The Space Interferometry Mission (SIM) will be the first in-space, long-baseline Michelson Stellar Interferometer. SIM will perform precision astrometry at the micro-arcsecond accuracy level, which will be used to characterize planetary systems around stars within about ten parsecs of Earth and address a number of other key astrophysics projects. This paper provides a broad overview of the SIM Mission. Topics covered include: the science objectives, key top level requirements, how the mission will be implemented (technical and programmatic), technology development status, an assessment of where the project is today, and prognosis for the future.

Optical and IR interferometry wil 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 in concert. The Jet Propulsion Laboratory along with its industry partners, Lockheed Martin and TRW, are addressing these challenges with a development program that plans to establish technology readiness for the SIM by the end of 2004.

In 2009, NASA's Origins Program will launch the Space Interferometry Mission (SIM), a 10-meter-baseline optical interferometry instrument, into an Earth-trailing solar orbit. This instrument will be comprised of four parallel optical interferometers whose prime mission objective is to perform astrometric measurements at unprecedented accuracy. Launched by the Space Shuttle and boosted into its final trajectory by an integral propulsion system, SIM will collect data for more than five years in the search for extra-solar system planets. NASA has assembled an integrated Jet Propulsion Laboratory (JPL)/Industry team comprised of TRW, Lockheed Martin, and Caltech to formulate a reference design to meet the SIM science objectives. Addressing unique technical challenges has proven to be a formidable task in numerous aspects of the system definition, from component development to system-level integration and test. Parallel activities to develop and test the necessary enabling technologies for SIM are coupled with the ongoing flight system design. The flight system design poses unique challenges in many areas, including geometric aspects of the layout, stability of the precision structure, thermal control, active vibration suppression, picometer-level laser metrology, etc. System-level trade studies that balance the requirements of the optics and metrology layouts and develop clean interfaces are presented herein. This paper also addresses the issues of the System Engineering processes and validation of performance specifications. Finally, this paper describes the current status of the SIM Reference System design.

One of the most critical technology requirements for the Space Interferometry Mission is that the difference in pathlength traveled by the starlight through each arm of the instrument be known with picometers of precision. SIM accomplishes this by using an internal laser metrology system to measure the optical path traveled by the starlight. The SIM technology program has previously demonstrated laser gauges with measurement accuracy below 10 picometers. The next challenge is to integrate one of these gauges into a full interferometer system and demonstrate that the system still operates at the required level. For SIM, the ultimate requirement is that the internal metrology system be able to give an accurate measure of the starlight internal path difference to about 150 picometers over its narrow-angle field, with a goal of 50 picometer accuracy. This accuracy must be maintained even as SIM's various active systems articulate the SIM optics and vary the SIM internal pathlengths. The Microarcsecond Metrology Testbed (MAM) is a full single-baseline interferometer coupled with a precision pseudostar, intended to demonstrate the level of agreement between starlight and metrology phase measurements needed to make microarcsecond-level measurements of stellar positions. MAM has been under development for several years and is now producing picometers-level consistency that translates into microarcseconds-level performance. This paper will present an overview of the MAM Testbed, together with recent results targeting the 150 picometer performance level required by SIM.

This paper describes the relationship between the Space Interferometer Mission (SIM) and the Micro-Arcsecond Measurement Testbed (MAM). MAM is necessary because differences exist between the starlight and the metrology measurements through the SIM instrument optical path. The goal of MAM is to establish the methodologies required to reduce these differences. The targeted SIM instrument performance requires these differences to be at the pico-meter level. Starlight and metrology difference errors can be grouped into two categories: field dependent and field independent errors. The field independent errors are either random (i.e., vibration) or drift (i.e., thermal mirror warping). The field dependent errors introduce differences between metrology and starlight that change as a function of 'look' angle. An example field dependent error is the different diffraction effects on the two beams as the delay line slews. SIM's fundamental error mitigation approach is to reduce the field dependent errors down to the prescribed error budget levels, then calibrate out the remaining field dependent portion. The paper describes the recipe to generate the MAM error budget. Since MAM is inherently a subset of SIM, the MAM testbed addresses a subset of the total SIM error budget. The paper describes the approach to determining the relevant MAM portion. In addition, it describes the derivation of the overall MAM error budget, including allocations for the pseudo star errors. The paper maps the SIM observing scenario to a MAM test measurement. A successful MAM measurement will be defined in terms of the actual measurements, the metric applied and its relationship to the MAM error budget.

This paper summarizes two different strategies envisioned for calibrating the systematic field dependent biases present in the Space Interferometry Mission (SIM) instrument. The Internal Calibration strategy is based on pre-launch measurements combined with a set of on-orbit measurements generated by a source internal to the instrument. The External Calibration strategy uses stars as an external source for generating the calibration function. Both approaches demand a significant amount of innovation given that SIM's calibration strategy requires a post-calibration error of 100 picometers over a 15 degree field of regard while the uncalibrated instrument introduces tens to hundreds of nanometers of error. The calibration strategies are discussed in the context of the wide angle astrometric mode of the instrument, although variations on both strategies have been proposed for doing narrow angle astrometry.

Real Time Control (RTC) for the Space Interferometry Mission will build on the real time core interferometer control technology under development at JPL since the mid 1990s, with heritage from the ground based MKII and Palomar Testbed Interferometer projects developed in the late '80s and early '90s. The core software and electronics technology for SIM interferometer real time control is successfully operating on several SIM technology demonstration testbeds, including the Real-time Interferometer Control System Testbed, System Testbed-3, and the Microarcsecond Metrology testbed. This paper provides an overview of the architecture, design, integration, and test of the SIM flight interferometer real time control to meet challenging flight system requirements for the high processor throughput, low-latency interconnect, and precise synchronization to support microarcsecond-level astrometric measurements for greater than five years at 1 AU in Earth-trailing orbit. The electronics and software architecture of the interferometer real time control core and its adaptation to a flight design concept are described. Control loops for pointing and pathlength control within each of four flight interferometers and for coordination of control and data across interferometers are illustrated. The nature of onboard data processing to fit average downlink rates while retaining post-processed astrometric measurement precision and accuracy is also addressed. Interferometer flight software will be developed using a software simulation environment incorporating models of the metrology and starlight sensors and actuators to close the real time control loops. RTC flight software and instrument flight electronics will in turn be integrated utilizing the same simulation architecture for metrology and starlight component models to close real time control loops and verify RTC functionality and performance prior to delivery to flight interferometer system integration at Lockheed Martin's Sunnyvale facility. A description is provided of the test environment architecture supporting the RTC path to flight.

To achieve micro-arcsecond astrometry, SIM's external metrology system must track the relative changes of three baseline vectors with a precision of tens of picometers over a one-hour time scale. The Kite testbed is designed to be the technology demonstration for a picometer-class external metrology truss. Four fiducials, two simple corner cubes and two triple corner cubes, ar arranged in a planar parallelogram configuration to allow a redundant measurement of truss deformations by six metrology gauges placed between the fiducials. Each metrology gauge is capable of 20-pm relative metrology accuracy and 10-μm absolute metrology accuracy, using a beam launcher capable of self-alignment at the arcsecond level. The Kite demonstration involves the articulation of one of the corner cubes to simulate SIM instrument geometrical changes while various performance metrics are evaluated based on the readings of the individual metrology gauges. The test performance metric compares the direct measurement of length changes by one metrology gauge against the computed estimate for the same based on the other five gauges.

Like all astrometric instruments, the Space Interferometry Mission (SIM) suffers from field-dependent errors requiring calibration. Diffraction effects in the delay line, polarization rotations on corner cubes, and beam walk across imperfect optics, all contribute to field-distortion that is significantly larger than is acceptable. The bulk of the systematic error is linear across the field - that is, it results in magnification and rotation errors. We show that the linear terms are inconsequential to the performance of SIM because they are inseparable from baseline length and orientation errors. One approach to calibrating the higher-order terms is to perform 'external' calibration; that is, SIM periodically makes differential measurements of a field of bright stars whose positions are not precisely known. We describe the requirements and constraints on the external calibration process and lay the groundwork for a specific procedure detailed in accompanying papers.

The Space Interferometry Mission (SIM) is fundamentally a one-dimensional instrument with a 15-degree field-of-regard. Mission objectives require a global reference grid of thousands of well-understood stars with positions known to 4 microarcseconds which will be used to establish the instrument baseline vector during scientific observations. This accuracy will be achieved by frequently observing a set of stars throughout the mission and performing a global fit of the observations to determine position, proper motion and parallax for each star. Each star will be observed approximately 200 times with about 6.5 stars per single instrument field on the sky. We describe the nature of the reference grid, the candidate objects, and the results of simulations demonstrating grid performance, including estimates of the grid robustness when including effects such as instrument drift and possible contamination of the grid star sample by undetected binaries.

