This PDF file contains the front matter associated with SPIE Proceedings Volume 7734, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The Keck Interferometer (KI) combines the two 10m diameter Keck telescopes providing milliarcsecond angular resolution. KI has unique observing capabilities such as sensitive K-band V², L-band V² and N-band nulling operations. The instrument status of the Keck Interferometer since the last SPIE meeting in 2008 is summarized. We discuss the performance of new visibility observing capabilities including L-band and self-phase referencing modes. A simultaneous dual-beam-combiner mode in the K and L-band has been demonstrated, nearly doubling operational efficiency for bright targets. Operational improvements including simplified reliable operations with reduced personnel resources are highlighted. We conclude with a brief review of the current and future developmental activities of KI. Details of ASTRA developments, nulling performance and science results are presented elsewhere at this conference.

The CHARA Array is a six-telescope optical/IR interferometer operated by the Center for High Angular Resolution Astronomy of Georgia State University and is located at Mount Wilson Observatory just to the north of Los Angeles California. The CHARA Array has the largest operational baselines in the world and has been in regular use for scientific observations since 2004. In this paper we give an update of instrumentation improvements, primarily focused on the beam combiner activity. The CHARA Array supports seven beam combiners: CHARA CLASSIC, a two-way high sensitivity K/H/J band system; CLIMB, a three-way K/H/J open air combiner, FLUOR, a two-way K band high precision system; MIRC, a four/six-way H/K band imaging system; CHAMP, a six way K band fringe tracker; VEGA, a four way visible light high spectral resolution system; and PAVO, a three-way visible light high sensitivity system. The paper will conclude with a review of science results obtained over the last few years, including our most recent imaging results.

The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities to the community. New configurations of the ATs array are discussed with the science users of the VLTI and implemented to optimize the scientific return. The monitoring and improvement of the different systems of the VLTI is a continuous work. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the instruments has been successfully installed and the commissioning is ongoing. The possibility for visiting instruments has been opened to the VLTI facility.

The Sydney University Stellar Interferometer (SUSI) has been enhanced by installation of the PAVO beam combiner, which uses an electron-multiplying CCD detector giving a fast, low-noise 2D readout. This allows PAVO to provide wide-band wavelength dispersed beam combination, which improves sensitivity and scientific productivity. PAVO also provides pupil segmentation which improves the instrumental fringe visibility. A remote operations facility has been established, which allows SUSI to be operated from Sydney or elsewhere. A new control system for the longitudinal dispersion corrector and siderostats is under development. Installation has commenced of a high precision differential astrometry system (MUSCA) which aims to detect planets in binary star systems.

The Magdalena Ridge Observatory Interferometer is a 10 x 1.4 meter aperture long baseline optical and near-infrared interferometer being built at 3,200 meters altitude on Magdalena Ridge, west of Socorro, NM. The interferometer layout is an equilateral "Y" configuration to complement our key science mission, which is centered on imaging faint and complex astrophysical targets. This paper serves as an overview and update on the status of the observatory and our progress towards first light and first fringes in 2012.

We present an update on the construction and integration of LINC-NIRVANA, a Fizeau-mode imaging interferometer for the Large Binocular Telescope (LBT). The LBT is a unique platform for interferometry, since its two, co-mounted 8.4 meter primary mirrors present an orientation-independent entrance pupil. This allows Fizeau-mode beam combination, providing 23-meter spatial resolution and 12-meter effective collecting area for panoramic imagery LINC-NIRVANA will sit at one of the shared, bent focal stations, receiving light from both mirrors of the LBT. The instrument uses visible wavelength radiation for wavefront control, and the near-infrared bands for science and fringe tracking. LINC-NIRVANA employs a number of innovative technologies, including multi-conjugated adaptive optics, state-of-the-art materials, low vibration mechanical coolers, active and passive control, and sophisticated software for data analysis. The instrument is in its final construction and integration phase. This paper reports on overall progress, including insights gained on large instrument assembly, software integration, science planning, and vibration control. A number of additional contributions to this conference focus on individual subsystems and integration-related issues.

Due to the recent dramatic technological advances, infrared interferometry can now be applied to new classes of objects, resulting in exciting new science prospects, for instance, in the area of high-mass star formation. Although extensively studied at various wavelengths, the process through which massive stars form is still only poorly understood. For instance, it has been proposed that massive stars might form like low-mass stars by mass accretion through a circumstellar disk/envelope, or otherwise by coalescence in a dense stellar cluster. Therefore, clear observational evidence, such as the detection of disks around high-mass young stellar objects (YSOs), is urgently needed in order to unambiguously identify the formation mode of the most massive stars. After discussing the technological challenges which result from the special properties of these objects, we present first near-infrared interferometric observations, which we obtained on the massive YSO IRAS 13481-6124 using VLTI/AMBER infrared long-baseline interferometry and NTT speckle interferometry. From our extensive data set, we reconstruct a model-independent aperture synthesis image which shows an elongated structure with a size of ~ 13 x 19 AU, consistent with a disk seen under an inclination of - 45°. The measured wavelengthdependent visibilities and closure phases allow us to derive the radial disk temperature gradient and to detect a dust-free region inside of 9.5 AU from the star, revealing qualitative and quantitative similarities with the disks observed in low-mass star formation. In complementary mid-infrared Spitzer and sub-millimeter APEX imaging observations we detect two bow shocks and a molecular outflow, which are oriented perpendicular to the disk plane and indicate the presence of a bipolar outflow emanating from the inner regions of the system.

The Infrared Spatial Interferometer (ISI) is a three telescope interferometer system that operates near 11 microns wavelength using heterodyne detection with CO₂ lasers as local oscillators. Stellar measurements have been made using consistent instrumentation for 20 years, allowing comparisons of stellar sizes of red giant and Mira stars over time intervals which are long in comparison to stellar luminosity periods. Recent visibility and closure phase measurements of the star Betelgeuse have been fitted to simple image models and these results have been added to the 17 year record of stellar observations. A new area of investigation of stellar properties at very high spectral resolution will begin in the 2010-2011 observing season. The design of a new digital spectrometer-correlator system is discussed. This system will obtain visibility measurements on-and-off individual spectral lines and the continuum, simultaneously.

Intensity interferometry permits very long optical baselines and the observation of sub-milliarcsecond structures. Using planned kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, inviting detailed studies of the shapes of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Signal-to-noise in intensity interferometry favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Prime candidate sources have been identified among classes of bright and hot stars. Observations are simulated for telescope configurations envisioned for large Cherenkov facilities, synthesizing numerous optical baselines in software, confirming that resolutions of tens of microarcseconds are feasible for numerous astrophysical targets.

The efficiency of the CHARA Array has proven satisfactory for a wide variety of scientific programs enabled by the first-generation beam combination and detector systems. With multi-beam combination and more ambitious scientific goals, improvements in throughput and efficiency will be highly leveraged. Engineering data from several years of nightly operations are used to infer atmospheric characteristics and raw instrumental visibility in both classic optical and single-mode fiber beam combiners. This information is the basis for estimates of potential gains that could be afforded by the implementation of adaptive optics. In addition to the very important partial compensation for higher order atmosphere-induced wavefront errors, the benefits include reduction of static and quasi-static aberrations, reduction of residual tilt error, compensation for differential atmospheric refraction, and reduction of diffractive beam propagation losses, each leading to improved flux throughput and instrumental visibility, and to associated gains in operability and scientific productivity.

The addition of new observational capabilities and continued sensitivity improvements have allowed observations with the Keck Interferometer to encompass new areas of astrophysics and expanded significantly the available sample size in areas which had been the focus of previous work. The technical details of the instrument techniques (including nulling, L-band and increased spectral resolution) are covered in other contributions to this conference. Here, we will highlight the astrophsyics enabled by these instruments including: a summary of the NASA Exo-zodical Dust Survey Key Project, observations across a range of dust temperatures with K and L-band measurements and faint target studies of active galactic nuclei and young stellar disks.

This paper presents the current status of the VEGA (Visible spEctroGraph and polArimeter) instrument installed at the coherent focus of the CHARA Array, Mount Wilson CA. Installed in september 2007, the first science programs have started during summer 2008 and first science results are now published. Dedicated to high angular (0.3mas) and high spectral (R=30000) astrophysical studies, VEGA main objectives are the study of circumstellar environments of hot active stars or interactive binary systems and a large palette of new programs dedicated to fundamental stellar parameters. We will present successively the main characteristics of the instrument and its current performances in the CHARA environment, a short summary of two science programs and finally we will develop some studies showing the potential and difficulties of the 3 telescopes mode of VEGA/CHARA.

The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements are affected by differential effects occurring between the four beams. We will shortly discuss the contributions of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as possible a user-friendly and efficient facility.

Based on the success of four-telescope imaging with the Michigan Infrared Combiner (MIRC) on the CHARA Array, our Michigan-based group will now upgrade our system to combine all six CHARA telescope simultaneously. In order to make this observationally efficient, we have had to improve a number of subsystems and commission new ones, including the new CHAMP fringe tracker, the introduction of photometric channels, the upgrading of the realtime operating systems, and the obvious hardware and software upgrades of the control system and the data pipeline. Here we will discuss the advantages of six-telescope operation, outline our upgrade plans and discuss our current progress.

The SIM Lite Astrometric Observatory (aka SIM Lite), a micro-arcsecond astrometry space mission, has been developed in response to NASA's indefinite deferral of the SIM PlanetQuest mission. The SIM Lite mission, while significantly more affordable than the SIM PlanetQuest mission concept, still addresses the full breadth of SIM science envisioned by two previous National Research Council (NRC) Astrophysics Decadal Surveys at the most stringent "Goal" level of astrometric measurement performance envisioned in those surveys. Over the past two years, the project has completed the conceptual design of the SIM Lite mission using only the completed SIM technology; published a 250 page book describing the science and mission design (available at the SIM website: http://sim.jpl.nasa.gov); been subject to an independent cost and technical readiness assessment by the Aerospace Corporation; and submitted a number of information responses to the NRC Astro2010 Decadal Survey. The project also conducted an exoplanet-finding capability double blind study that clearly demonstrated the ability of the mission to survey 60 to 100 nearby sun-like dwarf stars for terrestrial, habitable zone planets in complex planetary systems. Additionally, the project has continued Engineering Risk Reduction activities by building brassboard (form, fit & function to flight) version of key instrument elements and subjecting them to flight qualification environmental and performance testing. This paper summarizes the progress over the last two years and the current state of the SIM Lite project.

While the question of low cost / low science precursors is raised to validate the concepts of direct and nulling interferometry space missions, balloon payloads offer a real opportunity thanks to their relatively low cost and reduced development plan. Taking into account the flight capabilities of various balloon types, we propose in this paper, several concepts of payloads associated to their flight plan. We also discuss the pros and cons of each concepts in terms of technological and science demonstration power.

Astronomical studies at infrared wavelengths have dramatically improved our understanding of the universe. The relatively low angular resolution of these missions, however, is insufficient to resolve the physical scale on which mid-to far-infrared emission arises. We will build the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII), an eight-meter Michelson interferometer to fly on a high-altitude balloon. BETTII's spectral-spatial capability, provided by an instrument using double-Fourier techniques, will address key questions about the nature of disks in young star clusters and active galactic nuclei and the envelopes of evolved stars. BETTII will also lay the technological groundwork for future space interferometers.

The presence of large amounts of exozodiacal dust around nearby main sequence stars is considered as a potential threat for the direct detection of Earth-like exoplanets (exoEarths) with future space-based coronagraphic and interferometric missions. In this paper, we estimate the amount of exozodiacal light that can be tolerated around various stellar types without jeopardizing the detection of exoEarths with a space-based visible coronagraph or a free-flying mid-infrared interferometer. We also address the possible effects of resonant structures in exozodiacal disks. We then review the sensitivity of current ground-based interferometric instruments to exozodiacal disks, based on classical visibility measurements and on the nulling technique. We show that the current instrumental performances are not sufficient to help prepare future exoEarth imaging missions, and discuss how new groundor space-based instruments could improve the current sensitivity to exozodiacal disks down to a suitable level.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a structurally connected infrared space interferometer with 0.5 m diameter telescopes on a 12.5 m baseline, and is passively cooled to ~ 60K. The FKSI operates in the thermal infrared from 3-8 μm in a nulling (starlight suppressing) mode for the detection and characterization of exoplanets, debris disks, and extrasolar zodiacal dust levels. The FKSI will have the highest angular resolution of any infrared space instrument ever made with its nominal resolution of 40 mas at a 5 μm center wavelength. This resolution exceeds that of Spitzer by a factor of 38 and JWST by a factor of 5. The FKSI mission is conceived as a "probe class" or "mid-sized" strategic mission that utilizes technology advances from flagship projects like JWST, SIM, Spitzer, and the technology programs of TPF-I/Darwin. During the past year we began investigating an enhanced version of FKSI with 1-2 m diameter telescopes, passively cooled to 40K, on a 20-m baseline, with a sunshade giving a ± 45 degree Field-of-Regard. This enhanced design is capable of detecting and characterizing the atmospheres of many 2 Earth-radius super-Earths and a few Earth-twins. We will report progress on the design of the enhanced mission concept and current status of the technologies needed for this mission.

We present progress in the development of the monolithic achromatic nulling interference coronagraph (MANIC), an optic designed for enabling direct detection and characterization of exoplanetary systems around nearby stars. MANIC is a fully symmetric implementation of a rotational shearing interferometer consisting of fused quartz prisms and a symmetric beamsplitter optically contacted in an arrangement that geometrically flips the fields in the TR and RT arms about orthogonal axes such that upon recombination, a centro-symmetric, theoretically achromatic null is produced. In addition to a small inner working angle (⪅ 1λ/D), built-in alignment and stability are inherent benefits of the compact monolithic design, which make MANIC a competitive alternative to conventional discrete element nullers proposed for imaging exoplanetary environments. Following MANIC's initial fabrication, the path error between its TR and RT arms was measured. This measurement was used to fabricate compensator plates of varying thicknesses that were bonded to the optic to reduce dispersion imbalance, thereby improving broadband nulling performance. In performing this correction, initial OPD was reduced from 949 ± 44 nm to 63 ± 10 nm, which in the absence of any other asymmetries, corresponds to an increase in a 107 R-band (λ_c = 648 nm) nulling bandpass from monochromatic to 25%, or at the 106 level, from 5% to 50%. Current benchtop laser and polychromatic nulling strategies are described. The potential science return from using MANIC on a sub-orbital platform is discussed.

