This PDF file contains the front matter associated with SPIE Proceedings Volume 8445, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The exquisite precision delivered by interferometric techniques is rapidly being applied to more and more branches of optical astronomy. One particularly successful strategy to obtain structures at the scale of the diffraction limit is Aperture Masking Interferometry, which is presently experience a golden age with implementations at a host of large telescopes around the world. This startlingly durable technique, which turns 144 years old this year, presently sets the standard for the recovery of faint companions within a few resolution elements from the core of a stellar point spread function. This invited review will give a historical introduction and overview of the modern status of the technique, the science being delivered, and prospects for new advances and applications.

Aperture-masking interferometry allows diffraction-limited images to be recovered despite the turbulent atmo­ sphere. Here, this approach has been combined with polarimetry to form a novel technique allowing the dusty environments of mass-losing stars (so-called AGB stars) and proto-planetary and debris disks to be imaged, the characterisation of which is key to understanding the recycling of matter and the formation of new planetary systems. Polarimetric aperture-masking interferometry produces images by exploiting the fact that starlight scattered by circumstellar dust becomes strongly polarised. Essentially, aperture-masking allows access to the small spatial scales (rv1Omas) necessary while polarimetry allows light from the dust and star to be differentiated. Furthermore, measurements at multiple wavelengths allow dust grain sizes to be calculated using Mie scattering theory. Excellent results have already been obtained at near-IR wavelengths using the NACO instrument at the VLT. The next step is to leverage the higher spatial resolution and polarisation signal found in the visible, rather than near-IR. To this end, a new instrument allowing precision polarimetric aperture masking interferometry at 600-800nm is being developed for an 8m class telescope, details of which will also be presented.

Kernel-phase is a recently developed paradigm to tackle the classical problem of image deconvolution, based on an interferometric point of view of image formation. Kernel-phase inherits and borrows from the notion of closure-phase, especially as it is used in the context of non-redundant Fizeau interferometry, but extends its application to pupils of arbitrary shape, for diffraction limited images. It can therefore readily be (and is being) used to process existing archival data acquired by space borne telescopes (HST/NICMOS) as well as well corrected AO data from ground based telescopes. The additional calibration brought by kernel-phase boosts the resolution of conventional images and enables the detection of otherwise hidden faint features at the resolution limit and beyond, a regime often refered to as super-resolution. Kernel-phase analysis of archival data leads to new discoveries and/or improved relative astrometry and photometry. The paper also presents how the technique may influence the geometry of new interferometric arrays designed for imaging, dusting off a topic that has known little evolution for the past 40 years; and presents hints of a fast solution to the calibration of non-common path errors in AO systems, using direct focal plane based wavefront sensing.

High contrast imaging techniques such as aperture masking interferometry allow for the detection of faint companions such as substellar companions by utilizing light from the planet itself. This allows access to study a larger population of planetary companions as compared to the transit technique where only systems viewed edge on can be studied, for example. However, aperture masking has several shortcomings including, low throughputs, limited Fourier coverage, and leakage of residual atmospheric noise due to phase corrugations across each sub-apertures. These limitations can be overcome by remapping the pupil with single-mode waveguides. We present an integrated pupil remapping interferometer, known as Dragonfly, that aims to do just that. We discuss the progress we have made over the past year in developing a stable and robust instrument and elucidate challenges and the innovative solutions that were applied. Finally we discuss improvements to the instrument that will enable future scientific endeavors and outline the expected performance limitations.

Extrasolar planets are directly detected most easily when they are young and can have contrasts only a few hundred times fainter than their host stars at near- and mid- infrared wavelengths. However, planets and other solar-system scale structures around solar-type stars in the nearest star forming regions require the full diffraction limit of the world's largest telescopes, and can not be detected with conventional AO imaging techniques. I will describe the recent successes of long-baseline interferometry in detecting planetary-mass companions, focusing on the transitional disk system LkCa 15. I will outline why aperture-masking has been so successful in its resolution and sensitivity niche, and will outline the algorithms needed to calibrate the primary observable of closure/kernel phase to the level needed for extrasolar planet detection.