With a white-light interferometer (Fine Guidance Sensor 3) on the Hubble Space Telescope (HST) we have secured fringe scanning and fringe tracking observations to measure distances, orbits, and, hence, masses, for several nearby low-mass stars. We have made progress towards a more precise Mass-Luminosity Relation (MLR) for the lower Main Sequence. However, the MLR is a map whose low mass region is complicated by relative and absolute age and whose high-mass end is very poorly determined. To begin to disentangle these effects, and to obtain high-precision mass determinations throughout the Main Sequence, we will participate in the Space Interferometry Mission (SIM) to observe binary stars of all masses in five star clusters with a large range of well-known ages and chemical compositions. We will also observe a sample of stars throughout the Main Sequence. The unparalleled angular resolution and limiting magnitude of SIM will allow us to obtain masses precise to 1%.

The Space Interferometry Mission (SIM) spacecraft will be used to measure the proper motions for a sample of ~30 nearby galaxies. At this time there are no proper motion measurements of galaxies beyond the satellite systems of the Milky Way. With the capability of measuring absolute positions to 4 mas (microarcsecond) accuracy and a five-year baseline, SIM will be able to measure proper motions as small as 10 km/s over the Local Group and 40 km/s at 4 Mpc. The motion of each galaxy will be monitored by targeting 5-10 stars that are brighter than 20th magnitude. SIM measurements will lead to knowledge of the full 6-dimensional position and velocity vectors of each galaxy. In conjunction with gravitational flow modeling, improved total mass measurements of individual galaxies and the fractional contribution of dark matter to galaxies of the Local Group will be obtained. The project includes development of theoretical methods for orbital calculations.

We present the basic elements and first results of an end-to-end simulation package whose purpose is to test the validity of the Space Interferometer Mission design. The fundamental simulation time step is one millisecond, with substructure at 1/8 ms, and the total duration of the simulation is five years. The end product of a given 'wide-angle' astrometry run is an estimated grid star catalog over the entire sky with an accuracy of about 4 micro-arcseconds. SIMsim is divided into five separate modules that communicate via data pipes. The first generates the 'truth' data on the spacecraft structure and laser metrology. The second module generates uncorrupted fringes for the three SIM interferometers, based on the current spacecraft orientation, target stars' positions, etc. The third module reads out the CCD white light fringe data at specified times, corrupting that and the metrology data with appropriate errors. The data stream out of this module represents the basic data stream on the simulated spacecraft. The fourth module performs fringe-fitting tasks on this data, recovering the total path delay, and the fifth and final module inverts the entire metrology/delay dataset to ultimately determine the instantaneous path delay on a fiducial baseline fixed in space. (Pathlength feed forward is used every few milliseconds to re-position the interferometer to keep the fringes in the delay window.) The average of all such delays over an integration time (typically 30 s) is reported as one of several hundred thousand measured stellar delays over the five-year period, which are then inverted to produce the simulated catalog. Future plans include taking into account more sources of error from the SIM error budget and including narrow angle observations in the observing plan.

The Astrophysics of Reference Frame Tie Objects Key Science program will investigate the underlying physics of SIM grid objects. Extragalactic objects in the SIM grid will be used to tie the SIM reference frame to the quasi-inertial reference frame defined by extragalactic objects and to remove any residual frame rotation with respect to the extragalactic frame. The current realization of the extragalactic frame is the International Celestial Reference Frame (ICRF). The ICRF is defined by the radio positions of 212 extragalactic objects and is the IAU sanctioned fundamental astronomical reference frame. This key project will advance our knowledge of the physics of the objects which will make up the SIM grid, such as quasars and chromospherically active stars, and relates directly to the stability of the SIM reference frame. The following questions concerning the physics of reference frame tie objects will be investigated. What is the origin of optical emission in quasars? Are the optical photo-centers of quasars compact and positionally stable on the micro-arcsecond level? Are binary black hole mergers responsible for quasars? What is (are) the emission mechanism(s) responsible for generating radio emission in chromospherically active stars. What causes the transition of spherically symmetric Asymptotic Giant Branch (AGB) stars to asymmetric planetary nebulae (PNe)?

We will use the astrometric capabilities of the SIM to answer three key questions about active galactic nuclei: 1)Does the separation of the radio core and optical photocenter of quasars change on the same timescale as their photometric variability, or is the separation stable? 2)Does the most compact optical emission from an active galactic nucleus come from an accretion disk or from a relativistic jet? 3)Do the cores of galaxies harbor binary supermassive black holes remaining from galaxy mergers? We will compare the radio and optical positions of quasars used in the tie between optical and radio celestial reference frames. During the first year after launch, we will be able to show whether the frame tie will be limited by 'astrophysical noise'.

We have developed a technique that allows SIM to measure relative stellar positions with an accuracy of 1 micro-arcsecond at any time during its 5-yr mission. Unlike SIM's standard narrow-angle approach, Gridless Narrow Angle Astrometry (GNAA) does not rely on the global reference frame of grid stars that reaches full accuracy after 5 years. GNAA is simply the application of traditional single-telescope narrow angle techniques to SIM's narrow angle optical path delay measurements. In GNAA, a set of reference stars and a target star are observed at several baseline orientations. A linearized model uses delay measurements to solve for star positions and baseline orientations. A conformal transformation maps observations at different epochs to a common reference frame. The technique works on short period signals (P=days to months), allowing it to be applied to many of the known extra-solar planets, intriguing radio/X- ray binaries, and other periodic sources. The technique's accuracy is limited in the long-term by false acceleration due to a combination of reference star and target star proper motion. The science capability, 1 micro-arcsecond astrometric precision, is unique to SIM.

This paper describes the science scheduling and operation of the Space Interferometry Mission (SIM) as a science instrument. The SIM Science Team has defined a set of science programs to be observed with SIM. The scheduling and operation of SIM, and modeling of the expected performance, differs significantly from other astrometric or imaging space-based telescopes. A timeline of observations is developed using a scheduling unit called a 'tile' - a set of related science and reference star observations made while the instrument is inertially pointed. Global astrometry, for measuring stellar distances and velocities, is performed by observing multiple overlapping tiles covering the whole sky. Most observing program require many short observations over the life of the mission, which poses interesting challenges for scheduling tools in developing the science timeline. Development of these tools will be done by the Interferometry Science Center at Caltech. Planning tools will allow users to perform trades of the expected science performance, for instance, by varying the number of observations of a target against the time spent on an individual observation. The search for planets will require careful optimization of the selection of target and reference stars in a tile, and their observing sequence, to minimize instrumental systematic errors which may be a function of direction on the sky or due to thermally-induced drifts in the instrument.

An external calibration technique for SIM^1,2,3 involves measurement of calibration stars whose positions must already be known to an accuracy of 2 milliarcseconds. We demonstrate a procedure that effectively 'bootstraps' calibration star positions from an ab initio catalog to the required accuracy by observing them with the uncalibrated SIM instrument.

Astrometry in crowded fields is an important component of the science program of the Space Interferometry Mission (SIM). Resolving multiple point sources within the SIM beam, or imaging of complicated, extended source structures requires a (large) number of interferometer baselines. As the spacecraft design keeps evolving, the impact on various key projects needs to be studied. In this paper, we discuss the capabilities of the latest SIM design (with only two baselines available for science measurements) for measuring stellar proper motions in crowded fields. Using the nucleus of the Andromeda Galaxy (M 31) as a case study, we quantify the roll angle increment needed to enable such measurements with the reduced SIM baseline set. In particular, we demonstrate that SIM can measure Keplerian motion of luminous stars around the 300 million solar mass black hole in M 31, provided that the spacecraft roll angle can be chosen in increments of around 4 degrees or smaller.

ESA's DARWIN will be an interferometric mission carrying out high-resolution astrophysical observations as well as the detection/characterization of earthlike exoplanets. In this paper, the current status and development perspectives of the Darwin imaging mode are discussed. First, overall system aspects are addressed including expected sensitivity, and baseline reconfiguration needs. Subsequently, the current instrumental concept is reviewed. This is based on a phase-referencing architecture supporting simultaneous observation of the science object, and an off-axis reference target for OPD stabilization purposes. The reference and science beams are wavelength-multiplexed and propagate along a common path through the interferometer. The viability of the cophasing approach is discussed, with emphasis on crosstalk control for multiplexed beam transfer, real-time compensation of the astrometric OPD, and associated metrology requirements. Studies have shown that imaging capabilities can be implemented within the current nulling beam combiner concept, which avoids the complexity and cost of developing a dedicated imaging beam combiner spacecraft. However, this approach has important drawbacks for the imaging mission

We report on the progress in developing cryogenic delay lines and integrated optics components. These are some of the critical components needed to enable far-IR direct-detection interferometers. To achieve background-limited performance in the 40 to 400 μm region, th einterferometer optics and delay lines must be cooeld to near liquid Helium temperatures. Our cryogenic delay line designs incorporate a number of novel features and has been operated at liquid nitrogen temperatures. Our integrated optics effort has focued on producing single-mode spatial filters and beam combiners.