This paper examines how narrow-angle (NA) processing of data from the SIM Lite optical interferometry mission can be undertaken when realistic spacecraft and mission operational constraints are taken into account. Using end-to-end mission simulations we show that the goal of 1 μas single measurement accuracy (SMA) is obtainable, and hence the detection of earth-like planets is achievable with the SIM Lite mission.

Recently, the Keck interferometer was upgraded to do self-phase-referencing (SPR) assisted K-band spectroscopy at R ~ 2000. This means, combining a spectral resolution of 150 km/s with an angular resolution of 2.7 mas, while maintaining high sensitiviy. This SPR mode operates two fringe trackers in parallel, and explores several infrastructural requirements for off-axis phase-referencing, as currently being implemented as the KI-ASTRA project. The technology of self-phasereferencing opens the way to reach very high spectral resolution in near-infrared interferometry. We present the scientific capabilities of the KI-SPR mode in detail, at the example of observations of the Be-star 48 Lib. Several spectral lines of the cirumstellar disk are resolved. We describe the first detection of Pfund-lines in an interferometric spectrum of a Be star, in addition to Br γ. The differential phase signal can be used to (i) distinguish circum-stellar line emission from the star, (ii) to directly measure line asymmetries tracing an asymetric gas density distribution, (iii) to reach a differential, astrometric precision beyond single-telescope limits sufficient for studying the radial disk structure. Our data support the existence of a radius-dependent disk density perturbation, typically used to explain slow variations of Be-disk hydrogen line profiles.

The Astronomical Multi-BEam Recombiner (AMBER), has been operational at the Very Large Telescope Interferometer (VLTI) for many years. We present here some of the constant improvements we have been providing while still operating the instrument, with a heavy load of visitor and service observing programs, most of the nights of the year. In particular, we present here improvements regarding the spectral calibration and correction of the atmospheric loss in squared visibility due to path difference jitter, allowing the instrument to achieve greater precision.

The Keck Interferometer combines the two 10 m Keck telescopes as a long baseline interferometer. It is funded by NASA as a joint development among the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the NASA Exoplanet Science Institute. In February 2008, the 10 um nulling mode began a 32 night observing program with three key science teams to perform a survey of nearby stars for exozodiacal dust. This program has recently concluded, and has been followed by nuller observing on a variety of science topics through the standard proposal process. We provide a review and update of the nuller implementation, and describe the data reduction process, including the calibration approach. We then review the technical performance of the instrument based on the full key science data set, including sensitivity and systematic errors. We also provide some summary data on atmospheric effects applicable to the cophasing approach.

We present here a new observational technique, Phase Closure Nulling (PCN), which has the potential to obtain very high contrast detection and spectroscopy of faint companions to bright stars. PCN consists in measuring closure phases of fully resolved objects with a baseline triplet where one of the baselines crosses a null of the object visibility function. For scenes dominated by the presence of a stellar disk, the correlated flux of the star around nulls is essentially canceled out, and in these regions the signature of fainter, unresolved, scene object(s) dominates the imaginary part of the visibility in particular the closure phase. We present here the basics of the PCN method, the initial proof-of-concept observation, the envisioned science cases and report about the first observing campaign made on VLTI/AMBER and CHARA/MIRC using this technique.

The Magdalena Ridge Observatory Interferometer (MROI) has completed its design phase and is currently in the construction phase. The first telescope will be deployed at the MROI site in 2011. Five different vendors are involved in the design and fabrication of a unit telescope, and a much larger number for the full observatory. This paper addresses the steps that the MRO Interferometry project will undertake to integrate subsystems developed by different parties, through commissioning into an operational optical interferometer. Finally we present the commissioning plan to bring the interferometer to an operational mode. We have developed "performance verification milestones" that successively increase the "science readiness" of the interferometer and transitions to an operational phase.

The dual feed astrometric instrument software of PRIMA (PACMAN) that is currently being integrated at the VLTI will use two spatially modulated fringe sensor units and a laser metrology system to carry out differential astrometry. Its software and hardware compromises a distributed system involving many real time computers and workstations operating in a synchronized manner. Its architecture has been designed to allow the construction of efficient and flexible calibration and observation procedures. In parallel, a novel scheme of integrating M-code (MATLAB/OCTAVE) with standard VLT (Very Large Telescope) control software applications had to be devised in order to support numerically intensive operations and to have the capacity of adapting to fast varying strategies and algorithms. This paper presents the instrument software, including the current operational sequences for the laboratory calibration and sky calibration. Finally, a detailed description of the algorithms with their implementation, both under M and C code, are shown together with a comparative analysis of their performance and maintainability.

The dynamics of stars and gas undoubtedly shows the existence of a 4 million solar mass black hole at the center of the Milky Way: Sagittarius A* (SgrA*). Violent flare emission allows us to probe the immediate environment of the central mass. Near-infrared polarimetry now shows signatures of strong gravity that are statistically significant against randomly polarized red noise. Using these signatures we can derive spin and inclination information of SgrA*. A combined synchrotron self Compton (SSC) and adiabatic expansion model with source components peaking in the sub-mm domain can fully account for the observed flare flux densities and the time delays towards the (sub-)mm flares that have been reported in some cases. We discuss the expected centroid paths of the NIR images and summarize how the geometrical structure of the emitting region (i.e. spot shape, presence of a torus or spiral-arm pattern etc.) affects this centroid tracks. While most of the mentioned geometries are able to fit the observed fluxes, future NIR interferometry with GRAVITY at the VLT will break some of the degeneracies between different emission models. In this contribution we summarize several GRAVITY science cases for SgrA*. Our simulations propose that focusing GRAVITY observations on the polarimetry mode could reveal a clear centroid track of the spot(s). A non-detection of centroid shifts cannot rule out the multi-component model or spiral arms scenarios. However, a clear wander between alternating centroid positions during the flares will prove the idea of bright long-lived spots occasionally orbiting the central black hole.

GRAVITY is an adaptive optics assisted Beam Combiner for the second generation VLTI instrumentation. The instrument will provide high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the astronomical K-band for faint objects. We describe the wide range of science that will be tackled with this instrument, highlighting the unique capabilities of the VLTI in combination with GRAVITY. The most prominent goal is to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We present the preliminary design that fulfils the requirements that follow from the key science drivers: It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near-infrared wavefrontsensing adaptive optics; fringe-tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that 10 μas astrometry within few minutes is feasible for a source with a magnitude of m_K = 15 like Sgr A*, given the availability of suitable phase reference sources (m_K = 10). Using the same setup, imaging of m_K = 18 stellar sources in the interferometric field of view is possible, assuming a full night of observations and the corresponding UV coverage of the VLTI.

The ASTrometric and phase-Referenced Astronomy (ASTRA) project will provide phase referencing and astrometric observations at the Keck Interferometer, leading to enhanced sensitivity and the ability to monitor orbits at an accuracy level of 30-100 microarcseconds. Here we discuss recent scientific results from ASTRA, and describe new scientific programs that will begin in 2010-2011. We begin with results from the "self phase referencing" (SPR) mode of ASTRA, which uses continuum light to correct atmospheric phase variations and produce a phase-stabilized channel for spectroscopy. We have observed a number of protoplanetary disks using SPR and a grism providing a spectral dispersion of ~ 2000. In our data we spatially resolve emission from dust as well as gas. Hydrogen line emission is spectrally resolved, allowing differential phase measurements across the emission line that constrain the relative centroids of different velocity components at the 10 microarcsecond level. In the upcoming year, we will begin dual-field phase referencing (DFPR) measurements of the Galactic Center and a number of exoplanet systems. These observations will, in part, serve as precursors to astrometric monitoring of stellar orbits in the Galactic Center and stellar wobbles of exoplanet host stars. We describe the design of several scientific investigations capitalizing on the upcoming phase-referencing and astrometric capabilities of ASTRA.

ASTRA (ASTrometric and phase-Referencing Astronomy) is an upgrade to the existing Keck Interferometer which aims at providing new self-phase referencing (high spectral resolution observation of YSOs), dual-field phase referencing (sensitive AGN observations), and astrometric (known exoplanetary systems characterization and galactic center general relativity in strong field regime) capabilities. With the first high spectral resolution mode now offered to the community, this contribution focuses on the progress of the dual field and astrometric modes.

AMOS is in charge of the development of the unit telescopes for the MRO interferometer. This paper depicts the progress of the project and presents the results of the factory acceptance tests that were performed at AMOS facilities. Those tests are the earliest verifications of the telescope performance. AMOS has now extensive experience in testing small and large instruments, including optical testing, alignment, mechanical static, dynamic measurements, system identification, etc. It is this combination of various techniques of measurement that produce accurate and reliable results.

Here is presented the current outline and progress of MROI's automated alignment system design. Depending on the location of each of MROI's unit telescopes (UT), light can travel distances ranging from 460 to 660 meters via several reflections that redirect the beam's path through the beam relay system (BRS), delay line system (DLS), beam compressing telescope (BCR), switchyards and finally to the beam combiners (BC). All of these sub-systems comprise three major optical axes of the MROI which must be coaligned on a nightly basis by the AAS. The AAS consists of four subsystems: the primary fiducial-for beam injection, the UT tilt and shear measurement components (TASM), the BC TASM components, and the secondary fiducial-for quick alignment checks. All of these subsystems contribute to the unique design of the AAS which will allow for simultaneous measurements from the visible to the near-IR wavelengths, full automation, the capability to perform optical path difference (OPD) alignment and spectral calibration, making it cost effective and saving on realty in the beam combining area (BCA). The AAS is nearing completion and assembly of the various subsystems is expected to commence soon. The latest results on all of the following are reviewed here.

GRAVITY is a VLTI second generation instrument designed to deliver astrometry at the level of 10 μas. The beam transport to the beam combiner is stabilized by means of a dedicated guiding system whose specifications are mainly driven by the GRAVITY astrometric error budget. In the present design, the beam is monitored using an infrared acquisition camera that implements a mosaic of field, pupil and Shack-Hartmann images for each of the telescopes. Star and background H-band light from the sky can be used to correct the tip-tilt and pupil lateral position, within the GRAVITY specifications, each 10 s. To correct the beam at higher frequencies laser guiding beams are launched in the beam path, on field and pupil planes, and are monitored using position sensor detectors. The detection, in the acquisition camera, of metrology laser light back reflected from the telescopes, is also being investigated as an alternative for the pupil motion control.

In this paper, we present the results of three different studies of the Fomalhaut debris disk with infrared interferometry. First, VLTI/AMBER measurements are used to determine the position angle of the slightly oblate rapidly rotating photosphere by means of differential phase measurements across the Br-gamma photospheric line. This measurement allows us to confirm that the debris disk is located in the equatorial plane of its host star. Second, we use VLTI/VINCI to search for resolved near-infrared emission around the stellar photosphere, which would correspond to the presence of large amounts of hot dust grains located between the sublimation radius and the habitable zone. Our observations reveal a small excess of 0.88%±0.12% in K band relative to the photospheric flux. Finally, we use the Keck Interferometer Nuller in order to derive additional constraints on the nature of the resolved infrared emission. Our observations suggest a marginal detection of a circumstellar excess at 10 μm, which we use together with the VINCI detection to model the circumstellar emission. Preliminary results from this modeling effort are discussed.

Planets are believed to form in circumstellar disks around newly born stars at distances ranging from 0.1 to 10 AUs. This location corresponds to milli-arcsecond scales at the distance of the closest star forming regions and to temperatures ranging from a few hundred to a few thousand Kelvin. To conduct observations of close environments of such disks at the milli-arcsecond scale, infrared interferometry is a suitable tool that can be employed to observe T Tauri, FU Ori and Herbig Ae/Be stars. However, the data obtained so far consist of a small number of measurements which can only constrain theoretical models. With the advent of recent multi-aperture interferometers, the interferometric data can be used to reconstruct images independently of any parametric model, as is routinely done in the radio frequency range. On the other hand, in the optical range, not enough measurements are available to univocally reconstruct an image and some a priori must be introduced. In this contribution, we present systematic tests performed on the MiRA algorithm (an image reconstruction algorithm developed for optical interferometry) in order to evaluate the feasibility of the technique. The methodology allows deriving some practical rules for the user and has been applied to an YSO (HD 163296). I present the results of the image reconstruction, providing the first images of a complex YSO.

In this communication are presented some complements to a recent paper entitled "Simple Fourier optics formalism for high angular resolution systems and nulling interferometry" [1], dealing with imaging and nulling capacities of a few types of multi-aperture optical systems. Herein the characteristics of such systems in terms of Point Spread Function (PSF) and Field of View (FoV) are derived from simple analytical expressions that are further evaluated numerically for various configurations. We consider successively the general cases of Fizeau and Michelson interferometers, and those of a monolithic pupil, nulling telescope, of a nulling, Sheared-Pupil Telescope (SPT), and of a sparse aperture, Axially Combined Interferometer (ACI). The analytical formalism also allows establishing the exact Object-Image relationships applicable to nulling PSTs or ACIs that are planned for future space missions searching for habitable extra-solar planets.

