configuration is a function of the maximum frequency of interest (or sought resolution), and takes into account the diameter of the elementary telescopes. Simulations illustrate the usefulness of this criterion for designing a synthetic aperture optical instrument.