The x-ray band of the spectrum is the natural place to perform super-high resolution imaging of astronomical objects. Because x-ray sources can have very high surface brightness and interferometers can be made with very short baselines, x-ray interferometry has great potential. I will discuss MAXIM, the Micro-Arcsecond X-ray Imaging Mission and MAXIM Pathfinder, a coordinated pair of x-ray astronomy missions designed to exploit the potential of x-ray interferometry. We will show how it is possible to achieve huge gains in resolution using today's technology. The Pathfinder mission will achieve resolution of 100 micro-arcseconds and will image the coronae of the nearby stars. MAXIM, with a design specification of 0.1 micro-arcseconds, has the goal of imaging the event horizons of massive black holes. I will explain the architecture of the missions and describe the activities NASA is supporting in the area of x-ray interferometry.

Filled aperture telescopes can deliver a real, high Strehl image which is well suited for discrimination of faint planets in the vicinity of bright stars and against an extended exo-zodiacal light. A filled aperture offers a rich variety of PSF control and diffraction suppression techniques. Filled apertures are under consideration for a wide spectral range, including visible and thermal-IR, each of which offers a significant selection of biomarker molecular bands. A filled aperture visible TPF may be simpler in several respects than a thermal-IR nuller. The required aperture size (or baseline) is much smaller, and no cryogenic systems are required. A filled aperture TPF would look and act like a normal telescope - vendors and users alike would be comfortable with its design and operation. Filled aperture telescopes pose significant challenges in production of large primary mirrors, and in very stringent wavefront requirements. Stability of the wavefront control, and hence of the PSF, is a major issue for filled aperture systems. Several groups have concluded that these and other issues can be resolved, and that filled aperture options are competitive for a TPF precursor and/or for the full TPF mission. Ball, Boeing-SVS and TRW have recently returned architecture reviews on filled aperture TPF concepts. In this paper, I will review some of the major considerations underlying these filled aperture concepts, and suggest key issues in a TPF Buyers Guide.

Planet detection around a bright star is dependent on the resolution of the imaging system and the degree of light suppression of the star relative to the planet. We present a concept and a scaled precursor for a visible light Terrestrial Planet Finding (VTPF) mission. Its major feature is an imaging system for planet detection using a nulling interferometer behind a single aperture telescope. This configuration is capable of detecting earth-like planets with a 4m aperture using both imaging and spectroscopic imaging modes. We will describe the principles of the system, and show results of studies demonstrating its feasibility.

The Hubble Space Telescope's Fine Guidance Sensor FGS1r has been used to observe cool white dwarf stars with apparent magnitudes that are near the FGS's faint limit. We had expected to discover that about 10% of these stars are actually binary white dwarf systems. Furthermore, we expected the binaries to have angular separations much larger than the size of the FGS white light fringes, making them easy to resolve. Although we did find 10% of the stars to be binaries, most have angular separations less than 25 milli-arcseconds, well below the HST diffraction limit. Instead of two widely separated fringes, we observed fringes that displayed subtle differences, in amplitude and morphology, from those of point sources. A major complication for our program was the need to address and remove the effects of the detector's dark current, which for the faintest targets contributed up to 40% of the counts. This paper outlines the process we employed to extract the science from the data. Our scientific motivation is briefly discussed

'Densified-pupil multi-aperture imaging arrays', also called hypertelescopes, provide a path towards rich images obtained directly at the focal plane. They typically involve a large Fizeau arrangement, with a small attached 'pupil densifier' serving to gain luminosity at the expense of field. At scales ranging from kilometers to perhaps a million kilometers, such architectures appear of interest for stellar physics, galaxies, cosmology, and neutron star imaging with the larger sizes. Ground testing is initiated and space versions are proposed, particularly to NASA for its Terrestrial Planet Finder. The coronagraphic imaging achievable with this space version is expected to improve the detection sensitivity to attenuating the sky background contribution. Subsequent laser versions can in principle resolve the 'green spots' on an Earth seen at several parsecs. Current design work for a precursor array of 'flying mirrors' driven by solar sails in geostationary orbit will be presented.

An important step in the development of new concepts for imaging interferometry in space is to obtain a clear view of the imaging capabilities of the concept array for the intended set of target sources. This view needs to include an accurate rendition of the shape of the final point spread function, a photometrically accurate computation of the final images after restoration, and an evaluation of the image dynamic range for a realistic set of target image structures and brightness levels. An imaging simulator which provides for these features is a useful tool for the exploration of parameter space, and can support and help to guide the development phase of new space imaging interferometer concepts. The accomplishments and limitations of ground-based synthesis imaging both at radio and optical wavelengths provide the reference for an evaluation of the expected contributions from new space mission concepts. This paper presents a general framework for imaging simulators, both in Michelson and in Fizeau modes, and discusses briefly several implementations which we have created over the past few years. The first of these was designed to simulate the imaging capabilities of the (Michelson) Space Interferometry Mission (SIM) at optical wavelengths, and a version has recently been completed for the (Fizeau) Stellar Imager (SI) Optical/UV interferometer concept; these two simulators are described in more detail elsewhere in this session. We are also developing simulators for an imaging mode of the Mid-IR interferometer version of the Terrestrial Planet Finder (TPF-IR), for the Submillimeter Probe for the Evolution of Cosmic Structure (SPECS, and its precursor mission SPIRIT), and for the Mid-IR concept system Fourier-Kelvin Stellar Interferometer (FKSI).

The Wide-field Imaging Interferometry Testbed was designed to validate, experiment with, and refine the technique of wide field mosaic imaging for optical/IR interferometers. We offer motivation for WIIT, present the testbed design, and describe algorithms that can be used to reduce the data from a spatial and spectral Michelson interferometer. A conventional single-detector Michelson interferometer operating with narrow bandwidth at center wavelength lc is limited in its field of view to the primary beam of the individual telescope apertures, or ~λ_c/d_tel radians, where dtel is the telescope diameter. Such a field is too small for many applications; often one wishes to image extended sources. We are developing and testing techniques analogous to the mosaicing method employed in millimeter and radio astronomy, but applicable to optical/IR Michelson interferometers, in which beam combination is done in the pupil plane. An N_pix × N_pix array detector placed in the image plane of the interferometer is used to record simultaneously the fringe patterns from many contiguous telescope fields, effectively multiplying the field size by N_pix/2, where the factor 2 allows for Nyquist sampling. This technique will be especially valuable for interferometric space observatories, such as the Space Infrared Interferometric Telescope and the Submillimeter Probe of the Evolution of Cosmic Structure.

In preparation for the planet-finding missions DARWIN (ESA) and the Terrestrial Planet Finder (NASA) a range of precursor missions are being defined, aimed at testing and validating the technology needed to make the planet-finder missions feasible from a technology point of view. In Europe the SMART-2 mission is meant to test high critical technologies for the DARWIN and the gravitation wave mission LISA (ESA/NASA). The mission SMART-2 consists of two spacecraft. These two spacecraft will demonstrate the feasibility of formation flying related to the DARWIN mission. Furthermore SMART-2 will simulate a stellar interferometer by combining white light from the two spacecraft in an interferometric focus. Two fringe-tracking modes of operations will be tested. In the standard fringe-tracking mode an onboard optical delay line is commanded to keep the optical path difference within the coherence length of the combined light. In the second mode the optical path difference is equalised by commanding the FEEPS (Field Emission Electric Propulsion) thrusters. In both modes a range of metrology systems are needed to measure deviations from the nominal configuration of the two spacecraft. Here we report on the work related to metrology systems for the SMART-2 mission needed to measure the longitudinal distance with nanometer accuracy and the lateral position of one spacecraft with respect to the second spacecraft with 5 mm accuracy. We discuss the present concepts for the metrology systems for SMART-2 and we will elaborate on the possibility to integrate the different optical metrology systems into a single system reducing complexity, risks and mass.

A number of proposed space missions for high resolution imaging at wavelengths ranging from IR to UV call for ``dilute-aperture'' Fizeau-mode interferometers. We present here details of a software tool developed for high fidelity simulations of images obtained with such instruments. We show simulated images from the Stellar Imager, a mission concept being developed by NASA's GSFC to obtain high-resolution images of nearby stars in UV-optical wavelengths. Using the simulator, we study the capability of the proposed SI design to image stellar surfaces. We use the simulator to explore parameters of image quality such as resolution and dynamic range, and to evaluate proposed designs and the feasibility of science goals.