To date, about 17 hot Jupiters have been directly detected by photometric and/or spectroscopic observations. Only 2 of them, however, are non-transiting hot Jupiters and the rest are all transiting ones. Since non-transiting hot Jupiter systems are analogs of high contrast binaries, optical/infrared long baseline interferometers can resolve them and detect the planets if highly stable and precise closure phase measurements are obtained. Thus, this is a good opportunity for optical/infrared interferometers to contribute to the field of exoplanet characterization. To reach this goal, detailed calibration studies are essential. In this paper, we report the first results of our closure phase calibration studies. Specifically, we find strong closure phase drifts that are highly correlated with target positions, i.e., altitude and azimuth angle. The correlation is stronger with altitude. Our experiments indicate that the major cause of the drifts is probably longitudinal dispersion. We are able to find a strategy with multiple approaches to reduce this effect, and are able to model the closure phase drift with a quadratic function of both altitude and azimuth. We then use this model to calibrate the drifts, and test this new calibration scheme with the high contrast binary ε Per. Although we can find a better orbital solution with this new method, we have also found difficulties to interpret the orbit of ε Per, which may stem from possible mis-calibrations or the influence of the third component in the system. More investigations are definitely necessary to address this issue and to further confirm our calibration strategy.

Astronomical speckle imaging is a well established technique for obtaining diffraction limited images of binary and multiple stars, low contrast solar features and nearby extended objects such as comets and solar system planets, with large ground-based telescopes. We have developed a speckle masking code to reconstruct images of such objects from the corresponding specklegrams. This code uses speckle interferometry for estimating the Fourier amplitudes and bispectrum for estimating the Fourier phases. In this paper, we discuss a few technical issues such as: What is the photometric and astrometric accuracy that can be achieved with this code? What is the closest separation between the components of a binary star that can be clearly resolved with sufficient signal to noise ratio with this code? What is the maximum dynamic range? What kind of calibration schemes can be used in the absence of a bright calibrator close to the object of interest? We address these questions based on computer simulations. We present a few sample reconstructions from the real data obtained from the SOAR telescope. We also present the details of a technical feasibility study carried out with NACO-cube mode at the VLT.

Sub milli-arcsecond imaging in the visible band will provide a new perspective in stellar astrophysics. Even though stellar intensity interferometry was abandoned more than 40 years ago, it is capable of imaging and thus accomplishing more than the measurement of stellar diameters as was previously thought. Various phase retrieval techniques can be used to reconstruct actual images provided a sufficient coverage of the interferometric plane is available. Planned large arrays of Air Cherenkov telescopes will provide thousands of simultaneously available baselines ranging from a few tens of meters to over a kilometer, thus making imaging possible with unprecedented angular resolution. Here we investigate the imaging capabilities of arrays such as CTA or AGIS used as Stellar Intensity Interferometry receivers. The study makes use of simulated data as could realistically be obtained from these arrays. A Cauchy-Riemann based phase recovery allows the reconstruction of images which can be compared to the pristine image for which the data were simulated. This is first done for uniform disk stars with different radii and corresponding to various exposure times, and we find that the uncertainty in reconstructing radii is a few percent after a few hours of exposure time. Finally, more complex images are considered, showing that imaging at the sub-milli-arc-second scale is possible.

Experiments are in progress to prepare for intensity interferometry with arrays of air Cherenkov telescopes. At the Bonneville Seabase site, near Salt Lake City, a testbed observatory has been set up with two 3-m air Cherenkov telescopes on a 23-m baseline. Cameras are being constructed, with control electronics for either off- or online analysis of the data. At the Lund Observatory (Sweden), in Technion (Israel) and at the University of Utah (USA), laboratory intensity interferometers simulating stellar observations have been set up and experiments are in progress, using various analog and digital correlators, reaching 1.4 ns time resolution, to analyze signals from pairs of laboratory telescopes.

The use of a rotating-baseline nulling interferometer for exoplanet detection was proposed several decades ago, but the technique has not yet been fully demonstrated in practice. Here we consider the faint companion and exozodiacal disk detection capabilities of rotating-baseline nulling interferometers, such as are envisioned for space-based infrared nullers, but operating instead within the aperture of large single telescopes. In particular, a nulling interferometer on a large aperture corrected by a next-generation extreme adaptive optics system can provide deep interferometric contrasts, and also reach smaller angles (sub λ/D) than classical coronagraphs. Such rotating nullers also provide validation for an eventual space-based rotating-baseline nulling interferometer. As practical examples, we describe ongoing experiments with rotating nullers at Palomar and Keck, and consider briefly the case of the Thirty Meter Telescope.

High angular resolution images obtained with a hypertelescope can strongly constrain the radiative-hydrodynamics simulations of red supergiant (RSG) stars, in terms of intensity contrast, granulation size and temporal variations of the convective motions that are visible on their surface. The characterization of the convective pattern in RSGs is crucial to solve the mass-loss mechanism which contributes heavily to the chemical enrichment of the Galaxy. We show here how the astrophysical objectives and the array configuration are highly dependent to design a hypertelescope. For a given field of view and a given resolution, there is a trade-off between the array geometry and the number of required telescopes to optimize either the (u,v) coverage (to recover the intensity distribution) or the dynamic range (to recover the intensity contrast). To obtain direct snapshot images of Betelgeuse with a hypertelescope, a regular and uniform layout of telescopes is the best array configuration to recover the intensity contrast and the distribution of both large and small granulation cells, but it requires a huge number of telescopes (several hundreds or thousands). An annular configuration allows a reasonable number of telescopes (lower than one hundred) to recover the spatial structures but it provides a low-contrast image. Concerning the design of a pupil densifier to combine all the beams, the photometric fluctuations are not critical (Delta photometry < 50%) contrary to the residual piston requirements (OPD < λ/8) which requires the development of an efficient cophasing system to fully exploit the imaging capability of a hypertelecope.

The SIM Lite Astrometric Observatory will be the first space-based Michelson interferometer operating in the visible wavelength, with the ability to perform ultra-high precision astrometric measurements on distant celestial objects. SIM Lite data will address in a fundamental way questions such as characterization of Earth-mass planets around nearby stars. To accomplish these goals it is necessary to rely on a model-based systems engineering approach - much more so than most other space missions. This paper will describe in further detail the components of this end-to-end performance model, called "SIM-sim", and show how it has helped the systems engineering process.

The dynamic stability of white light fringes formed on the guide and science interferometers in SIM-Lite along with the pointing stability of each arm of each interferometer affect the visibility of fringes and the length of the fringe camera integration time for the observatory. Hence, tight fringe and pointing stability requirements are needed to reduce science interferometer camera integration times, which in turn help increase the all important instrument's observing efficiency. The SIM-Lite Instrument Dynamics and Controls (D&C) System Architecture deals with such dynamic issues through a "tailored" system dynamics design complemented by a comprehensive active control system. The SIM-Lite on-orbit System architecture is described in this paper. Key roles played by the resulting D&C System are also established, while the system design is clearly linked to the four nominal phases of on-orbit operations for the observatory (Tile to Tile slew & settling, guide star acquisition, science observation, & science interferometer retargeting). Top driving requirements dictating system interferometric-baseline stability and repeatability, instrument pointing stability, and fringe stability are discussed here together with the resulting high level Error Budget. Key system sensitivities and currently known D&C related design challenges are also discussed.

SIM-Lite missions will perform astrometry at microarcsecond accuracy using star light interferometry. For typical baselines that are shorter than 10 meters, this requires to measure optical path difference (OPD) accurate to tens of picometers calling for highly accurate calibration. A major challenge is to calibrate the star spectral dependency in fringe measurements - the spectral calibration. Previously, we have developed a spectral calibration and estimation scheme achieving picometer level accuracy. In this paper, we present the improvements regarding the application of this scheme from sensitivity studies. Data from the SIM Spectral Calibration Development Unit (SCDU) test facility shows that the fringe OPD is very sensitive to pointings of both beams from the two arms of the interferometer. This sensitivity coupled with a systematic pointing error provides a mechanism to explain the bias changes in 2007. Improving system alignment can effectively reduce this sensitivity and thus errors due to pointing errors. Modeling this sensitivity can lead to further improvement in data processing. We then investigate the sensitivity to a model parameter, the bandwidth used in the fringe model, which presents an interesting trade between systematic and random errors. Finally we show the mitigation of calibration errors due to system drifts by interpolating instrument calibrations. These improvements enable us to use SCDU data to demonstrate that SIM-Lite missions can meet the 1pm noise floor requirement for detecting earth-like exoplanets.

We present the start of the ground alignment plan for the SIM Lite Instrument. We outline the integration and alignment of the individual benches on which all the optics are mounted, and then the alignment of the benches to form the Science and Guide interferometers. The Instrument has a guide interferometer with only a 40 arc-seconds field of regard, and 200 arc-seconds of alignment adjustability. This requires each sides of the interferometer to be aligned to a fraction of that, while at the same time be orthogonal to the baseline defined by the External Metrology Truss. The baselines of the Science and Guide interferometers must also be aligned to be parallel. The start of these alignment plans is captured in a SysML Instrument System model, in the form of activity diagrams. These activity diagrams are then related to the hardware design and requirements. We finish with future plans for the alignment and integration activities and requirements.

The Guide-2 telescope (G2T) is an important subsystem of the new SIM Lite Astrometric Observatory. In this paper we present system identification experiments, design and implementation of the G2T stellar pointing loop that achieves milliarcsecond resolution of spacecraft attitude. Special emphasis was placed on characterization and modeling of PZT hysteresis since this nonlinearity plays an important part in the control loop performance. Power spectral densities of the star image centroids were use to evaluate the pointing loop performance with and with out the presence of simulated ACS disturbances injected via a fast steering mirror (FSM).

The SIM Lite Astrometric Observatory is a mission concept for a space-borne instrument to perform micro-arcsecond narrow-angle astrometry to search 60 to 100 nearby stars for Earth-like planets, and to perform global astrometry for a broad astrophysics program. The main enabling technology development for the mission was completed during phases A & B. While the project is waiting for the results of the ASTRO2010 Decadal Survey to proceed into flight implementation, the instrument team is currently converting the developed technology onto flight-ready engineering models. These key engineering tasks will significantly reduce the implementation risks during the flight phases C & D of the mission. The main optical interferometer components, including the astrometric beam combiner (ABC), the fine steering mechanism (FSM), the path-length control and modulation optical mechanisms (POM & MOM), focal plane camera electronics (ATC & FTC), camera cooling cryo-heat pipe, and the siderostat mechanism are currently under development. Main assemblies are built to meet flight requirements and have been or will be subjected to flight qualification level environmental testing (random vibration and thermal cycling) and performance testing. The Spectral Calibration Development Unit (SCDU), a white light interferometer testbed has recently demonstrated how to perform the spectral calibration of the instrument. The Guide 2 Telescope testbed (G2T) has demonstrated the 50 micro-arcsecond angle monitoring capability required by SIM Lite to perform astrometry. This paper summarizes recent progress in engineering risk reduction activities.

SIM Lite is a space-borne stellar interferometer capable of searching for Earth-size planets in the habitable zones of nearby stars. This search will require measurement of astrometric angles with sub micro-arcsecond accuracy and optical pathlength differences to 1 picometer by the end of the five-year mission. One of the most significant technical risks in achieving this level of accuracy is from systematic errors that arise from spectral differences between candidate stars and nearby reference stars. The Spectral Calibration Development Unit (SCDU), in operation since 2007, has been used to explore this effect and demonstrate performance meeting SIM goals. In this paper we present the status of this testbed and recent results.

The SIM-Lite project has designed and built PZT driven precision optical mechanisms for pointing and optical pathlength phasing. This paper will discuss the designs, the flight qualification, and performance in a space representative environment of these dynamic optical devices. We will also discuss performance of the strain gauges bonded to the PZTs of the SIM Fine Steering Mirror.

Interest in pupil-remapping interferometry, in which a single telescope pupil is fragmented and recombined using fiber optic technologies, has been growing among a number of groups. As a logical extrapolation from several highly successful aperture masking programs underway worldwide, pupil remapping offers the advantage of spatial filtering (with single-mode fibers) and in principle can avoid the penalty of low throughput inherent to an aperture mask. However in practice, pupil remapping presents a number of difficult technological challenges including injection into the fibers, pathlength matching of the device, and stability and reproducibility of the results. Here we present new approaches based on recently-available photonic technologies in which coherent threedimensional waveguide structures can be sculpted into bulk substrate. These advances allow us to miniaturize the photonic processing into a single, robust, thermally stable element; ideal for demanding observatory or spacecraft environments. Ultimately, a wide range of optical functionality could be routinely fabricated into such structures, including beam combiners and dispersive or wavelength selective elements, bringing us closer to the vision of an interferometer on a chip.

Detection and observation of gravitational waves requires extremely accurate displacement measurement in the frequency range 0.03 mHz to 1 Hz. The Laser Interferometer Space Antenna (LISA) mission will attain this by creating a giant interferometer in space, based on free floating proof masses in three spacecrafts. Due to orbit evolution and time delay in the interferometer arms, the direction of transmitted light changes. To solve this problem, a picometer stable Point-Ahead Angle Mechanism (PAAM) was designed, realized and successfully tested. The PAAM concept is based on a rotatable mirror. The critical requirements are the contribution to the optical path length (less than 1.4 pm / rt Hz) and the angular jitter (less than 8 nrad / rt Hz). Extreme dimensional stability is achieved by manufacturing a monolithical Haberland hinge mechanism out of Ti₆Al₄V, through high precision wire erosion. Extreme thermal stability is realized by placing the thermal center on the surface of the mirror. Because of piezo actuator noise and leakage, the PAAM has to be controlled in closed-loop. To meet the requirements in the low frequencies, an active target capacitance-to-digital converter is used. Interferometric measurements with a triangular resonant cavity in vacuum proved that the PAAM meets the requirements.

MATISSE is a mid-infrared spectro-interferometer combining beams of up to four telescopes of the ESO VLTI providing phase closure and image reconstruction using interferometric spectra in the LM and N band. This paper presents the opto-mechanical design of the two cold benches containing several types of cryogenic mechanisms (shutter, Tip/Tilt) used for cryogenic alignment. Key aspects are detailed such as the highly integrated opto-mechanical approach of the design in order to guarantee component stability and accuracy specifications in the order of nanometers and arcseconds.