We report here on some of the major astronomical observations obtained by the Keck Interferometer Nuller (KIN), the high dynamic range instrument recombining the Keck Telescopes at wavelengths of 8 to 13 microns. A few science targets were observed during the commissioning phase (2004-2007). These early observations aimed at demonstrating the KIN’s ability to spatially resolve and characterize circumstellar dust emission around a variety of targets, ranging from evolved stars to young debris disks. Science operations started then in 2008 with the more demanding KIN exozodi key science programs, augmented by observations of YSOs and hot debris disks between 2009 and 2011. The last KIN observations were gathered in 2011B, and the interpretation of some of the results depicted here is still preliminary (exo-zodi survey) or pending (complicated behavior observed in YSOs). We discuss in particular the initial results of the KIN’s exo-zodi observations, which targeted a total of 40 nearby main sequence single stars. We look for trends in this sample, searching for possible correlations between the measured KIN excesses and basic stellar properties such as spectral type or the presence of dust inferred from separate observations.

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer designed to fly on a high altitude balloon. BETTII uses a double-Fourier Michelson interferometer to simultaneously obtain spatial and spectral information on science targets; the long baseline permits subarcsecond angular resolution, a capability unmatched by other far-infrared facilities. Here, we present key aspects of the overall design of the mission and provide an overview of the current status of the project. We also discuss briefly the implications of this experiment for future space-based far-infrared interferometers.

Interferometry in space has marked advantages: long integration times and observation in spectral bands where the atmosphere is opaque. When installed on separate spacecraft, it also has extended and flexible baselines for better filling of the uv plane. Intensity interferometry has an additional advantage, being insensitive to telescope and path errors, but is unfortunately much less light-sensitive. In planning towards such a mission, we are experimenting with some fundamental research issues. Towards this end, we constructed a system of three vehicles floating on an air table in formation flight, with an autonomous orbit control. Each such device holds its own light collector, detector, and transmitter, to broadcast its intensity signal towards a central receiving station. At this station we implement parallel radio receivers, analogue to digital converters, and a digital three-way correlator. Current technology limits us to ~1GHz transmission frequency, which corresponds to a comfortable 0.3m accuracy in light-bucket shape and in its relative position. Naïve calculations place our limiting magnitude at ~7 in the blue and ultraviolet, where amplitude interferometers are limited. The correlation signal rides on top of this huge signal with its own Poisson noise, requiring a very large dynamic range, which needs to be transmitted in full. We are looking at open questions such as deployable optical collectors and radio antennae of similar size of a few meters, and how they might influence our data transmission and thus set our flux limit.

Interferometry is an affordable way to bring the benefits of high resolution to space far-IR astrophysics. We summarize an ongoing effort to develop and learn the practical limitations of an interferometric technique that will enable the acquisition of high-resolution far-IR integral field spectroscopic data with a single instrument in a future space-based interferometer. This technique was central to the Space Infrared Interferometric Telescope (SPIRIT) and Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) space mission design concepts, and it will first be used on the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). Our experimental approach combines data from a laboratory optical interferometer (the Wide-field Imaging Interferometry Testbed, WIIT), computational optical system modeling, and spatio-spectral synthesis algorithm development. We summarize recent experimental results and future plans.

The Wide-Field Imaging Interferometry Testbed (WIIT) is a wide-field spectral imaging Michelson interferometer developed at the NASA/Goddard Space Flight Center. WIIT is operational and effectively demonstrates imaging and spectroscopy over fields-of-view larger than the narrow primary beam footprint of a conventional Michelson interferometer. At the heart of this technique is the "double-Fourier" approach whereby the apertures and a delay line are both moved to collect interferograms over a 2D wide field detector grid simultaneously; one interferogram per detector pixel. This aggregate set of interferograms, as a function of baseline and delay line, is algorithmically processed to construct a single spatial-spectral cube with angular resolution approaching the ratio of the wavelength to the longest baseline. Herein we develop the mathematical spatial-spectral imaging model and the baseline processing algorithm and show results using simulated data and using WIIT testbed data.