Fourier Transform Spectrometers usually consist of a single telescope with the beam split into two paths prior to the focal plane. The beams form a Michelson interferometer with beam recombination occurring at the focal plane. The path length of one beam is varied in order to scan through the white light fringe packet while a series of images is collected. Fourier transforming each pixel of the image across the series results in a spectral data cube of the scene. We propose using a multiple telescope FIzeau Imager for collecting Fourier Transform Spectrometer data. The path lengths through one telescope are varied while a series of images is collected. The processing is similar to the standard IFTS with some modification due to the necessity of image restoration. We present preliminary results from a laboratory multiple telescope FIzeau Imaging system.

Future high-resolution space telescoeps will use discontinuous apertures, either primary segments or sub-telescopes. One of the most critical points in the operation of such instruments will be the cophasing of the sub-apertures. Cophasing sensors are available on ground interferometers, allowing sub-aperture piston/tip/tilt measurement on unresolved or slightly resolved starts. But when observing a very extended object, such as the Earth as seen from space, no reference star can be found in the field. In this case, the cophasing measurement must be derived from the observed object itself, which is a major issue for extended objects. Phase diversity is one of the very few solutions to this problem. Phase diversity consists in the joint estimation of the object and the instrument aberrations from the analysis of several images obtained with different but perfectly known aberrations, for example the focal image and a slightly defocused image. Theoretical analysis and numerical simulations were carried out to investigate how our phase diversity algorihtm behaves when estimating sub-aperture piston and tip/tilt in various conditions. Our study shows that the aperture configuration has a major impact on performance. For diluted apertures, when the optical transfer function has zeros within the frequency domain of interest, it can be shown that at least two possible solutions can be derived by phase diversity. For redundant apertures, when several pairs of sub-aperture contribute to the same spatial frequency, the piston or tip/tilt estimation is degraded for sub-apertures contributing to redundant frequencies.

The Wide-Field Imaging Interferometry Testbed (WIIT) will provide valuable information for the development of space-based interferometers. This laboratory instrument operates at optical wavelengths and provides the ability to test operational algorithms and techniques for data reduction of interferometric data. Here we present some details of the system design and implementation, discuss the overall performance of the system to date, and present our plans for future development of WIIT. In order to make best use of the interferometric data obtained with this system, it is critical to limit uncertainties within the system and to accurately understand possible sources of error. The WIIT design addresses these criteria through a number of ancillary systems. The use of redundant metrology systems is one of the most important features of WIIT, and provides knowledge of the delay line position to better than 10 nm. A light power detector is used to monitor the brightness of our light sources to ensure that small fluctuations in brightness do not affect overall performance. We have placed temperature sensors on critical components of the instrument, and on the optical table, in order to assess environmental effects on the system. The use of these systems provides us with estimates of the overall system uncertainty, and allows an overall characterization of the results to date. These estimates allow us to proceed forward with WIIT, adding rotation stages for 2-D interferometry. In addition, they suggest possible avenues for system improvement. The possibility exists to place WIIT inside an environmentally controlled chamber within the Diffraction Grating Evaluation Facility (DGEF) at Goddard in order to provide maximum control over environmental conditions. Funding for WIIT is provided by NASA Headquarters through the ROSS/SARA Program and by the Goddard Space Flight Center through the IR&D Program.

The MAXIM Pathfinder (MP) and Stellar Imager (SI) missions are under study to do 100 microarcsecond resolution imaging for a number of different targets using interferometers divided over formation flying spacecrafts. One of the most challenging technical hurdles for these missions is to have an independent directional reference in the sky to use for target acquisition and tracking. This directional reference will guide the placement of separate free flying elements of the interferometers to have ~<30 microarcseconds of alignment with the target. This paper will discuss some of the specific challenges as well as some possible options to explore for achieving this alignment.

The Spectral Astrometry Mission is a space-mission concept that uses simultaneous, multiple-star differential astrometry to measure exo-solar planet masses. The goal of SAM is to measure the reflex motions of hundreds of nearby (~50 pc) F, G and K stars, relative to adjacent stars, with a resolution of 2.5 micro-arcsec. SAM is a new application of Spectral Interferometry (SI), also called Externally Dispersed Interferometry (EDI), that can simultaneously measure the angular difference between the target and multiple reference stars. SI has demonstrated the ability to measure a λ/20,000 white-light fringe shift with only lambda/3 baseline control. SAM's structural stability and compensation requirements are therefore dramatically reduced compared to existing long-arm balanced-arm interferometric astrometry methods. We describe the SAM's mission concept, long-baseline SI astrometry method, and technical challenges to achieving the mission.

The Space Interferometry Mission (SIM) has some very tight stability requirements that drive the thermal control approach well beyond the traditional spacecraft thermal control regime. The precision support structure will be constructed of composite materials with a quite low coefficient of thermal expansion (CTE) on the order of 10^-7/K. Even then, the temperature variations of the structure cannot exceed about 0.2°C. For the main optical elements, which will be fabricated of ultra-low expansion glass, the temperature stability must be such that the temperature gradient through the glass cannot vary by more than a couple of millikelvin through the 5 cm thickness over a one hour period. The laser metrology system, which measures motions on the order of a few tens of picometers, contains some sensitive optical elements whose temperature variations cannot exceed a few tens of microkelvin. This paper will describe how the SIM thermal control designers have addressed some of these very challenging requirements.

The Space Interferometry Mission (SIM), planned for launch in 2009, will measure the positions of celestial objects to an unprecedented accuracy of 4.0 microarcseconds. In order to achieve this accuracy, which represents an improvement of almost two orders of magnitude over previous astrometric measurements, a ten-meter baseline interferometer will be flown in space. NASA challenges JPL and its industrial partners, Lockheed Martin and TRW, to develop an affordable mission. This challenge will be met using a combination of existing designs and new technology. Performance and affordability must be balanced with a cost-conscious Systems Engineering approach to design and implementation trades. This paper focuses on the Lockheed Martin-led Starlight (STL) and Metrology (MET) subsystems within the main instrument of SIM. Starlight is collected by 35cm diameter telescopes to form fringes on detectors. To achieve the stated accuracy, the position of these white-light fringes must be measured to 10-9 of a wavelength of visible light. The STL Subsystem consists of siderostats, telescopes, fast steering mirrors, roof mirrors, optical delay lines and beam combiners. The MET Subsystem is used to measure very precisely the locations of the siderostats with respect to one another as well as to measure the distance traveled by starlight from the siderostat mirrors and reference corner cubes through the system to a point very close to the detectors inside the beam combiners. The MET subsystem consists of beam launchers, double and triple corner cubes, and a laser distribution system.

Optical systems, which operate over a wide range of Fresnel numbers, are often times performance-limited by diffraction effects. In order to characterize such effects at the 40-100 picometer level, a diffraction testbed has been built which has the capability of measuring diffraction effects at this level. Concurrently, mathematical diffraction modeling tools have been developed that propagate an input wavefront through an optical train, while retaining amplitude and phase information at a grid resolution sufficient for yielding picometer-resolution diffraction test data. This paper contains a description of this diffraction hardware testbed, the diffraction modeling approach, and a comparison of the modeled and hardware test results, which then serves as validation of the diffraction modeling methodology.

Future space-based optical interferometers such as the Space Interferometer Mission require fringe stabilization to the level of nanometers in order to produce astrometric data at the micro-arc-second level. Even the best attitude control system available to date will not be able to stabilize the attitude of a several thousand pound spacecraft to a few milli-arc-seconds. Active pathlength control is usually implemented to compensate for attitude drift of the spacecraft. This issue has been addressed in previous experiments while tracking bright stars. In the case of dim stars, as the sensor bandwidth falls below one hertz, feedback control will not provide sufficient rejection. However, stabilization of the fringes from a dim-star down to the nanometer level can be done open loop using information from additional interferometers looking at bright guide stars. The STB3 testbed developed at the Jet Propulsion Laboratory features three optical interferometers sharing a common baseline, dynamically representative to the SIM interferometer. An artificial star feeding the interferometers is installed on a separate optics bench. Voice coils are used to simulate the attitude motion of the spacecraft by moving the entire bench. Data measured on STB3 show that fringe motion of a dim star due to spacecraft attitude changes can be attenuated by 80 dB at 0.1Hz without feedback control, using only information from two guide stars. This paper describes the STB3 setup, the pathlength feed-forward architecture, implementation issues and data collected with the system.