Kilometric-scale optical imagers seem feasible to realize by intensity interferometry, using telescopes primarily erected for measuring Cherenkov light induced by gamma rays. Planned arrays envision 50-100 telescopes, distributed over some 1-4 km². Although array layouts and telescope sizes will primarily be chosen for gamma-ray observations, also their interferometric performance may be optimized. Observations of stellar objects were numerically simulated for different array geometries, yielding signal-to-noise ratios for different Fourier components of the source images in the interferometric (u, v)-plane. Simulations were made for layouts actually proposed for future Cherenkov telescope arrays, and for subsets with only a fraction of the telescopes. All large arrays provide dense sampling of the (u, v)-plane due to the sheer number of telescopes, irrespective of their geographic orientation or stellar coordinates. However, for improved coverage of the (u, v)-plane and a wider variety of baselines (enabling better image reconstruction), an exact east-west grid should be avoided for the numerous smaller telescopes, and repetitive geometric patterns avoided for the few large ones. Sparse arrays become severely limited by a lack of short baselines, and to cover astrophysically relevant dimensions between 0.1-3 milliarcseconds in visible wavelengths, baselines between pairs of telescopes should cover the whole interval 30-2000 m.

We review the status of hardware developments related to the Linc-Nirvana optical path difference (OPD) control. The status of our telescope vibration measurements is given. We present the design concept of a feed-forward loop to damp the impact of telescope mirror vibrations on the OPD seen by Linc-Nirvana. At the focus of the article is a description of the actuator of the OPD control loop. The weight and vibration optimized construction of this actuator (aka piston mirror) and its mount has a complex dynamical behavior, which prevents classical PI feedback control from delivering fast and precise motion of the mirror surface. Therefore, an H-; optimized control strategy will be applied, custom designed for the piston mirror. The effort of realizing a custom controller on a DSP to drive the piezo is balanced by the outlook of achieving more than 5x faster servo bandwidths. The laboratory set-up to identify the system, and verify the closed loop control performance is presented. Our goal is to achieve 30 Hz closed-loop control bandwidth at a precision of 30 nm.

The Fringe and Flexure Tracker System (FFTS) of the LINC-NIRVANA instrument is designed to monitor and correct the atmospheric piston variations and the instrumental vibrations and flexure at the LBT during the NIR interferometric image acquisition. In this contribution, we give an overview of the current FFTS control design, the various subsystems, and their interaction details. The control algorithms are implemented on a realtime computer system with interfaces to the fringe and flexure detector read-out electronics, the OPD vibration monitoring system (OVMS) based on accelerometric sensors at the telescope structure, the piezo-electric actuator for piston compensation, and the AO systems for offloading purposes. The FFTS computer combines data from different sensors with varying sampling rate, noise and delay. This done on the basis of the vibration data and the expected power spectrum of atmospheric conditions. Flexure effects are then separated from OPD signals and the optimal correcting variables are computed and distributed to the actuators. The goal is a 120 nm precision of the correction at a bandwidth of about 50 Hz. An end-to-end simulation including models of atmospheric effects, actuator dynamics, sensor effects, and on-site vibration measurements is used to optimize controllers and filters and to pre-estimate the performance under different observation conditions.

The Large Binocular Telescope Interferometer (LBTI) has been developed and tested and is almost ready to be installed to LBT mount. In preparation for installation, testing of the beam combination and phasing of the system have been developed. The testing is currently in progress. The development of a telescope simulator for LBTI has allowed verification of phasing and alignment with a broad band source at 10 microns². Vibration tests with the LBTI mounted to the LBT were carried out in July 2008, with both seismic accelerometers and an internal optical interferometric measurement. The results have allowed identification of potential vibration sources on the telescope. Plans for a Star Simulator that illuminates each LBT aperture at the prime focus with two artificial point sources derived from a single point source via fiber optics are presented. The Star Simulator will allow testing of LBTI with the telescope and the adaptive secondaries in particular. Testing with the Star Simulator will allow system level testing of LBTI on the telescope, without need to use on-sky time. Testing of the Star Simulator components are presented to verify readiness for use with the LBTI.

We present the latest status of the fringe detecting algorithms for the LINC-NIRVANA FFTS (Fringe and Flexure Tracker System). The piston and PSF effects of the system from the top of the atmosphere through the telescopes and multi-conjugate AO systems to the detector are discussed and the resulting requirements for the FFTS outlined.

GRAVITY is a second generation instrument for the VLTI. It will combine four telescopes in the K band and perform fringe tracking on stars as faint as 10 magnitude with a lambda/8 accuracy, thus counterbalancing atmospheric piston and UTs longitudinal vibrations, despite flux drop-outs due to residual tip-tilt jitter. To achieve such a performance, new developments have to be tested. We have developed a complete simulator so as to improve algorithms and establish an efficient fringe tracking strategy. In addition, a prototype of the fringe tracker for GRAVITY is being built up in order to demonstrate the results of this simulator. We present here the current status of these developments, achieved by simulating realistic tracking at VLTI.

We report first results obtained from observations using a PRIMA FSU (Fringe Sensor Unit) as a fringe tracker for MIDI on the VLTI when operating with the 1.8-m ATs. Interferometric observations require the correction of the disturbance in the optical path induced by atmospheric turbulence ("piston"). The PRIMA FSU is able to compensate for such disturbances in real-time which makes it a suitable facility to stabilize the fringe signal for other VLTI instruments, like AMBER, MIDI or later MATISSE. Currently, the atmospheric coherence time in the N band (8 to 13 μm) observed by MIDI, as well as the thermal background in this band, require a minimum target flux of 20 Jy and a correlated flux of 10 Jy (in PRISM/HIGH SENSE mode and using the ATs under standard conditions) to allow self-fringe-tracking and data reduction. However, we show that if the fringes are stabilized by the FSU, coherent integration allows a reliable data reduction even for the observation of faint targets (F_corr <10 Jy) with MIDI at standard detector exposure times. We were able to measure the correlated flux of a 0.5 Jy source, which pushes the current limits of MIDI down to regions where numerous new targets become accessible on ATs. For faint object observations we will discuss the usage of VISIR photometry for calibration purposes. The observational tests done so far and the obtained results represent a first step towards Phase Referenced Imaging with the VLTI in the mid-infrared.

Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and determine their angular separation to a precision of 10 μas in only 5 minutes. To perform the astrometric measurement with such a high accuracy, the differential path length through the VLTI and the instrument has to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.

The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.

The Polarization-Based Collimated Beam Combiner efficiently produces pairwise interference between beams from multiple telescopes. An important feature is achieving "Photometric Symmetry" whereby interference measurements have no first-order sensitivity to wavefront perturbations (or photometric variations following spatial filtering) which otherwise entail visibility measurements with increased error, bias, and nonlinearity in phase determination. Among other proposed applications, this topology has been chosen as the basis for the design of the NOVA Fringe Tracker (NFT), a proposed 4 or 6 telescope second-generation fringe tracker for the VLTI. The NFT takes advantage of the photometric symmetry thus achieved making it capable of tracking on stars resolved beyond the first visibility null, as well as interfering a telescope beam with one which is 20 times brighter, a design goal set by ESO. By not requiring OPD modulation for interferometric detection, the detector exposure time can be increased without performance reduction due to time skew nor is sensitivity reduced by removing optical power for photometric monitoring, and use of two-phase interferometric detection saves one half of the photons being diverted for detection of the other two (mainly) unused quadrature phases. The topology is also proposed for visibility measuring interferometers with configurations proposed for the achievement of balanced quadrature or 3-phase interferometric detection. A laboratory demonstration confirms >>100:1 rejection of photometric crosstalk in a fringe tracking configuration where atmospheric OPD fluctuations were simulated using a hair dryer. Tracking with a 30:1 intensity ratio between the incoming beams was performed while rejecting large introduced photometric fluctuations.

In a few years, the second generation instruments of the Very Large Telescope Interferometer (VLTI) will routinely provide observations with 4 to 6 telescopes simultaneously. To reach their ultimate performance, they will need a fringe sensor capable to measure in real time the randomly varying optical paths differences. A collaboration between LAOG (PI institute), IAGL, OCA and GIPSA-Lab has proposed the Planar Optics Phase Sensor concept to ESO for the 2^nd Generation Fringe Tracker. This concept is based on the integrated optics technologies, enabling the conception of extremely compact interferometric instruments naturally providing single-mode spatial filtering. It allows operations with 4 and 6 telescopes by measuring the fringes position thanks to a spectrally dispersed ABCD method. We present here the main analysis which led to the current concept as well as the expected on-sky performance and the proposed design.

POPS (Planar Optical Phase Sensor) is a second-generation fringe tracker for the Very Large Telescope Interferometer (VLTI), intended to simultaneously measure the cophasing and coherencing errors of up to six Unit Telescopes (UT) or Auxiliary Telescopes (AT) in real time. The most promising concepts are probably based on the utilization of Integrated Optics (IO) components, and were the scope of a Phase A study led by Observatoire de Grenoble (LAOG). Herein is described a tentative design built around a multi-axial IO chip whose fringes are dispersed downstream on a detector array, and a Chromatic Phase Diversity algorithm presented in another paper of this conference . We depict the foreseen opto-mechanical, detection and software implementations, and provide numerical results from a realistic simulation model in terms of group and phase delay measurement accuracy and limiting magnitudes in the K band. The ultimate performance of the method is discussed and compared with the original 2^nd generation VLTI fringe tracker requirements.

Performing long coherent integrations is now widely accepted as one of the best methods for improving the signal-to-noise ratio of fringe measurements. There are two basic ways of carrying out coherent integration. One method, real-time coherent integration, stabilizes the fringes on a detector in real time using a separate detector and feedback loop. In order for this to work, the fringes must be stabilized (nominally to less than one radian) and the response-time of the fringe-tracking loop must be less than a coherence time. The other method, post-processing coherent integration, records the fringe and instrument data with minimum integration and assembles the coherently integrated visibilities after the fact. While recording fringe data for post-processing coherent integration it is only necessary to stabilize to less than the coherence length of the individual channels. In terms of fringe stabilization, in the absence of read-noise, post-processing performs significantly better than real-time coherent integration, one the order of a factor two smaller fringe tracking error. This results in improved SNR, reduced integration time, and the ability to coherently integrate on fainter targets. In cases of sufficiently large detector read noise the situation can change to the point where real-time coherent integration produces better SNR. Real-time coherent integration is thus the less efficient of the two, and should only be employed when detector read noise prevents post-processing coherent integration.

Z CMa is a young binary system consisting of an Herbig primary and a FU Ori companion. Both components seem to be surrounded by active accretion disks and a jet was associated to the Herbig B0. In Nov. 2008, K. Grankin discovered that Z CMa was exhibiting an outburst with an amplitude larger than any photometric variations recorded in the last 25 years. To study the innermost regions in which the outburst occurs and understand its origin, we have observed both binary components with AMBER/VLTI across the Br emission line in Dec. 2009 in medium and high spectral resolution modes. Our observations show that the Herbig Be, responsible for the increase of luminosity, also produces a strong Br emission, and they allow us to disentangle from various origins by locating the emission at each velocities through the line. Considering a model of a Keplerian disk alone fails at reproducing the asymmetric spectro-astrometric measurements, suggesting a major contribution from an outflow.

As for large interferometers, the crucial point to solve is the cophasing issue. Indeed, the cophasing device must be both efficient and independent of the number of telescopes allowing a large capture range and accurate piston measurements while being easy to implement. We developed such a cophasing method named the Chromatic Phase Diversity (CPD) method. Actually, using three spectral channels, the CPD method can determine the piston errors without ambiguity and on a range much larger than ± half a wave. This method makes it possible to work whether in coherencing mode or in cophasing mode. We designed and implemented this method on the SIRIUS test bench1 at the Observatoire de la Cote d'Azur, France. We present the instrument design, the results obtained with the CPD method. The performances such as the achieved capture range, the accuracy of the piston values extraction and the attainable magnitude are described and analyzed.

For ground-based optical interferometry, the simple specification of high surface quality flat relay mirrors is not the end of the story for obtaining high quality fringes. The Navy Prototype Optical Interferometer array transports the stellar radiation from six primary collectors through a 10-reflection vacuum relay system, resulting in six separate combinable wavefronts. The surface error of each of the 60 relay mirrors is specified to be no greater than 32 nm peak-to-valley for fabrication purposes. However, once mounted in the 10-element optical train the errors from each mirror do not necessarily cancel one another, but can add and increase the resultant wavefront distortion for that path. This leads to fringe contrast reduction, reduced ability to fringe track, and a reduction in the limiting magnitude of observable objects. Fortunately, the total wavefront distortion for each train can be measured, calibrated, and nullified by using a phaseshifting interferometer combined with a compliant static deformable mirror and control system. In this paper we describe a system that mitigates the resultant wavefront distortion.

We present the Fiber Coupler subsystem of the future VLTI instrument GRAVITY. GRAVITY is specifically designed to deliver micro-arcsecond astrometry and deep interferometric imaging. The Fiber Coupler is designed to feed the light from a science and a reference object into single-mode fibers. The Fiber Coupler consists of four independent units. The units de-rotate the FoV. A motorized half-wave plate allows rotating the liner polarization axis. Each unit provides actuators for fast piston actuation, tip-tilt correction and pupil stabilization for one of the beams from four VLT telescopes. The actuators are operated in closed-loop. Together with a dedicated Laser Guiding System, this allows to stabilize the beams and maximize the coherently coupled light. The fast piston actuator provides the crucial fringe tracking capability at a bandwidth of >220Hz. A special roof prism design allows to either split the FoV or to serve as a 50/50 beam splitter without changing the optical path. This offers the possibility of on-axis as well as off-axis fringe tracking. The optical train consists solely of mirrors, which ensures an achromatic behavior and maximum throughput. The sophisticated optical design compensates for aberrations which are introduced by off-axis parabolic mirrors. This allows to achieve Strehl ratios of >95% across the FoV.

Laser-based metrology has been identified as an enabling technology in the deployment of large, spaceborne observatories, where nanometer-level knowledge of fiducial displacement drives overall system performance. In particular, Nd:YAG NPRO (non-planar ring oscillator) based lasers have received considerable attention in this application because of their inherent high coherence at wavelengths of interest (1064 and 1319nm). However, the use of NPRO based lasers in decade long space missions is limited by typical 800nm-band pump laser diode wearout and random failure rates. Therefore, reliably achieving multi-hundred milliwatt NPRO power over prolonged mission lifetimes requires innovative pump architectures. In this paper we present a pump architecture capable of supporting continuous NPRO operation over 5.5yrs at 300mW with reliability exceeding 99.7%. The proposed architecture relies on a low-loss, high port count, all-fiber optical coupler to combine the outputs of multiple single-mode pump laser diodes. This coupler is capable of meeting the exacting environmental requirements placed by a space mission, such as SIM Lite.

The possibility of interferometrically coupling large telescopes using single-mode (SM) fibers is a very attractive one, especially at topographically complex and culturally sensitive astronomical observing sites such as the Mauna-Kea summit in Hawaii. The 'OHANA project (Optical Hawaiian Array for Nanoradian Astronomy) aims to link up seven of the large telescopes on Mauna-Kea. The concept of using SM fiber links for interferometry has been demonstrated using the two W. M. Keck telescopes. A beam-combiner and optical delay line has been installed at the Canada France Hawaii Telescope (CFHT) to link up Gemini North and CFHT. In order to test the CFHT beam-combiner without making use of CFHT and Gemini observing time, the idea of using two small, 20 cm aperture telescopes to inject starlight into the 'Ohana interferometer fibers was devised. This project, dubbed 'OHANA-Iki, is also exploring the concept of a "soft" optical interferometer, specifically one in which the telescopes are easily movable and would not require the heavy, fixed infrastructure found in conventional freespace interferometers such as the VLTI.

The Wide-Field Imaging Interferometry Testbed (WIIT) at NASA's Goddard Space Flight Center and a computational model of the testbed were developed to demonstrate and learn the practical limitations of techniques for wide-field spatial-spectral ("double Fourier") interferometry. WIIT is an automated and remotely operated system, and it is now producing substantial amounts of high-quality data from its state-of-the-art operating environment, Goddard's Advanced Interferometry and Metrology Lab. In this paper, we discuss the characterization and operation of the testbed and present recently acquired data. We also give a short description of the computational model and its applications. Finally, we outline future research directions. A companion paper within this conference discusses the development of new widefield double Fourier data analysis algorithms.

The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry. One of the concepts is based on dispersive plates made of three materials which where fully characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the development of the achromatic phase shifters and report on the current status of the test bench.

In this paper we compare the performance of multi and single-mode interferometry for the estimation of the phase of the complex visibility in presence of detector, photon and atmospheric noises. We show that, despite the loss of flux occurring when injecting the light in the single-mode component, the spatial filtering properties of such single-mode devices often enable higher performance than multimode concepts. In the high flux regime speckle noise dominated, single-mode interferometry is always more efficient, and its performance is significantly better when the correction provided by adaptive optics becomes poor, by a factor of 2 and more when the Strehl ratio is lower than 10%. In detector noise regime, multimode interferometry reaches better performance, yet the gain never exceeds 20%, which corresponds to the percentage of photon loss due to the injection in the guides. We finally show that single-mode interferometry is also more robust to the turbulence in both cases of fringe tracking and phase referencing, at the exception of narrow field of views. We conclude that fringe trackers built using single-mode optics should be considered as a solution both practical and competitive.

LINC-NIRVANA (LN) is a Fizeau interferometer that will provide coherent images in the near-IR combining the beams from the two Large Binocular Telescope (LBT) arms, by adopting a Multi-Conjugate Adaptive Optics system (MCAO) that allows for atmospheric turbulence compensation. We applied the software AIRY-LN for the simulation and the reconstruction of LN images in order to investigate the dependence of the image quality from the magnitude of the star used for the PSF extraction. A good knowledge of this dependence is a crucial point for the LN observations, especially for the extragalactic target where the presence of a bright psf-star within the LN field of view of 10×10 arcsec is not granted. Our results, although still preliminary, show that while from the morphological point of view the use of psf-star up to a magnitude of 18 is still acceptable, from the photometric point of view the use of psf-star fainter than Ks ~ 16 mag could cause considerable problems.

I developed a new imaging algorithm allowing the combination of multi-channel interferometric data in a single image. The method is applicable to sources emitting optically thick radiation where the dependence of the source structure on wavelength can therefore be described by a single parameter, i.e. the (local) effective temperature. The result of the imaging process is an intensity map as well as a map of the effective temperature across the source. The advantage of this method over the independent imaging of data in each channel is the benefit of a much better DB ("dirty beam", also PSF) when combining the aperture coverage of each channel over a wide range of wavelengths. I present results from applications to composite spectrum binaries and stellar surfaces of rotating stars based on simulated data. My imaging algorithm is based on the well known CLEAN method, adding the effective temperature as an additional parameter for each image pixel. This information can be used to scale the PSF in each channel with the blackbody law and subtract it from the combined residual map. Once a cleaned map has been obtained (using regular phase self-calibration techniques) and thus a set of phase-corrected visibilities, individual channel maps are obtained by running the CLEAN restricted to non-zero pixels. The cleaned channel maps together with the total flux spectrum can then be used to estimate the blackbody temperature for each non-zero image pixel, and used for the next iteration of the modified CLEAN algorithm.

Imaging large intensity gradients for rapid rotators, complicated spot patterns on rotating RS CVn binaries, and diffuse disk emission around Young Stellar Objects requires advanced imaging methods beyond the current image reconstruction paradigms in optical/infrared interferometry. We report on the ongoing development of SQUEEZE, a flexible software for optical and infrared interferometry capable of simultaneous model-fitting and image reconstruction, and possessing useful new capabilities such as imaging on a spheroid. We treat the problem of the local minimas due to the "missing phase" and explore the use of non-pixel-based reconstructions within the Bayesian evidence and compressed sensing frameworks.

This paper presents some methods being developped for relaxing the underdetermination of the image reconstruction from interferometric data. We consider, in a first part, the advantages of using spectro-differential data for having a more accurate and complete set of complexe visibilities. We formulate some regularization criteria along the spectral dimension, in order to express some prior knowledge on the correlation between the brightness distributions in different wavelength. These spectral prior terms are inspired by, and combinable with, some spatial regularization functions already in use in existing Image Reconstruction sofwares. We also show that the interferometric image reconstruction problem can benefit from being reformulated as a sparse approximation problem in redundant dictionaries. The dictionary is composed from union of representation bases, whose atoms correspond to geometric features of the image. Different bases (e.g. impulsions, wavelets, discrete cosine transform) correspond to different features. The sparse approximation approach consists in selecting the geometrical features that best explain the interferometric data, by imposing that only a few such features should be necessary to reconstruct the image. Simulations showing images reconstructed using this method are presented.

We present a novel spectral imaging method for characterization of exoplanets. This method uses 4 collecting telescopes, in a pattern similar to TPF-I or Darwin, combined with phase chopping. Focusing on contiguous observing wavelengths in space, the (u, v) plane can be simultaneously filled by the use of the contiguous observing wavelengths instead of continuously rotating the baselines. For a target comprising a star and a planet, observations on two baselines are sufficient to extract an image of the planetary system and a spectrum of the planet. Our simulations show that this new method allows us to detect an analog Earth around a Sun-like star at 10pc and to acquire its spectrum over the wavelength range from 8 to 18μm.

The small angular distance (<100 mas) and the huge flux ratio (10⁷) between an Earth-like exoplanet in the socalled habitable zone and its host star makes it very difficult to direct image such systems. Nulling interferometry consists of a very powerful technique that combines destructively the light from two or more collectors to dim the starlight and to reveal faint companions in its vicinity. We have developed a new nulling experiment based on the fiber nuller principle. This fully symmetric reflective nulling bench aims at testing broadband nulling in both H and K bands as well as characterizing photonic fibers for modal filtering. We present in this paper the design, the development as well as preliminary results of the experiment.

Nulling interferometry is still a promising method to characterize spectra of exoplanets. One of the main issues is to cophase at a nanometric level each arm despite satellite disturbances. The bench PERSEE aims to prove the feasibility of that technique for spaceborne missions. After a short description of PERSEE, we will first present the results obtained in a simplified configuration: we have cophased down to 0.22 nm rms in optical path difference (OPD) and 60 mas rms in tip/tilt, and have obtained a monochromatic null of 3 · 10^-5 stabilized at 3•10^-6. The goal of 1 nm with additional typical satellite disturbances requires the use of an optimal control law; that is why we elaborated a dedicated Kalman filter. Simulations and experiments show a good rejection of disturbances. Performance of the bench should be enhanced by using a Kalman control law, and we should be able to reach the desired nanometric stability. Following, we will present the first results of the final polychromatic configuration, which includes an achromatic phase shifter, perturbators and optical delay lines. As a conclusion, we give the first more general lessons we have already learned from this experiment, both at system and component levels for a future space mission.

We present the results of the fourth Optical/IR Interferometry Imaging Beauty Contest. The contest consists of blind imaging of test data sets derived from model sources and distributed in the OI-FITS format. The test data consists of spectral data sets on an object "observed" in the infrared with spectral resolution. There were 4 different algorithms competing this time: BSMEM the Bispectrum Maximum Entropy Method by Young, Baron & Buscher; RPR the Recursive Phase Reconstruction by Rengaswamy; SQUEEZE a Markov Chain Monte Carlo algorithm by Baron, Monnier & Kloppenborg; and, WISARD theWeak-phase Interferometric Sample Alternating Reconstruction Device by Vannier & Mugnier. The contest model image, the data delivered to the contestants and the rules are described as well as the results of the image reconstruction obtained by each method. These results are discussed as well as the strengths and limitations of each algorithm.

The Optical Long Baseline Interferometry News (OLBIN) is a website and forum for scientists, engineers, and students who share a common interest in long-baseline stellar interferometry. Through OLBIN you will find links to projects devoted to stellar interferometry, as well as news items, recent papers and preprints, notices of upcoming meetings, and resources for further research. This paper describes the history of the website, how it has evolved to serve the community, and the current plans for its future development. The website can be found at http://olbin.jpl.nasa.gov/.

The Apodizing Phase Plate (APP) is a simple optic that provides coronagraphic suppression of diffraction without the need for any focal plane occulters. We present the design of a broadband APP (the BAPP) for LMIRCam that is optimised for the direct imaging of cool extrasolar giant planets around nearby stars at thermal infrared wavelengths. These optics have a high throughput and require no precision alignment. We cover earlier results using a chromatic APP, the basic principle and manufacture of the optic.

In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer (NPOI). Coherent integration relies on being able to phase reference interferometric measurements, which in turn relies on making measurements at multiple wavelengths. We first discuss the generalized group-delay approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration can be used to perform very precision measurement of stellar diameters. The phase of the complex visibility is particularly attractive as a data product because it is not biased in the same way as visibility amplitudes. We discuss the relative SNR of triple-product phases and single-baseline phases. We then demonstrate how singlebaseline phases can be used to make accurate measurements of magnitude differences and separations of binary stars.

In the literature you could find requirements for nulling IR interferometry in terms of nulling ratio nl and stability of this nulling ratio σ_nl for the observation of a exo-Earth gravitating at 1au of a solar type star located at 10pc from the Earth: nl = 10^-5 between 7 and 20um and σ_nl = 3•10^-9 over 10 days.¹ These requirements are very demanding. We report on studies made to obtain the technological requirements in terms of nulling ratio and stability of this nulling ratio in function of the targeted planets and stars. Finally we will present methods of dithering developed for the NULLTIMATE testbench to achieve to the maximum stability.

A unique statistical data analysis method has been developed for reducing nulling interferometry data. The idea is to make use of the statistical distributions of the fluctuating null depths and beam intensities to retrieve the astrophysical null depth in the presence of fluctuations. The approach yields an accuracy much better than is possible with standard data reduction methods, because the accuracy of the null depth is not limited by the sizes of the phase and intensity errors but by the uncertainties on their statistical distributions. The result is an improvement in the instrumental null depth measurement limit of roughly an order of magnitude. We show in this paper that broadband null depths of 10^-4 can be measured in the lab with our infrared Fiber Nuller without achromatic phase shifters. On sky results are also dramatically improved, with measured contrasts up to a couple of 10^-4 with our instrument mounted on the Hale telescope at the Palomar Observatory. This statistical analysis is not specific to our instrument and may be applicable to other interferometers.

Integrated optic, stellar interferometric two-beam and three-beam combiners have been designed and fabricated for operation in the L band (3 μm - 4 μm). The devices have been realized in titanium-indiffused, x-cut lithium niobate substrates, and on-chip electro-optic fringe scanning has been demonstrated. White light fringes were produced in the laboratory using the two-beam combiner integrated with an on-chip Y-splitter. Furthermore, a new design for next-generation, broadband, achromatic, astronomical, beam combiners operating in the N band (8 μm - 12 μm) is proposed and analyzed. The design is based on adiabatic mode-evolution couplers fabricated using Ge/Si heterostructure rib waveguides.

We present a review of our activities on PERSEE (Pégase Experiment for Research and Stabilization of Extreme Extinction) at Observatoire de la Côte d'Azur (OCA). PERSEE is a laboratory testbench aiming at achieving a stabilized nulling ratio better than 10^-4 in the astronomical bands K and M, in presence of flight-representative spacecraft perturbations. The bench has been jointly developed by a Consortium of six French institutes and companies, among which OCA was responsible for the star simulator and of the opto-mechanical studies, procurement and manufacturing of the optical train. In this communication are presented the alignment and image quality requirements and the optomechanical design of the illumination module and main optical train, including a periscope Achromatic Phase Shifter (APS), tip-tilt mirrors used to introduce and then compensate for dynamic disturbances, delay lines, beam compressors and fiber injection optics. Preliminary test results of the star simulator are also provided.