At first glance the SIM concept seems deceptively simple. After all, its primary task is just to lock two interferometers on to a couple of bright guide stars and let the third integrate photons from a dim 'science' star to gather astrometric data. The difficulty is of course in the details of the performance requirements and overall integration and test risk associated with their verification. The challenge is to provide a meaningful verification of the SIM functionality on the ground that can be extrapolated to show satisfaction of the on-orbit performance requirements. The associated difficulties relate to the ability to provide a reasonable simulation of (1) the space environment with all the implications and (2) the creation of simulated target stars for each interferometer input optics that meet the associated wave front characteristic and star position knowledge requirements. The difficulty and complexity of the simulation of target stars itself is a major development challenge and program risk. In order to reduce this risk early development test beds are created to evolve the optical verification concept and build the actual devices needed for the flight system performance evaluation. The role of the PKT and FAST (Flight System Astrometric Test bed) test beds and their influence on flight integration risk reduction and test process is presented.

The Kite testbed at JPL is being developed to demonstrate relative metrology on a four-cornered metrology truss with six gauges to 10's of picometers. The solution to the truss equation requires accurate knowledge of the direction cosines between the fiducials, which is derived from the absolute distances that the gauges measure. The absolute distance accuracy is required at better than 10μm in order for relative metrology to measure picometer accuracy. In this paper, a technique called switched heterodyne interferometry is described and implemented. The implementation is in a calibration testbed which is used to develop the hardware and software to perform this technique, as well as to calibrate the interferometric beamlaunchers which are part of the metrology gauges. Some early tests were successful, but recent tests have had more noise in the measurement than acceptable. The reasons for this are under investigation.

Visible interferometry at µarc-second accuracy requires measurement of the interferometric baseline length and orientation at picometer accuracy. The optical metrology instruments required for these interferometers must achieve accuracy on order of 1 to 10 picometers. This paper discusses the progress in the development of optical interferometers for use in distance measurement gauges with systematic errors below 100 picometers. The design is discussed as well as test methods and test results.

The Stellar Interferometry Mission (SIM) and particularly one of its testbeds require compound optical pieces the construction and qualification of which, in turn, require very high-precision absolute surface metrology gauges. In this paper, the details of the design, construction and performance of a triplet of interferometers capable of performing the required measurements are presented.

To accomplish micro-arcsecond astrometric measurement, stellar interferometers such as SIM require the measurement of internal optical path length delay with an accuracy of ~10 picometers level. A novel common-path laser heterodyne interferometer suitable for this application was proposed and demonstrated at JPL. In this paper, we present some of the experimental results from a laboratory demonstration unit and design considerations for SIM's internal metrology beam launcher.

The Precision Structure Subsystem (PSS) for the Space Interferometry Mission (SIM) is a large composite structure designed to house the interferometer optics in a structurally and thermally stable environment on orbit. The resulting design requirements of the PSS must be weighed against the demands of the baseline launch vehicle: the Space Shuttle. While a Shuttle launch provides new opportunities for the mission, it also presents new challenges. Many of these chal-lenges are reflected in the design of the PSS, including structural stability for supporting the optics on orbit, launch vehi-cle interface considerations (acoustic and stress loads), minimization of launch mass to provide maximum payload to orbit, thermal control to achieve necessary structural stability and a stable thermal environment for the optics, and isola-tion of the optics mounts from jitter sources and microdynamics effects. Many of these design challenges result in inherently conflicting requirements on the design of the PSS. Drawing on our experience with large composite structures such as the Chandra X-ray Observatory, TRW has created a conceptual design for this structure that addresses these challenging requirements. This paper will describe that conceptual design including trades and analyses that led to the design.

SIM System Testbed 3 (STB3) features three optical interferometers sharing a common baseline, as a dynamic representation of the SIM instrument. An artificial star feeding the interferometers is installed on a separate optics bench. All three interferometers use photons captured by avalanche photo diodes (APDs) to measure the position and quality of fringes, and additional pointing precision is achieved by fast steering mirrors (FSMs) that keep the star images centered on the beam combining optics using a CCD camera. Each interferometer uses internal metrology to measure changes in its optical pathlength. External metrology beams measure changes in the baseline vector. This system acquires and tracks white light fringes with one interferometer, while the other two acquire and track laser light fringes representing the bright guide stars that will be used by SIM. The white light source represents a dim star that cannot supply enough photons for the Science interferometer to lock onto fringes in closed-loop mode; instead it operates open-loop, using pathlength corrections fed to it from the two guide interferometers and the external metrology subsystem to reject disturbances and maintain the fringes. This tracking mode is known as Pathlength Feed Forward (PFF). The precise real-time behavior required to achieve this result is implemented by a complex set of interacting software control loops. This paper describes how these loops take advantage of the benefits of the RTC Core architecture, and how they work together to accomplish STB3's objectives.

The Space Interferometer Mission (SIM) demands extremely precise and well-characterized laser metrology gauges (also called beam launchers) to monitor the internal and external optical delay quantities which are required for astrometric measurements. In general, any space-based sparse aperture system will require laser metrology gauges for high-bandwidth sensing of phasing errors. Lockheed Martin has aggressively pursued a technology development program for high-accuracy, space-qualified laser gauge systems. Part of this effort is focused on making compact, lightweight, low-power consumption, relatively inexpensive beam-launcher units using integrated-optics components. This paper will describe the design, laboratory implementation, performance, and error analysis for an integrated-optic based laser gauge that was constructed in FY 2000-2001 using commercially available heterodyne interferometer optics and electronics, combined with commercial fiber-optic cables and splitters. In order to provide for heterodyne mixing between the signals in the reference and measurement arms of the gauge, polarization-maintaining (PM) fiber components were used. The PM fiber lengths were matched to within 0.5 mm to avoid differential thermal effects in the measurement and reference arms. Steps were also taken to minimize the cyclic phase error due to polarization leakage, and the residual cyclic errors were measured. While not meeting the extreme picometer-level measurement accuracy requirements of SIM, the gauge can distinguish optical path differences to better than a 10 nm accuracy, which is sufficient for many space applications.

The Space Interferometry Mission (SIM) is a space-based long baseline optical interferometer designed to perform precision astrometry to an unprecedented accuracy. Highly accurate white light fringe estimation is an important enabling technology for the success of SIM. To accomplish this, the combined light from the two arms of the interferometer is sent through a prism so that fringes formed at different wavelengths are dispersed across a number of spectral channels. The relative optical path difference (OPD) between the two arms is modulated so that an estimate of the phase in each of the channels can be obtained using phase shifting interferometry (PSI) techniques. The present paper presents several of the difficulties encountered in white light fringe estimation for SIM, and offers a number of alternatives for mitigating them.

Future space-based optical interferometers, such as the Space Interferometer Mission, require fringe measurements at the picometer level in order to produce astrometric data at the micro-arc-second level. More specifically, both the position of the central starlight fringe and the change in the internal optical path of the interferometer must be measured to a few tens of picometers. The internal path is measured with a small metrology beam, whereas the starlight fringe position is estimated with a CCD sampling a large concentric annular beam. One major challenge for SIM is to align the metrology beam with the starlight beam to ensure consistency between these two sensors. The Micro-Arcsecond Metrology testbed (MAM) developed at the Jet Propulsion Laboratory features an optical interferometer with a white light source, all major optical components of a stellar interferometer, and heterodyne metrology sensors. The experiment is installed inside a large vacuum chamber in order to mitigate atmospheric and thermal disturbances. Both the white light and metrology sensors have been proven to work independently at the required levels. The next step is to integrate them as a micro-arc-second capable system. A complex alignment sequence has been developed in order to match the absolute tilt and shear of the metrology and starlight paths to 1 micro-radian and 10 micrometers respectively. This paper describes the MAM optical setup, the alignment process, the contribution of the fine alignment to the final performance, and how they relate to SIM.

Future space-based optical instruments such as the Space Interferometer Mission have vibration-induced error allocations at the levels of a few nano-meters and milli-arc-seconds. A dual stage passive isolation approach has been proposed using isolation first at the vibration-inducing reaction wheels, and a second isolation layer between the bus portion of the space vehicle (the backpack) and the optical payload. The development of the backpack isolator is described, with unit transmissibility results for individual isolator struts. The dual stage isolation approach is demonstrated on a dynamically feature-rich, 7-meter structural testbed (STB3). A new passive suspension that mitigates ground vibrations above 0.4 Hz has been integrated into the testbed. A series of OPD performance predictions have been made using measured transfer functions. These indicate that the 5-nm dynamic OPD allocation is within reach using the dual isolator approach. Demonstrating these low response levels in a noisy air environment has proven to be difficult. We are sequentially executing a plan to mitigate acoustic transmission between backpack and flight structure, as well as developing techniques to mitigate effects of background acoustic noise.