We have designed and constructed a prototype of Photometric Channels for Michigan Infrared Combiner (MIRC). Photometric Channels provide direct real-time measurements of fluxes from individual telescopes, improving the precision of visibility and close phase calibration. While this prototype has increased the MIRC data quality since the commissioning in August 2009, it leaves several issues unresolved, such as the changing polarization of starlight due to the CHARA beam train. We are planning to make an improvement of the prototype in conjunction with the six-beam combiner upgrade in the summer 2011.

This paper presents the first empirical measurement of the K¹-band effective wavelength and bandwidth of the CHARA Classic beam combiner on the CHARA Array. Prior to this work, the accepted effective wavelength value used for CHARA Classic data (2.1501μm) came from a model of the system; it was not derived from measurements done on the system directly. We employ two data collection methods for our observations: using the Optical Path Length Equalizer (OPLE) cart to scan through the interference fringes and using the dither mirror to scan through the fringes. The two observational methods yield similar effective wavelength measurements (2.141±0.003μm with the OPLE cart and 2.136±0.002μm with the dither mirror). Both of these results are lower than the previously adopted effective wavelength value, but by less than 0.7%. The bandwidth values measured by the two methods differ from each other by almost 5% (0.334 ± 0.002μm with the OPLE cart and 0.351±0.003μm with the dither mirror). Our results establish the first estimate of the uncertainty in the effective wavelength and bandwidth.

LINC-NIRVANA is the near-infrared Fizeau interferometric imaging camera for the Large Binocular Telescope (LBT). For an efficient interferometric operation of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is mandatory: It is a real-time servo system that allows to compensate atmospheric and instrumental optical pathlength differences (OPD). The thereby produced time-stable interference pattern at the position of the science detector enables long integration times at interferometric angular resolutions. As the development of the FFTS includes tests of control software and robustness of the fringe tracking concept in a realistic physical system a testbed interferometer is set up as laboratory experiment. This setup allows us to generate point-spread functions (PSF) similar to the interferometric PSF of the LBT via a monochromatic (He-Ne laser) or a polychromatic light source (halogen lamp) and to introduce well defined, fast varying phase offsets to simulate different atmospheric conditions and sources of instrumental OPD variations via dedicated actuators. Furthermore it comprises a piston mirror as actuator to counteract the measured OPD and a CCD camera in the focal plane as sensor for fringe acquisition which both are substantial devices for a fringe tracking servo loop. The goal of the setup is to test the performance and stability of different control loop algorithms and to design and optimize the control approaches. We present the design and the realization of the testbed interferometer and comment on the fringe-contrast behavior.

Characterisation, mitigation and correction of telescope vibrations have proven to be crucial for the performance of astronomical infrared interferometers. The project teams of the interferometers for the LBT, LINC-NIRVANA and LBTI, and LBT Observatory (LBTO) have embarked on a joint effort to implement an accelerometer-based vibration measurement system distributed over the optical elements of the LBT. OVMS, the Optical Path Difference and Vibration Monitoring System will serve to (i) ensure conditions suitable for adaptive optics (AO) and interferometric (IF) observations and (ii) utilize vibration information, converted into tip-tilt and optical path difference data, in the control strategies of the LBT adaptive secondary mirrors and the beam combining interferometers. The system hardware is mainly developed by Steward Observatory's LBTI team and its installation at the LBT is underway. The OVMS software development and associated computer infrastructure is the responsibility of the LINC-NIRVANA team at MPIA Heidelberg. Initially, the OVMS will fill a data archive provided by LBTO that will be used to study vibration data and correlate them with telescope movements and environmental parameters thereby identifiying sources of vibrations and to eliminate or mitigate them. Data display tools will help LBTO staff to keep vibrations within predefined thresholds for quiet conditions for AO and IF observations. Later-on real-time data from the OVMS will be fed into the control loops of the AO systems and IF instruments in order to permit the correction of vibration signals with frequencies up to 450 Hz.

A two stage blocking system is implemented in the GRAVITY science and the fringe tracking spectrometer optical design. The blocking system consists of a dichroic mirror and a long wave band-pass filter with the top level requirements of high transmission of the science light in the K-Band (1.95 - 2.5 μm) region and high blocking power optical density (OD) ≥ 8 for the metrology laser wavelength at 1.908 μm. The laser metrology blocking filters have been identified as one critical optical component in the GRAVITY science and fringe tracker spectrometer design. During the Phase-B study of GRAVITY we procured 3 blocking filter test samples for demonstration and qualification tests. We present the measurements results of an effective blocking of the metrology laser wavelength with a long wave band-pass filter at OD=12.

Gravity is a 2nd generation interferometric instrument for VLTI. It will combine 4 telescopes in dual feed in the K band to study general relativity effects around the Galactic Center black hole. The concept of Gravity is based on two equivalent beam combiner instruments: the scientific one fed by the science target (Sgr A*) and the fringe tracker fed by a bright reference star (See Gillessen et al.¹). Both beam combination instruments are based on silica on silicon integrated optics (IO) component glued to fluoride glass fiber array. The beam combiners are implemented in a cryogenic vessel cooled at 200°K and back-illuminated by a high power laser used for metrology (Bartko et al.²). This paper is dedicated to the description of the development of the integrated beam combiner assembly.

Operating on 6 interferometric baselines, i.e. using all 4 unit telescopes (UTs) of the Very Large Telescope Interferometer (VLTI) simultaneously, the 2^nd generation VLTI instrument GRAVITY will deliver narrow-angle astrometry with 10μas accuracy at the infrared K-band. At this angular resolution, GRAVITY will be able to detect the positional shift of the photo-center of a flare at the Galactic Center within its orbital timescale of about 20 minutes, using the observed motion of the flares as dynamical probes of the gravitational field around the supermassive black hole Sgr A*. Within the international GRAVITY consortium, the 1. Physikalische Institut of the University of Cologne is responsible for the development and construction of the two spectrometers of the camera system: one for the science object, and one for the fringe tracking object. In this paper we present the phase-B optical design of the two spectrometers as it got derived from the scientific and technical requirements and as it passed the preliminary design review (PDR) at the European Southern Observatory (ESO) successfully in late 2009.

The hydrogen emission line is a defining characteristic of young stellar objects probing the planet forming regions of the disks. The limiting sensitivity of current interferometers has precluded it's detailed study. We'll review our current understanding of hydrogen emission, recent results and project the science that can be achieved with sensitive interferometers such as the PRIMA off-axis mode and GRAVITY.

PIONIER is a 4-telescope visitor instrument for the VLTI, planned to see its first fringes in 2010. It combines four ATs or four UTs using a pairwise ABCD integrated optics combiner that can also be used in scanning mode. It provides low spectral resolution in H and K band. PIONIER is designed for imaging with a specific emphasis on fast fringe recording to allow closure-phases and visibilities to be precisely measured. In this work we provide the detailed description of the instrument and present its updated status.

Multiple Application Curvature Adaptive Optics (MACAO) systems are used at the coud´e focus of the unit telescopes (UTs) at the La-Silla Paranal Observatory, Paranal, to correct for the wave-front aberrations induced by the atmosphere. These systems are in operation since 2005 and are designed to provide beams with 10 mas residual rms tip-tilt error to the VLTI laboratory. We have initiated several technical studies such as measuring the Strehl ratio of the images recorded at the guiding camera of the VLTI, establishing the optimum setup of the MACAO to get collimated and focused beam down to the VLTI laboratory and to the instruments, and ascertaining the data generated by the real time computer, all aimed at characterizing and improving the overall performance of these systems. In this paper we report the current status of these studies.

The ESO Very Large Telescope Interferometer (VLTI) offers the unique access to the combination of the four 8-meter Unit Telescopes (UT) of Cerro Paranal. The quality of the scientific observations in interferometric mode is strongly related to the stability of the optical path difference (OPD) between the telescopes. Vibrations at the level of the telescopes and affecting the mirrors were shown to be an important source of perturbation for the OPD. ESO has thus started an important effort on the UTs and VLTI to tackle this effect. Active controls based on accelerometers and phase measurements have been developed to provide real-time correction of the variation of OPD introduced by vibrations. Systematic studies and measurement of the sources of vibration (instruments, wind, telescope altitude, ...) have been performed. Solutions to reduce the vibrations via design modification and/or new operation configurations are studied and implemented. To ensure good operational conditions, the levels of vibrations are regularly monitored to control any environmental change. This document will describe the modifications implemented and foreseen and give a status of the VLTI-UT vibrations evolution.

We compare the quality of interferometric image reconstructions for two different sets of data: square of the visibility plus closure phase (e.g. AMBER like case) and square of the visibility plus visibility phase (e.g. PRIMA+AMBER or GRAVITY like cases). We used the Multi-aperture image Reconstruction Algorithm for reconstructions of test cases under different signal-to-noise ratios and noisy data (squared visibilities and phases). Our study takes into account noise models based on the statistics of visibility, phase and closure phase. We incorporate the works developed by Tatulli and Chelly (2005) on the noise of the power-spectrum and closure phase in the read-out and photon noise regimes,1 and by Colavita (1999) on the signal-to-noise ratio of the visibility phase.2 The final images were then compared to the original one by means of positions and fluxes, computing the astrometry and the photometry. For the astrometry, the precision was typically of tens of microarcseconds, while for the photometry, it was typically of a few percent. Although both cases are suitable for image restorations of real interferometric observations, the results indicate a better performance of phase referencing (V² + visibility phase) in a low signal-to-noise ratio scenario.

Optical long-baseline interferometric data is commonly calibrated with respect to an external calibrator, which is either an unresolved source or a star with a known angular diameter. A typical observational strategy involves acquiring data in a sequence of calibrator-target pairs, where the observation of each source is obtained separately. Therefore, the atmospheric variations that have time scales shorter than the cadence between the target-calibrator pairs are not always fully removed from the data even after calibration. This results in calibrated observations of a target star that contain unknown quantities of residual atmospheric variations. We describe how Monte Carlo simulations can be used to assess quantitatively the impact of atmospheric variations on fitted model parameters, such as angular diameters of uniform-disk models representing semi- and fully-resolved single stars.

Two identical three-way beam combiners have been installed at the CHARA Array. The new setup is an extension of the two-beam pupil plain combiner, which has been in use thus far. Using the new beam combiners we can now obtain phase closure data in H, K or J band on two sets of three telescopes. A new optical design has been implemented to image the six outputs of the combiners onto six separate pixels in the infrared detector array. The new optical arrangement provides reduced background and spatial filtering. The magnitude limit of this beam combiner has reached 7.8 in K magnitude mainly as a result of better image quality by the new infrared camera optics.

We have detected a satellite via optical interferometry for the first time, using a 16 m baseline of the Navy Prototype Optical Interferometer (NPOI) to observe the geostationary communications satellite DirecTV-9S during the "glint" seasons of February-March 2008 and 2009 when the sun-satellite-NPOI geometry was favorable for causing specular reflections from geostationary satellites. We used the US Naval Observatory Flagstaff Station 1 m telescope to generate accurate positions for steering the NPOI. Stars are the easiest targets for optical/infrared interferometers because of their high surface brightness. Low surface brightness targets are more difficult: if they are small enough not to be resolved out by typical baselines, they are likely to be too faint to produce detectable fringes in an atmospheric coherence time. The 16 m NPOI baseline, the shortest available at the time of our observations, resolves out structures larger than ~ 1.5 m at the geostationary distance, while a typical size for the solar panel arrays is 2 m x 30 m. Our detection indicates that a small fraction of the satellite glinted, not surprising given that the solar panels are not accurately flat. Our fringe data are consistent with a two-component image consisting of a 1 to 1.3 m higher surface brightness component and a significantly larger lower surface brightness component. The brightness of the glints (2.^m 4 and ~ 1.^m 5 for the two detections in March 2009) and the size scale suggest that the compact component has an albedo of 0.06 to 0.13, while the larger-scale component is much darker, if circular geometry is assumed.

We present simulated observations of surface features on Red Supergiant (RSG) stars and clumpy dust structures surrounding Active Galactic Nuclei (AGN) with the Magdalena Ridge Observatory Interferometer (MROI). These represent two of the classes of astrophysical targets enumerated in the MROI Key Science Mission that are typical of the types of complex astrophysical phenomena that the MROI has been designed to image. The simulations are based on source structures derived from recent theoretical models and include both random and systematic noise on the measured Fourier data (visibility amplitudes and closure phases) consistent with our expectations for typical such targets observed with the MROI. Image reconstructions, obtained using the BSMEM imaging package, are presented for 4-, 6- and 8- telescope implementations of the array. Although a rudimentary imaging capability is demonstrated with only 4 telescopes, the detailed features of targets are only reliably determined when at least 6 telescopes are present. By the tine 8 telescope are used, the reconstructed images are sufficiently faithful to allow the discrimination between competing models, confirming the design goal of the MROI, i.e. to offer model-independent near-infrared imaging on sub-milliarcsecond scales.

Optical long baseline interferometry is a technique that has generated almost 850 refereed papers to date. The targets span a large variety of objects from planetary systems to extragalactic studies and all branches of stellar physics. We have created a database hosted by the JMMC and connected to the Optical Long Baseline Interferometry Newsletter (OLBIN) web site using MySQL and a collection of XML or PHP scripts in order to store and classify these publications. Each entry is defined by its ADS bibcode, includes basic ADS informations and metadata. The metadata are specified by tags sorted in categories: interferometric facilities, instrumentation, wavelength of operation, spectral resolution, type of measurement, target type, and paper category, for example. The whole OLBIN publication list has been processed and we present how the database is organized and can be accessed. We use this tool to generate statistical plots of interest for the community in optical long baseline interferometry.

We present the results of Navy Prototype Optical Interferometer observations of the binary stars θ² Tauri and HR7955. These data are reduced using standard methods, as well as coherent integration, and were fitted using three different methods to measure the separation and position angle of the components, and their magnitude differences. We used the traditional technique of fitting the V2's, triple amplitudes and triple phases, we also fitted the baseline phases obtained through coherent integration, and measured the separation of the components directly on images reconstructed using complex visibilities and phase self calibration. We find that fitting baseline phases produces the highest precision. The results obtained from imaging are similar to these, although with higher uncertainties, while the traditional method has the lowest precision. We attribute this result to the fact that the traditional method combines multiple measurements, e.g. triple phases, thus increasing the errors and reducing the amount of information that can be fitted. We also obtain a preliminary fit to the orbit of HR7955.

Close binary stars like spectroscopic binaries create a completely different environment than single stars for the evolution of a protoplanetary disk. Dynamical interactions between one star and protoplanets in such systems provide more challenges for theorists to model giant planet migration and formation of multiple planets. For habitable planets the majority of host stars are in binary star systems. So far only a small amount of Jupiter-size planets have been discovered in binary stars, whose minimum separations are 20 AU and the median value is about 1000 AU (because of difficulties in radial velocity measurements). The SIM Lite mission, a space-based astrometric observatory, has a unique capability to detect habitable planets in binary star systems. This work analyzed responses of the optical system to the field stop for companion stars and demonstrated that SIM Lite can observe exoplanets in visual binaries with small angular separations. In particular we investigated the issues for the search for terrestrial planets in P-type binary-planetary systems, where the planets move around both stars in a relatively distant orbit.

In this paper we describe a new technique of circular aperture apodization. It is produced by interferometry. The light, which is diffracted by a circular aperture, is split into two beams of different amplitudes where one of them has undergone an homothety to change its radial dimensions, by using an afocal optical system. The two beams are then combined coherently to form an apodized point spread function (PSF). This procedure allows us to reduce the diffraction wings of the PSF with different reduction factors depending on the combination parameters.

In the study of faint, extended sources at high resolving power, interferometry offers significant etendue advantages relative to conventional dispersive grating spectrometers. A Spatial Heterodyne Spectrometer (SHS) is a compact format two-beam interferometer that produces wavenumber dependent 2-D Fizeau fringe pattern from which an input spectrum can be obtained via a Fourier transform. The sampled bandpass of SHS is limited by the highest spatial frequency that can be sampled by the detector, which is typically less than 10 nm. This limitation has made these instruments useful primarily for studies of single emission line features or molecular bands. To date there have been few broadband implementations. We describe here continuing progress toward development of a broadband tunable SHS (TSHS) that is based on an all-reflective format where a single grating operates simultaneously as a beam-splitter, dispersive element, and beam combiner. The narrow spectral coverage of the TSHS is moved to different tuning wavenumbers by adjusting the angle of the pilot mirrors that guide the interfering beams through the optical path, thus slewing the acceptance band over a much broader spectral range. Our present effort involves a breadboard laboratory prototype of a secondgeneration TSHS in which we address several technical limitations of an earlier version. In particular the new design reduces wavefront distortions on the pilot mirrors, solves problems with magnification and focus of the fringe localization plane onto the detector, and addresses the variability in sensitivity and resolving power limitations of using a single grating over a large bandpass.

We report progress on the United States Naval Observatory, Navy Prototype Optical Interferometer, Astrometric Catalog (UNAC). This catalog uses observations from eight astrometric observation runs (Jan. 2005 - Nov. 2009) at the Navy Prototype Optical Interferometer (NPOI). The goal of the first release of the UNAC is to provide an astrometric catalog of at least 100 bright (V < 6) stars with precise positions accurate to < 16 milliarcseconds. In this paper we report on some of the data processing methods used to obtain absolute astrometric positions from optical interferometer data. We also discuss plans for assessing the accuracy of our interferometrically derived absolute astrometric positions.

We present the angular diameters, physical radii, and effective temperatures of five stars observed using both the Palomar Testbed Interferometer (PTI) and the Navy Prototype Optical Interferometer (NPOI). These preliminary results are part of a larger project focused on measuring the angular diameters of the 62 stars that have been observed with both instruments. We plan to achieve diameter accuracies of 0.5% or better through the combination of infrared- and visible-wavelength data from PTI and the NPOI, respectively. The difference in limb-darkening effects between the two wavelength regimes, together with other external parameters, will allow us to test the atmospheric models on which the limb-darkening corrections are based. High quality angular diameters for these stars will also lead to more accurate physical diameter measurements and effective temperature determinations.

The LINC-NIRVANA wavefront sensors are in their AIT phase. The first Ground-layerWavefront Sensor (GWS) is shaping in the Adaptive Optics laboratory of the Astronomical Observatory of Padova, while both the Mid- High Wavefront Sensors (MHWSs) have been aligned and tested as stand-alone units in the Observatory of Bologna (MHWS#1 aligned to LINC-NIRVANA post focal relay optics). LINC-NIRVANA is a Fizeau infrared interferometer equipped with advanced, MultiConjugated Adaptive Optics (MCAO) for the Large Binocular Telescope. The aim of the instrument is to allow true interferometric imagery over a 10" square Field of View (FoV), getting the sensitivity of a 12m telescope and the spatial resolution of a 22.8m one. Thanks to the MCAO concept, LINC-NIRVANA will use up to 20 Natural Guide Stars (NGS) which are divided, according to Layer-Oriented Multiple Field of View technique, between the GWSs and the MHWSs. To find such a large number of references, the AO systems will use a wide FoV of 6' in diameter and the light coming from the references used by each WFS will optically sum on its CCD camera. The MHWSs will detect the deformations due to the high layers and will select up to 8 NGSs in the inner 2' FoV. The GWSs, instead, will reconstruct the deformations introduced by the lower atmosphere, which was found out to be the main source of seeing. Their peculiarity is the highest number of references (up to 12) ever used in a single instrument, selected in an annular 2'-6' FoV.

Ultra-stable Monolithic Michelson interferometer can be an ideal reference for highprecision applications such as RV measurement in planet searching and orbit study. The advantages include wide wavelength range, simple sinusoidal spectral format, and high optical efficiency. In this paper, we report that a monolithic Michelson interferometers has been in-house developed with minimized thermal sensitivity with compensation tuning. With a scanning white light interferometer, the thermal sensitivity is measured ~ 6x10^-7/°C at 550 nm and it decreases to zero near 1000 nm. We expect the wideband wavelength reference source to be stabilized better than 0.3 m/s for RV experiments

We present the laboratory demonstration of a very high-dynamic range imaging instrument FIRST (Fibered Imager foR Single Telescope). FIRST combines the techniques for aperture masking and a single-mode fiber interferometer to correct wavefront errors, which leads to a very high-dynamic range up to 106 around very near the central object (~ λ/D) at visible to near-infrared wavelengths. Our laboratory experiments successfully demonstrated that the original image can be reconstructed through a pupil remapping system. A first on-sky test will be performed at the Lick Observatory 3- m Shane telescope for operational tests in the summer of 2010.

We put forward an innovative scheme allowing multiple beam combination and closure-phase retrieval by means of a three dimensional array of coupled optical waveguides. Light propagation in an array of evanescently coupled waveguides is similar to conventional diffraction, however it is bound to a system with finite degrees of freedom. We have demonstrated that the latter feature allows relating uniquely the intensity pattern at the end of a 3x3 waveguide array to the amplitude and relative phases of three monochromatic fields coupled to suitable input waveguides. The method is scalable to arbitrary large arrays of telescopes and baselines.

We present a fixed delay interferometer to be installed in IR-ET (Infra-Red Exoplanets Tracker). We introduce the design, fabrication and testing processes. In particular, we present a new methodology of computing the fundamental limit of radial velocity (RV) measurement given by photon noise for DFDI (Dispersed Fixed Delay Interferometer) method as opposed to conventional echelle method. The new method is later used to determine the optical path difference (OPD) of the IR-ET interferometer. In addition, we introduce a novel method of monitoring the stability of the interferometer for IR-ET in broad-band using fourier-transform white-light scanning interferometry technique. The new method can be potentially expanded and applied to thermo-optic effect measurement if temperature control system is introduced into the experiment. The thermal response of the optical system is 3500 m/s/°C. We find that the RV calibration precision of 'Bracketing' method is 1.74 m/s without temperature control.

In the astrophysical context of the search for Earth-like extrasolar planets, an important research effort has been done for the realization of single-mode integrated optics devices for mid-infrared space-based interferometry. Preparatory projects like FKSI [3], where rejection of high order modes is required to a level better than 40dB, will need photonic devices that achieve modal filtering and beam combination in the mid-IR band. In this context, we present results on midinfrared planar integrated optic beam combiners characterized at LAOG using chalcogenide and silver halide materials. We show results on FTS measurements, allowing to determine the single mode spectral domain, as well as interference fringes obtained from Y-junctions realized on these materials.

LINC-NIRVANA is the IR Fizeau interferometric imager that will be installed within a couple of years on the Large Binocular Telescope (LBT) in Arizona. Here we present a particular sub-system, the so-called Patrol Camera (PC), which has been now completed, along with the results of the laboratory tests. It images (in the range 600-900 nm) the same 2 arcmin FoV seen by the Medium-High Wavefront Sensor (MHWS), adequately sampled to provide the MHWS star enlargers with the positions of the FoV stars with an accuracy of 0.1 arcsec. To this aim a diffraction-limited performance is not required, while a distortion free focal plane is needed to provide a suitable astrometric output. Two identical systems have been realized, one for each single arm, which corresponds to each single telescope. We give here the details concerning the optical and mechanical layout, as well as the CCD and the control system. The interfaces (mainly software procedures) with LINC-NIRVANA (L-N) are also presented.

The Sydney University Stellar Interferometer uses embedded processors to control each siderostat station as well as other major components of the instrument. The maintenance of the original controllers has become a significant issue and we set out to design a new system that would be inexpensive, suitable for the relatively harsh operating environment and simple to maintain. We have demonstrated that the new system works satisfactorily and we are currently replacing the existing controllers with new ones.

We report on final fabrication tests for the dielectric coatings for the Magdalena Ridge Observatory Interferometer (MROI) fringe tracking beam combiner. The broadband anti-reflection (1.1 μm to 2.4 μm) and beamsplitter (1.49 μm to 2.31 μm) coatings required have been designed with both optical and mechanical constraints in mind. Not only do these coatings have very low optical losses, but they induce minimal bending of their substrates, thereby giving very low fringe contrast reductions. Performance tests on the deposited coatings at our manufacturers (Optical Surface Technologies, Albuquerque, NM) demonstrate reflection losses of less than 0.5% over the full bandpass of the AR coating, and deviations from the desired 50:50 intensity ratio of the beamsplitter coating of no more than 2%. When combined with the measured wavefront perturbations these results imply that the total fringe visibility losses induced by imperfect coating quality will be no more than 2% for all outputs of the MROI fringe tracking beam combiner.

We report on the testing of the modulators within the MROI fringe tracking beam combiner. Modulation in the beam combiner will be performed via modulators introducing an optical path difference in increments of λ/4 into the beams. Knowledge of the path difference introduced needs to be accurate to within 1!. To achieve this accuracy, the modulators are characterized and the desired step waveform optimized through a Fourier analysis technique. Control is implemented in an FPGA embedded system and performance will be monitored by means of a slow loop Fourier algorithm. Details of the progress on characterization, optimization and future implementation are presented here.

While doing optical study in an instrument similar to the interferometers dedicated to the Very Large Telescope (VLT), we have to take care of the pupil and focus conjugations. Modules with artificial sources are designed to simulate the stellar beams, in terms of collimation and pupil location. They constitute alignment and calibration tools. In this paper, we present such a module in which the pupil mask is not located in a collimated beam thus introducing Fresnel diffraction. We study the instrumental contrast taking into account the spatial coherence of the source, and the pupil diffraction. The considered example is MATISSE, but this study can apply to any other instrument concerned with Fresnel diffraction.

Hybrid sol-gel technology was used for fabrication of prototypes of coaxial two, three and four telescopes beam combiners for astronomical applications. These devices were designed for the astronomical J-band and have been characterized using an optical source with emission centered at 1265 nm and with a spectral FWHM of 50 nm. Interferometric characterization of the two, three and four beam combiners, showed average contrasts respectively higher than 98%, 96% and 95%. Interferometric spectral analysis of the beam combiners revealed that the chromatic differential dispersion is the main contributor to the observed contrast decay in the latter cases. The laser direct writing technique was used for fabrication of a coaxial two beam combiner on sol-gel material; it showed a contrast of 95%. The measured high contrast fringes confirm that the procedures used lead to performant IO beam combiners. These results demonstrate the capabilities of the hybrid sol-gel technology for fast prototyping of complex chip designs for astronomical applications.

MAMMUT (Mirror vibrAtion Metrolology systeM for the Unit Telescope) is an ESO funded feasibility project for the development of a fiber interferometer prototype designed for optical path laser-metrology along the optical train of the Unit Telescopes (UT) of the Very Large Telescope Interferometer (VLTI). Fast mechanical vibrations originating in the VLTI cause fast variations of the optical path difference between two arms of the stellar interferometer, thus reducing the contrast of measured interference fringes. MAMMUT aims at monitoring in real time the optical path variations inside the Coudé train of the UT, for active control purposes. MAMMUT features a 250-meter-long optical fiber which can be used to deliver and inject a laser beam at 1353 nm into the UT. The injected beam can be dropped from the telescope in the Coudé room and interfered with a phase reference, provided by the second 250-meter-long arm of the fiber interferometer. The optical path variations are measured by means of an active homodyne scheme. Coherence between the beam at the injection point and the phase reference is provided by active fiber stabilization, made possible by the implementation of an internal metrology channel in MAMMUT. Here we present the initial laboratory performance results of the MAMMUT prototype, which will be able to sense optical path variations of +/- 5 μm with sub-10 nm precision within a bandwidth of at least 100 Hz.