TThe SIM Interferometry Test Bed 3 (STB3) is the spaceborne-stellar-interferometer simulator for SIM, built and operating at JPL. Its construction details and performance are described elsewhere in this conference. The test bed consists of an interferometer system built on a large optical table, and a star simulator built on another large optical table placed directly across it. The optical tables float on independent, air-filled suspension legs simulating the SIM spacecraft and the distant stars it is to observe. In order to demonstrate the performance requirements, a novel attitude control system (ACS) has been built and installed on the STB3. In this paper, the details of the design, construction and performance of the attitude control system are presented. The attitude control system has been used to meet certain SIM requirements. An example of this performance test is also included.

The astrometric performance of the SIM relies on precise measurements of the optical pathlength difference of the starlight through the arms of the interferometers that comprise the SIM instrument, and on precise relative distance betweeen a set of fiducials that define the baselines of the interferometers. The accuracy of these measurements can be affected by various phenomena. Some of them are time-dependent, while others are relatively static and repeatable. In this work we are concerned with the instrument errors of the latter type and in their compensation. In particular, a procedure for on-orbit calibration of the instrument error function is defined, and a proof of concept of its viability is presented. On a given grid of stars, the proposed procedure generates approximations of the gradient of the instrument error function at a discrete set of field points corresponding to the star locations via a specialized set of maneuvers of the spacecraft. These gradient approximations are then used to estimate the error function via a least squares procedure in a manner that is very analogous to the wavefront reconstruction problem in adaptive optics systems. An error analysis of the procedure is presented providing further insights into the connections between instrument errors and the grid reduction solution. Finally, numerical results are presented on a randomly generated grid of stars that demonstrate the feasibility of the method.

Picometer scale optical metrology specifications for the Space Interferometry Mission require precision calibration functions involving the optical and orientation characteristics of corner cube retroreflectors. Accurate knowledge of such parameters as the index of refraction of the reflective coating, dihedral between facets, and the orientation of the retroreflector with respect to the interrogating metrology beam and its polarization state is critical. Knowledge errors result in optical path differences that are shown to be on the order of nanometers. These sensitivities are determined from Zemax-generated models and measured parameters. Due to the stringent requirements of SIM, accurate and consistent experimental measurements of corner cube characteristics are required for improved calibration of mission metrology systems. Initial dihedral measurements to within 0.05 arcsecond and refractive indices to within 1% are obtained and integrated into the models.

The IR Space Interferometer Darwin is an integral part of ESA's Cosmic Vision 2020 plan, intended for a launch towards the middle of next decade. It has been the subject of a feasibility study and is now undergiogn technological development. The scientific scope is aimed towards developing a system that could carry out the search for, and characterization of Earth-like planets orbiting other stars. A secondary objective is to carry out imaging of astrophysical objects with unprecedented spatial resolution. The implementation of Darwin is based on the new technique of 'nulling interferometery', in the mid-IR and becomes the culmination of a decade of technology- and science precursor missions. Darwin is also foreseen to be carrie dout in an international context.

Several concept of space missions dedicated to the direct detection and analysis of extrasolar planets are based on nulling interferometry principle. This principle, which is theoretically very promising requires the capability of propagating and combining beams with very high accuracy in term of amplitude phase and polarization. In order to validate the principle of nulling interferometry, it is necessary to develop laboratory techniques of recombination. In this paper, we present a new test bench that should allow measuring rejection rate up to 105 in a large spectral band between 2 and 4 microns.

Nulling interferometry at mid-infrared wavelengths holds promise for finding and characterizing Earth-like planets that orbit nearby stars. By strongly suppressing light from a nearby star, the instrument becomes sensitive enough for direct detection of planets orbiting that star. A compound nulling interferometer (combining light from more than 2 telescopes) is needed for these searches, in order to achieve adequate light suppression across the full disk of the star. We present an error analysis of quasi-static and chopping variants of a four element nulling interferometer, including the dependence on amplitude, delay, baseline length, and telescope pointing errors.

The Terrestrial Planet Finder (TPF) mission is aimed at providing direct images of Earth-like planets orbiting nearby stars and characterizing their atmospheres (low resolution spectroscopy). The BOEING/SVS hypertelescope concept, NRLA (Non-Redundant Linear Array), uses a 35m baseline interferometric rotating array of six 2.3-meter telescopes operating in the infrared (7 to 12 microns) to produce wide field images of exoplanetary systems. The full (u,v) plane coverage of the array offers very good imaging capabilities, which is essential to unambiguously confirm the detection of planets, and also provides an outstanding capability for high resolution/high dynamic range imaging for general astrophysics. Thanks to a novel approach combining pupil densification, phase mask coronagraphy and pupil redilution, this concept combines wide field of view imaging and interferometric nulling of the central star. We first briefly present the techniques used by this concept (phase mask coronagraphy, pupil densification and redilution, aperture synthesis imaging) and demonstrate how they can be used to overcome the limitations commonly encountered by interferometers (low (u,v) plane coverage, small field of view, low dynamical range). A complete computer simulation of the concept has been written and is used to study the performance of the array for exoplanet imaging and spectroscopy. We show that with this concept, detection (S/N=5) of Earth-like planets at 10pc with a 5 microns spectral bandwidth can be achieved in less than an hour (for a 100% quantum efficiency).

The reduction of the thermal background emission from the local and exozodiacal dust clouds is a critical element for the success of ESA's space mission, DARWIN. Internal modulation, a technique using fast signal chopping, isolating the planetary signal from these noise sources, was proposed by Mennesson and Léger. In this paper, a short review of internal modulation is given, and new configurations with internal modulation are proposed to reduce the complexity of the beam-combining optics. A modification to the implementation of internal modulation is then investigated. It provides similar performance with a single detector and a greatly simplified optical layout: the number of beam-combiners is reduced by a factor of about two. The principle of inherent modulation is different from internal modulation in that no sub-interferometers are used: different phase shifts are applied to the input beams before recombination such that an asymmetric transmission map is obtained directly, without plus or minusπ/2 modulation as used in internal modulation. By combining the phase shifts and the input beams differently a transposed transmission map is obtained, allowing the signal to be chopped. During operations, multiplexing between the two interferometers is performed, such that at any time only one interferometer is being used.

Mid-infrared (10 micron band) interferometry from space is a promising technique for extrasolar planet detection and characterization. However, technology development in several areas is needed before a search for terrestrial planets can be performed with an interferometer. A key capability of such an instrument is the achievement of a deep (~1E-06), stable, broadband (~1 octave) interferometric null, with dependence on sky angle of quartic or broader. This performance sets requirements on amplitude, delay, polarization, and pointing (wavefront tilt) matching between different apertures of the interferometer. The wavefront quality must be ~1/1000 of a wavelength rms, probably requiring a high performance spatial filter. An additional technology challenge is to reject scattered sunlight and thermal emission from each telescope at the beam combiner optics and detector. This stray radiation will arrive at small angles to the starlight beams, making suppression difficult. The current status and suggested development path of these technologies will be discussed.

Nulling interferometry is a direct method to detect earth-like planets. To determine whether a planet is earth-like spectrometry is performed on a broadband infra-red (l = 4-20 mm) input signal from the planet. The star signal in this region is roughly 10⁶ times stronger than the planet signal. Nulling interferometry should decrease the broadband star signal by about this factor of 10⁶. This can be performed using an achromatic phaseshifter based on dispersive elements. The design of a complete breadboard under an ESA contract including a prism based (eight prisms in total) dispersive achromatic phaseshifter is presented including error budget and implied tolerances on the mechanical components. Measurements with this breadboard resulted in nulling depths of 3.5.10⁵ for polarized laser light and just below 10³ for polarized visible broadband light in the wavelength range of 530-750nm.

The StarLight flight project was designed to demonstrate the key technologies of spaceborne long-baseline stellar interferometry and precision formation flying for potential use on the Terrestrial Planet Finder (TPF) and other future astrophysics missions. Interferometer performance validation could be achieved over a 6-12 month period by obtaining several hundred fringe visibility amplitude measurements for stars in the band 600-1000 nm for a variety of stellar visibilities, magnitudes, and baselines. Interferometery could be performed both in a 1 meter fixed-baseline combiner-only mode and in a two-spacecraft formation mode. In formation mode, the combiner spacecraft would remain at the focus of a virtual parabola, while the collector spacecraft assumed various positions along the parabola such that the two arms of the interferometer remained equal over a variety of separations and bearing angles. Challenges to be encountered in flight include high-bandwidth inter-spacecraft stellar and metrology pointing control, alingment and shear correction, delay and delay-rate estimation, visibility calibration, and robust fringe trackign in the presence of local and inter-spacecraft dynamics. This paper is based on the StarLight project design-capture of March 2002 and will describe the StarLight Interferometer System architecture and selected operational concepts.