The atmospheric piston simulator is an integral part of the calibration unit of LINC-NIRVANA, the Fizeau interferometric imager for the Large Binocular Telescope. The calibration unit will be necessary to align and set up the different opto - mechanical subsystems of the instrument. It will assist in (1) the alignment of the optics via reference fibers; (2) establishing zero optical path difference using a balanced fiber splitter; (3) flat fielding of the detectors with an integrating sphere; (4) correction of the non-common path aberrations using a fiber-based phase diversity source; and (5) calibration of the adaptive optics with a rotating reference fiber plate. Substantial testing and verification of the fringe tracker under as realistic as possible conditions in the lab is desirable, since the performance of the fringe tracker will ultimately determine the high angular resolution imaging capability of LINC-NIRVANA as a whole. We are therefore also constructing an atmospheric piston simulator working in the J and H photometric bands. As with many of the other calibration unit sub-systems, our design concept is mainly fiber based. Opto - electronic phase modulators will be used to introduce the piston sequences. The control system of the piston modulators will allow for easy implementation of different vibration power spectra. This will enable us to test and demonstrate the capabilities of the fringe tracker under realistic conditions.

LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). In close cooperation with the Adaptive Optics systems of LINC-NIRVANA the Fringe and Flexure Tracking System (FFTS) is a fundamental component to ensure a complete and time-stable wavefront correction at the position of the science detector in order to allow for long integration times at interferometric angular resolutions. In this contribution, we present the design and the realization of the ongoing FFTS laboratory tests, taking into account the system requirements. We have to sample the large Field of View and to follow the reference source during science observations to an accuracy of less than 2 microns. In particular, important tests such as cooling tests of cryogenic components and tip - tilt test (the repeatability and the precision under the different inclinations) are presented. The system parameters such as internal flexure and precision are discussed.

Mozurkewich presented a beam combiner design suitable for use with a large number of apertures. Such a system has applications both in space and as a camera for single-aperture, ground-based telescopes. We implemented a version of this design which accommodates ~ 90 apertures and resolves the fringes into more than 100 spectral channels. The resulting system, which has been tested in the laboratory, provides over 50,000 simultaneous, independent fringe measurements. Implementation details and test results are presented.

We report on the mechanical design currently performed at the Magdalena Ridge Observatory Interferometer (MROI) and how the construction, assembly, integration and verification are planned towards commissioning. Novel features were added to the mechanical design, and high level of automation and reliability are being devised, which allows the number of reflections to be kept down to a minimum possible. This includes unit telescope and associated enclosure and transporter, fast tip-tilt system, beam relay system, delay line system, beam compressor, automated alignment system, beam turning mirror, switchyard, fringe tracker and vacuum system.

We present the procedure used to optically align the CHARA telescopes. We show that the beam quality, delivered by the CHARA telescopes E1, E2 and W2, is significantly better now than in 2008. RMS wavefront error is about 200 nm. The astigmatism observed in W1 is more likely due to a combination of a mechanical problem in the mounting and misalignment. We present wavefront quality results from four telescopes. Further beam quality improvements can be expected when the second part of the alignment procedure (tuning) will be carried out later this year.

The MROI fringe tracking beam combiner will be the first fringe instrument for the interferometer. It was designed to utilize the array geometry and maximize sensitivity to drive the interferometer for faint source imaging. Two primary concerns have driven the design philosophy: 1) maintaining high throughput and visibilities in broadband polarized light, and 2) mechanical stability. The first concern was addressed through tight fabrication tolerances of the combiner substrates, and custom coatings. In order to optimize mechanical stability, a unique modular design approach was taken that minimizes the number of internal adjustments. This paper reports initial laboratory fringe and stability measurements.

Fringe-tracking is a critical issue for modern astronomical ground interferometry. The incoming generation of fringe trackers is intended to simultaneously co-phase a large number of telescopes, bringing new questions related to control and redundancy. In this paper, a new control architecture is proposed for the 4/6-telescope Second Generation Fringe Tracker. Such a system is currently under study for the Very Large Telescope Interferometer (VLTI). The main features of the proposed solution lie in the explicit handling of many aspects that is rarely taken into account altogether. These are: delay in the transmission of set-points, coupling between the different delay lines and redundancy in the measurement. The proposed solution is based on Linear Quadratic Regulator design together with an extended dynamic observer that recovers the first moments of the disturbance dynamics (atmospheric pistons). The resulting multi-variable solution enables in particular to tune the different baselines control-related weighting coefficients according to the quality of the related measurements. While state-space framework underlines the proposed control design methodology, the resulting controller can still be expressed in a transfer function form to fit classical implementation scheme generally adopted in the targeted application community. The efficiency of the proposed solution is illustrated through dedicated simulations involving realistic data.

LINC-NIRVANA (LN) is a German/Italian interferometric beam combiner camera for the Large Binocular Telescope. Due to homothetic imaging, LN will make use of an exceptionally large field-of-view. As part of LN, the Fringe-and-Flexure-Tracker system (FFTS) will provide real-time, closed-loop measurement and correction of pistonic and flexure signals induced by the atmosphere and inside the telescope-instrument system. Such compensation is essential for achieving coherent light combination over substantial time intervals (~ 10min.). The FFTS is composed of a dedicated near-infrared detector, which can be positioned by three linear stages within the curved focal plane of LN. The system is divided into a cryogenic (detector) and ambient (linear stages) temperature environment, which are isolated from each other by a moving baffle. We give an overview of the current design and implementation stage of the FFTS opto-mechanical and electronic components. We present recent important updates of the system, including the development of separated channels for the tracking of piston and flexure. Furthermore, the inclusion of dispersive elements will allow for the correction of atmospheric differential refraction, as well as the induction of artificial dispersion to better exploit the observational-conditions parameter space (air mass, brightness).

The delay lines for the Magdalena Ridge Observatory Interferometer in New Mexico are required to provide up to 380m optical path delay with an OPD jitter of better than 15nm, in vacuum, using a single adjustable stroke. In order to meet these demanding requirements in a cost-effective manner a unique combination of techniques has been used in the design and construction of the delay line trolley which operates continuously within 190m of evacuated pipe. These features include contactless delivery of power and control signals, active control of the cat's eye optics and the use of composite materials to achieve good thermal stability. A full-size prototype trolley has been built and fully tested and the first production trolley is under construction. We describe the system's key design features and review the construction and alignment of the delay line trolley. Results obtained with the trolley operating in an evacuated 20m-long test rig under the full range of conditions required for successful astronomical observations are presented. An OPD jitter of typically 10nm is achieved over the total tracking velocity range from 0 to 15mm/s.

Measurements from long-baseline interferometry are commonly analysed in terms of the power spectrum and the bispectrum (or triple-product) of the fringe patterns, as these estimators are invariant in the presence of phase instabilities. At low light levels, photon and detector noise give rise to systematic "bias" in the power spectrum and bispectrum. This paper extends previous work on computing the expected biases and variances for these quantities by introducing a general method which can be applied to any fringe-encoding scheme where the measurement equation is linear and to measurements affected by a combination of photon noise and detector noise. We apply our method to a number of interesting practical cases, including systems with unevenly-sampled fringe patterns and in the presence of read noise.

PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) is an ESO/VLTI instrument designed for phase-referenced imaging and narrow-angle astrometry, dedicated to exoplanet detection. The astrometric datareduction software (ADRS) is a key component of the system, calculating very precise (~ 10 μas) differential angular separations projected on the sky. For an interferometer with a baseline of 100 m, this separation corresponds to measuring the (differential) optical path difference with a precision of 5 - 15 nanometers. This precision can only be achieved with careful calibration of the instrument, including effects that are irrelevant for almost any other scientific application. PRIMA is currently being commissioned on Paranal, and we expect to obtain the first astrometric data in September 2010. These data will provide a new insight into the operation and calibration of the instrument.

We present the third release of the AMBER data reduction software by the JMMC. This software is based on core algorithms optimized after several years of operation. An optional graphic interface in a high level language allows the user to control the process step by step or in a completely automatic manner. Ongoing improvement is the implementation of a robust calibration scheme, making use of the full calibration sets available during the night. The output products are standard OI-FITS files, which can be used directly in high level software like model fitting or image reconstruction tools. The software performances are illustrated on a full data set of calibrators observed with AMBER during 5 years taken in various instrumental setup.

The advent of GPU hardware and associated software libraries for scientific computing renders possible acceleration of parallelisable problems by a typical factor of 10-100. We present the first GPU-accelerated and open source image reconstruction software for optical/infrared interferometry, making use of the OpenCL library. Finally we evaluate how this improvement in speed may translate in terms of improvement in image reconstruction quality for currently computationnally intensive algorithms.

The JMMC1 Calibrator Workgroup has long developed methods to ascertain the angular diameter of stars, and provides this expertise in the SearchCal2 software. SearchCal dynamically finds calibrators near science objects by querying CDS3 hosted catalogs according to observational parameters. Initially limited to bright objects (K magnitude ≤ 5.5), it has been upgraded with a new method providing calibrators without any magnitude limit but those of queried catalogs. We introduce here a new static catalog of stellar diameters, containing more than 38000 entries, obtained from SearchCal results aggregation on the whole celestial sphere, complete for all stars with HIPPARCOS4 parallaxes. We detail the methods and tools used to produce and study this catalog, and compare the static catalog approach with the dynamical querying provided by SearchCal engine. We also introduce a new Virtual Observatory service, enabling the reporting of, and querying about, stars flagged as "bad calibrators" by astronomers, adding this ever-growing database to our SearchCal service.

In the course of fulfilling its mandate, the Spectral Calibration Development Unit (SCDU) testbed for SIM-Lite produces copious amounts of raw data. To effectively spend time attempting to understand the science driving the data, the team devised computerized automations to limit the time spent bringing the testbed to a healthy state and commanding it, and instead focus on analyzing the processed results. We developed a multi-layered scripting language that emphasized the scientific experiments we conducted, which drastically shortened our experiment scripts, improved their readability, and all-but-eliminated testbed operator errors. In addition to scientific experiment functions, we also developed a set of automated alignments that bring the testbed up to a well-aligned state with little more than the push of a button. These scripts were written in the scripting language, and in Matlab via an interface library, allowing all members of the team to augment the existing scripting language with complex analysis scripts. To keep track of these results, we created an easilyparseable state log in which we logged both the state of the testbed and relevant metadata. Finally, we designed a distributed processing system that allowed us to farm lengthy analyses to a collection of client computers which reported their results in a central log. Since these logs were parseable, we wrote query scripts that gave us an effortless way to compare results collected under different conditions. This paper serves as a case-study, detailing the motivating requirements for the decisions we made and explaining the implementation process.

This article is focused on the two-level control system of ODL, which are divided into bottom layer control of linear motor and upper layer control of Piezoelectric Transducer(PZT).This ODL are designed to compensate geometrical optical path difference, which results from the earth rotation, and other disturbances, with high-accuracy and real time. Based on the PLC of PMAC controller, the linear motor tracks the trajectory of the simulated optical path difference to compensate roughly. PZT then compensates the rest error measured by ZLM almost real time. A detailed fulfillment of this method is shown in the article, and the first result data is produced. The result implies that this method is efficient. This article offers the reference for the ODL development with the practical high accuracy of compensation.

The SIM Lite Astrometric Observatory is to perform narrow angle astrometry to search for Earth-like planets, and global astrometry for a broad astrophysics program, for example, mapping the distribution of dark matter in the Galaxy. The new SIM Lite consists of two Michelson interferometers and one star tracking telescope. The main six-meter baseline science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction of a target star. The Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two orthogonal directions. A testbed has been built to demonstrate star-tracking capability of the G2T concept using a new interferometric angle metrology system. In the presence of simulated 0.2 arcsecond level of expected spacecraft attitude control system perturbations, the measured star-tracking capability of the G2T testbed system is less than 43 micro-arcsecond during single narrow angle observation.

The SIM-Lite astrometric interferometer will search for Earth-size planets in the habitable zones of nearby stars. In this search the interferometer will monitor the astrometric position of candidate stars relative to nearby reference stars over the course of a 5 year mission. The elemental measurement is the angle between a target star and a reference star. This is a two-step process, in which the interferometer will each time need to use its controllable optics to align the starlight in the two arms with each other and with the metrology beams. The sensor for this alignment is an angle tracking CCD camera. Various constraints in the design of the camera subject it to systematic alignment errors when observing a star of one spectrum compared with a start of a different spectrum. This effect is called a Color Dependent Centroid Shift (CDCS) and has been studied extensively with SIM-Lite's SCDU testbed. Here we describe results from the simulation and testing of this error in the SCDU testbed, as well as effective ways that it can be reduced to acceptable levels.

The most stringent astrometric performance requirements on NASA's SIM(Space Interferometer Mission)-Lite mission will come from the so-called Narrow-Angle (NA) observing scenario, aimed at finding Earth-like exoplanets, where the interferometer chops between the target star and several nearby reference stars multiple times over the course of a single visit. Previously, about 20 pm NA error with various shifts was reported1. Since then, investigation has been under way to understand the mechanisms that give rise to these shifts. In this paper we report our findings, the adopted mitigation strategies, and the resulting testbed performance.

Main brassboard Michelson interferometer components have been recently developed for the future flight phase implementations of SIM Lite mission. These brassboard components include two fine steering mirrors, pathlength modulation and cyclic averaging optics and astrometric beam combiner assembly. Field-independent performance tests will be performed in a vacuum chamber using two siderostats in retro-reflecting positions and a white light stimulus. The brightness and color dependence of the angle and fringe tracking performance will be measured. The performance of filtering algorithms will be tested in a simulated spacecraft attitude control system perturbation. To demonstrate capability of a dim star observation, the angle and fringe tracking CCD sensors are cooled to -110 C using a cold diode heat pipe system. The new feed-forward control (angle and path-length) algorithms for the dim star observation will be tested as well. In this paper, we will report the recent progress toward the integration and performance tests of the brassboard interferometer.