The StarLight mission is designed to validate the technologies of formation flying and stellar interferometry in space. The mission consists of two spacecraft in an earth-trailing orbit that formation-fly over relative ranges of 40 to 600m to an accuracy of 10 cm. The relative range and bearing of the spacecraft is sensed by a novel RF sensor, the Autonomous Formation Flyer sensor, which provides 2cm and 1mrad range and bearing knowledge between the spacecraft. The spacecraft each host instrument payloads for a Michelson interferometer that exploit the moving spacecraft to generate variable observing baselines between 30 and 125m. The StarLight preliminary design has shown that a formation-flying interferometer involves significant coupling between the major system elements - spacecraft, formation-flying control, formation-flying sensor, and the interferometer instrument. Mission requirements drive innovative approaches for long-range heterodyne metrology, optical design, glint suppression, formation estimation and control, spacecraft design, and mission operation. Experimental results are described for new technology development areas.

There are many advantages to space-based interferometry, but monolithic, single-spacecraft platforms set limits on the collecting area and baseline length. These constraints can be overcome by distributing the optical elements of the interferometer over a system of multiple spacecraft flying in precise formation, opening up new realms of angular resolution and sensitivity. While the principles of interferometry are the same as for structurally-connected systems, formation-flying interferometers must integrate a wide range of technologies to provide an optically stable platform capable of finding, tracking and measuring fringes. This paper discusses some of the key differences between formation-flying and structurally-connected interferometers, including formation configurations, controlling beam shear, station-keeping, and the importance of delay and delay rate estimation in determining the instrument sensitivity. Proposed future formation-flying interferometer missions include the Terrestrial Planet Finder (TPF), Darwin, the Submillimeter Probe of the Evolution of Cosmic Structure (SPECS), the Stellar Imager, the Micro-Arcsecond Xray Imaging Mission (MAXIM), and its precursor, MAXIM Pathfinder. In addition, Life Finder and Planet Imager have been identified as two formation-flying missions capable of detailed characterization of habitable exo-planets. The parameters for these missions are compared and described briefly.

As part of The National Aeronautics and Space Administration's (NASA's) endeavor to push the envelope and go where we have never been before, the Space Science Enterprise has laid out a vision which includes several missions that revolutionize the collection of scientific data from space. Many of the missions designed to meet the objectives of these programs depend heavily on the ability to perform space-based interferometry, which has recently become a rapidly growing field of investigation for both the scientific and engineering communities. While scientists are faced with the challenges of designing high fidelity optical systems capable of making detailed observations, engineers wrestle with the problem of providing space-based platforms that can permit this data gathering to occur. Observational data gathering is desired at a variety of spectral wavelengths and resolutions, calling for interferometers with a range of baseline requirements. Approaches to configuration design are as varied as the missions themselves from large monolithic spacecraft to multiple free-flying small spacecraft and everything in between. As will be discussed, no one approach provides a ?panacea? of solutions rather each has its place in terms of the mission requirements. The purpose here is to identify the advantages and disadvantages of the various approaches, to discuss the driving factors in design selection and determine the relative range of applicability of each design approach.

The StarLight mission aimed to place the first formation flying optical interferometer into space in year 2006. Utilizing two spacecraft to form a long baseline Michelson interferometer, it would measure white light fringes on a number of partially resolved stars of magnitudes >5 in the wavelength range 600 to 1000 nm. The interferometer baseline is variable between 30 and 125 m, and also has a fixed 1.3 m mode. The spacecraft are flown in a parabolic geometry which requires an optical delay line to build up more than 14 m of delay on one arm of the interferometer. To obtain high fringe visibility, starlight wavefront, pointing and intensity must be preserved through 22 reflections from mirrors and beamsplitters. The alignment of a total of 27 optics is maintained through careful thermal design and the use of two actuated mirrors on each arm. This paper describes the optical layout, including the beam combiner design which allows star tracking, optical system alignment and fringe formation on a single CCD. The effects of diffraction of the starlight transferred from a distant spacecraft and from optical surface imperfections are modeled. Other contributors to the visibility budget and the resulting variation of fringe visibility across the focal plane are discussed.

The LISA experiment has six telescopes, in three spacecraft, in orbit about the sun. There is a continuous laser link between all of the spacecraft. Because of the large, 5 million kilometer distances, between the spacecraft and the need to perform picometer level interferometry and the fact that the optical system is dynamic precludes the use of standard optical codes in the design and analysis of this optical system. A description of the mathematical approach used to model the optics, in the spacecraft in orbit, is presented and the ability of this model to analyze requirements is discussed.

Separated spacecraft interferometry is a candidate architecture for several future NASA missions. The Formation Interferometer Testbed (FIT) is a ground based testbed dedicated to the validation of this key technology for a formation of two spacecraft. In separated spacecraft interferometry, the residual relative motion of the component spacecraft must be compensated for by articulation of the optical components. In this paper, the design of the FIT interferometer pointing control system is described. This control system is composed of a metrology pointing loop that maintains an optical link between the two spacecraft and two stellar pointing loops for stabilizing the stellar wavefront at both the right and left apertures of the instrument. A novel feedforward algorithm is used to decouple the metrology loop from the left side stellar loop. Experimental results from the testbed are presented that verify this approach and that fully demonstrate the performance of the algorithm.

We describe a simple approach to laser frequency stabilization for the metrology subsystem for NASA's StarLight mission, a space-based separated-spacecraft stellar interferometer. The current design of the laser frequency stabilization is based on monitoring the transmitted light through a reference cavity. Currently our free-running lasers do not meet the frequency stability requirements of the mission (100 Hz/root(Hz) between 1 and 1000 Hz) because of the up to 600 m length difference in the two arms of the interferometer. We need additional three orders of magnitude reduction of the frequency noise power spectral density in that frequency regime to meet the 11 nm accuracy requirement for the metrology system. Because we need only a modest improvement in the frequency stability, we plan to use a simple transmit/reflect architecture in which the laser frequency is locked to one side of the cavity resonance peak. The frequency stabilization system measures the transmitted light portion of a Fabry-Perot cavity and compares it to a stable reference voltage to generate the feedback signal. This signal is controlling the laser frequency using the NPRO laser PZT and crystal temperature actuators, therefore keeping the transmitted light level on the photo detector constant. This is equivalent to keeping the laser frequency stable. Because this system measures the transmitted light level it is sensitive to laser power fluctuations. One remedy to this problem is to monitor the reflected light from the cavity as well and use the ratio transmitted/reflected as the sensor signal. The residual frequency noise in our system was measured with respect to a stabilized laser light that was frequency stabilized using Pound-Drever-Hall stabilization.

The Space Interferometry Mission's (SIM) shared-baseline astrometric interferometer System Test Bed 3 (STB3) has been constructed at JPL. STB3's objective is to use two of its interferometers (guides) for low frequency (0 to 1 Hz) fringe stabilization in the third one (science). This approach - being proposed for the first time in the context of space based observatories - is needed given the dim nature of science stars to be observed by SIM. Fringe stability is mostly affected by the low frequency attitude motion of the test bed's instrument table, with the inevitable exception of instrument vibration, thermal drift, and atmospheric fluctuations. Relative changes in table attitude cause optical path changes in the guide interferometers, which are tracked, linearly combined and fed forward to the science interferometer's active delay line to stabilize its optical path. This technique for tracking fringes in the science interferometer is possible because the position of the guide stars relative to the science star is well known. This open loop fringe tracking technique is dubbed Path-length Feed Forward, or PFF. In STB3, current fringe stability in the science interferometer using the PFF technique is at 50 to 60 nanometers RMS (from 0 to 500 Hz). Compare this to 15 to 20 nm RMS fringe stability in the guide interferometers, which operate in closed loop mode. Vibration, thermal drift and atmospherics in the science and guide interferometers are largely eliminated with the use of an internal metrology system. By design, mechanical vibrations are above the bandwidth of the interferometer system, and are passively rejected. Nevertheless, the internal metrology system can easily reject current low-level vibrations in STB3 down to the 6-nanometer RMS level. Fringe tracking error in the science interferometer due to atmospherics is currently about 40 nanometers RMS at frequencies below 1.0 Hz. In SIM, the error in this low frequency band must be no more than 6 nm RMS. This error arises because the optical path stabilized by the internal metrology system is not equal to taht of the starlight, so not all atmospheric fluctuations in the starlight path can be stabilized. Therefore, there is a need to reduce the strength of atmospheric fluctuations or to filter them from the PFF command. In STB3 the strength of atmospheric fluctuations is already reduced with the use of optical path enclosures, which brought these fluctuations down from ~170nm RMS to their current levels of ~66nm RMS with a spread of 20nm. Simulations show that signal to noise ratios are generally not sufficient to filter atmospheric errors on-line.

StarLight, a NASA/JPL mission originally scheduled for launch in 2006, proposed to fly a two spacecraft visible light stellar interferometer. The Formation Interferometer Testbed (FIT) is a ground laboratory at JPL dedicated to validating technologies for StarLight and future formation flying spacecraft such as Terrestrial Planet Finder. The FIT interferometer achieved first fringes in February 2002. In this paper we present our status and review progress towards fringe tracking on a moving collector target.

This paper presents a modeling methodology used to predict the performance of a flexible structure, such as a space telescope, in the presence of an on-board vibrational disturbance source, such as a reaction wheel assembly (RWA). Both decoupled and coupled analysis methods are presented. The decoupled method relies on blocked RWA disturbances, measured with the RWA hardmounted to a rigid surface. The coupled method corrects the blocked RWA disturbance boundary conditions using 'force filters' which depend on estimates of the interface accelerances of the RWA and spacecraft. Both methods were validated on the Micro-Precision Interferometer testbed at the Jet Propulsion Laboratory. Experimental results are encouraging, indicating that both methods provide sufficient accuracy compared to measured values; however, the coupled method provides the best results when the gyroscopic nature of the spinning RWA is captured in the RWA accelerance model. Additionally, the RWA disturbance cross spectral density terms are found to be influential.

During the preliminary design phase of space-based interferometer missions, observational requirements need to be translated into dynamical accuracy requirements on the optical components. The first part of this paper presents a methodology that specifies allowable statistical variances on the optical path difference in order to achieve a specified mean level of null depth for a nulling interferometer. These dynamical requirements can then be used as inputs to controller design processes which ensures that the closed-loop system satisfies the performance requirements. The second part of this paper describes a staging control design tool that optimally uses a suite of actuators to reject disturbances and analyzes the performance limitations as a function of actuator constraints. The particular actuator constraints considered here are saturation limit, resolution level, and the operational bandwidth of each actuator. As an example, the control design tool is applied to an example optical delay line problem yielding a feedback control law which ensures nanometer level stabilization of optical path difference for the interferometer. This benchmark problem allows the control design tool to demonstrate its capabilities on a system with stringent dynamical requirements.

Laser induced, micro-chemical etching is a promising new technology that can be used to fabricate three dimensional structures many millimeters across with micrometer accuracy. Laser micromachining possesses a significant edge over more conventional techniques. It does not require the use of masks and is not confined to crystal planes. A non-contact process, it eliminates tool wear and vibration problems associated with classical milling machines. At the University of Arizona we have constructed the first such laser micromaching system optimized for the fabrication of THz and far IR waveguide and quasi-optical components. Our system can machine many millimeters across down to a few microns accuracy in a short time, with a remarkable surface finish. This paper presents the design, operation and performance of our system, and its applications to waveguide devices for sub millimeter and far IR interferometry.

The control system architecture and vibration mitigation approach for a Terrestrial Planet Finder (TPF) mission based on structurally connected interferometers are defined. The spacecraft configurations investigated and associated control and operational requirements are presented. Disturbance sources are identified and their relevance assessed. Results of dynamics analysis are presented, as well as a description of the dynamic models and simulations used to predict on-orbit performance. An assessment of the maturity of the technologies for control and vibration mitigation is provided. Analysis results indicate that pointing and path-length control requirements for a TPF mission based on structurally connected interferometers with baselines from 9 to 80 meters can be achieved using a conventional spacecraft attitude control system combined with active pointing and path-length control and a vibration mitigation approach that does not rely on structural damping.

The success of interferometry in space depends on the development of lasers that can survive launch conditions and the challenging space environment during missions that could last five years or more. This paper describes the fabrication of a rugged, laser-welded package for a 200mW, monolithic diode-pumped solid-state Nd:YAG laser operating at 1319nm. Environmental testing shows that the laser withstands non-operational thermal cycles over a temperature range from -20°C to 55°C, and 22.3 g-rms of random vibration, with little or no degradation of laser output power or performance. The novel packaging method employs a specially designed housing to which multi-mode or single-mode polarization-maintaining fiber pigtails can be aligned and laser-welded into place. To further enhance reliability, a redundant pumping system called the Multi-Fiber Pump Ferrule (MFPF) was developed and implemented. The MFPF allows multiple laser diode pump modules to be aligned to the laser crystal simultaneously, in order to accommodate either parallel or standby pump redundancy. This compact, lightweight design is well suited for space flight applications and the laser-welded technique can easily be adapted to a number of other fiber optic and electro-optic devices in which critical optical alignments must be maintained in a harsh environment.

In order for data products from WIIT to be as robust as possible, the alignment and mechanical positions of source, receiver, and detector components must be controlled and measured with extreme precision and accuracy, and the ambient environment must be monitored to allow environmental effects to be correlated with even small perturbations to fringe data. Relevant detailed anatomy of many testbed components and assemblies are described. The system of displacement measuring interferometers (DMI), optical encoders, optical alignment tools, optical power monitors, and temperature sensors implemented for control and monitoring of the testbed is presented.

MAM is a dedicated systems-level testbed that combines the major SIM subsystems including laser metrogy, pointing, and pathlength control. The testbed is configured as a modified Michelson interferometer for the purpose of studying the white-light fringe measurement processes. This paper will compare the performance of various algorithms using the MAM data, and will aid in our recommendation of how the SIM flight system should process the science and guide interferometer data.

The micro-arcsecond metrology testbed (MAM) provides a testing ground for SIM to perform optical path difference measurements with picometer (pm) precision. Because of imperfect optics and non-ideal laser sources it is inevitable that the cyclic bias is one of the major error sources for SIM. Many experiments have been conducted to diagnose and to characterize cyclic bias in the laser gauges, and in white light fringe detection. Our data analysis indicates that cyclic bias in MAM has a predictable frequency and a relatively stable amplitude. It has been proposed to use phase measurements at different wavelengths to solve for the cyclic bias. The experiment results have shown that the cyclic bias in SAVV are reduced from nm level to the level of hundred picometers. Besides the cyclic bias the effective wavelengths of spectral channels have to be calibrated also. At present, a new method using FFT technique and new metrology gauge demonstrates that the wavelength determination has a precision of 10^-4. The spectrometer in MAM is stable. The changes of effective wavelengths in a few weeks is about one nanometer, or less. Systematic biases above must be periodically calibrated.

The design of precision optical gauges for metrology is facilitated by predictions of performance by means of computer simulation. For example, consequences of design parameter variations can be investigated before committing to the time and expense of hardware fabrication or procurement. Once the components are assembled, puzzling test results may be understood by simulating various misalignments. Classical geometric optical ray trace analysis of these systems is not capable of predicting diffraction effects which are important in determining the ultimate achievable metrology precision. We present details of a simulation approach which we have developed for the diffraction analysis of such systems. Optical signals are represented as two-dimensional scalar fields, and paraxial scalar diffraction theory is used to calculate the propagation of signals -- through various optical elements, such as apertures, lenses and corner cubes - to detector focal planes. The simulation includes coherent combination of signal and local reference wavefronts at the focal plane, and modeling the measurement of optical phase by heterodyne detection - a capability critical to optical metrology. We discuss the capabilities, limitations and sources of error inherent in our approach; present the results of modeling one or more metrology systems which are currently under laboratory development; indicate how simulation can identify sources and magnitudes of measurement error; and show correspondence of simulations with laboratory measurement.

Simple laser interferometers for testing of the Space optical systems are discussed. An interferometer consists of a lens in front of a helium-neon gas laser, flat pellicle beamsplitter, a tiny reference flat mirror, and optics under test. A light of the laser passes the lens and the beamsplitter to produce the point like image at the surface of the reference flat mirror. The beam is split by the beamsplitter into a reference beam and an analyzer beam. The analyzer beam is reflected by the system under test, and it crosses the beamsplitter and recombines with the reference wavefront to produce Twyman-Green fringes in a viewing screen. The reference wavefront is reflected by the reference flat mirror and by the beamsplitter. The interferometer is not affected by displacements and tilts of the lens, the beamsplitter, and the reference flat mirror, that is why it is especially recommended for the Space optics testing on the orbit. The interferometer is also very simple, small, light, and cheap, and easily can be manufactured in quantity.

This paper addresses the issue of modeling the white light fringe. We developed analytic technique for extracting the phase, visibility and amplitude information as needed for interferometric astrometry with the Space Interferometry Mission (SIM). The model accounts for a number of instrumental and physical effects and is able to compensate for a number of operational regimes. In particular, we were able to obtain general solution for polychromatic phasors and address properties of unbiased fringe estimators in the presence of noise. For demonstration purposes we studied the case of rectangular bandpass filter with two different methods of optical path difference (OPD) modulation -- stepping and ramping OPD modulations. A number of areas of further studies relevant to instrument design and simulations are outlined and discussed.