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 modes. The instrument improvements and status of the Keck Interferometer since the 2010 SPIE meeting are summarized. We discuss the current capabilities of the KI, operational improvements, and the science from the KI during the past two years. We will conclude with a brief note on the closure of the KI facility. Details of dual field phase referencing developments and nulling science results are presented elsewhere at this conference.

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 and increase sensitivity. New configurations of the ATs have been offered and are frequently discussed with the science users of the VLTI and implemented to optimize the scientific return. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the instruments is being commissioned. The visitor instrument PIONIER is now fully operational and bringing imaging capability to the VLTI. The current status of the VLTI is described with successes and scientific results, and prospects on future evolution are presented.

Ground-baseed long baselinne interferomeeters have lonng been limiteed in sensitiviity by the shoort integration periods imposed by atmospheric tuurbulence. Thee first observaation fainter thhan this limit wwas performedd on January 222, 2011 when the Keck Interferommeter observedd a K=11.5 taarget, about onne magnitude fainter than iits K=10.3 limmit. This observation wwas made posssible by the Duual Field Phase Referencing instrument of the ASTRA pproject: simultaaneously measuring thhe real-time efffects of the atmmosphere on a nearby bright guide star, andd correcting foor it on the fainnt target, integration tiime longer thaan the turbulennce time scale are made possible. As a preelude to this ddemonstration, we first present the implementatioon of Dual FField Phase RReferencing onn the interferoometer. We tthen detail itss on-sky performance focusing on tthe accuracy oof the turbulennce correction, and on the reesulting fringe contrast stabiility. We conclude witth a presentatioon of early resuults obtained wwith Laser Guidde Star AO andd the interferommeter.

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. Since January 2011 the integration and individual testing of the different subsystem has come to a necessary minimum. At the same time the astrometric commissioning phase has begun. In this contribution we give an update on the status of the facility and present some highlights and difficulties on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and operational aspects. In particular, within the context of the latter we are going to present a modified mode of operation that scans across the fringes. We will show that this mode, originally only intended for calibration purposes, facilitates the detection of dual-fringes.

Rotation plays a crucial role in the shaping and evolution of a star. Widely incorporated into early and late-stage stellar models, rotational effects remain poorly understood in main-sequence stars, mainly due to the absence of observations challenging contemporary models. The Precision Astronomical Visible Observations (PAVO) instrument, located at the Center for High Angular Resolution Astronomy (CHARA) array, provides the highest angular resolution yet achieved (0.3 mas) for stars V=8 magnitude and brighter. We describe instrumental techniques and advances implemented in PAVO@CHARA to observe heavily resolved targets and yield well calibrated closure phases which are key milestones on the pathway to delivery of the first-ever image in the visible of fast-rotating main-sequence star.

The CHARA Array is a six-telescope optical/IR interferometer managed by the Center for High Angular Resolution Astronomy of Georgia State University and located at Mount Wilson Observatory in the San Gabriel Mountains overlooking Pasadena, California. The CHARA Array has the longest operational baselines in the world and has been in regular use for scientific observations since 2005. 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. We also present an overview of science results obtained over the last few years, including some recent imaging results.

The visitor instrument PIONIER provides VLTI with improved imaging capabilities and sensitivity. The in- strument started routinely delivering scientic data in November 2010, that is less than 12 months after being approved by the ESO Science and Technical Committee. We recall the challenges that had to be tackled to design, built and commission PIONIER. We summarize the typical performances and some astrophysical results obtained so far. We conclude this paper by summarizing lessons learned.

In the framework of the understanding of extrasolar systems, the study of host stars is a fundamental point. We need to understand the link between them and the presence of companions, i.e. what makes a star becoming a host star. In this perspective, we used the instrument called VEGA, situated at Mount Wilson (California) on the CHARA array to perform optical interferometric measurements. Interferometry at visible wavelengths allows reaching very high spatial frequencies well adapted for very small (less than 1 millisecond of arc) angular diameters. Therefore, we can access limb darkening measurements which is one of the very few directly measurable constraints on the structure of the atmosphere of a star. From this we can derive stars fundamental parameters. A precise measurement within spectral lines is also a very powerful tool to study the temperature and density structure of the atmosphere of distant stars. Besides, the detection of exoplanets is also related to this method. Combined with the radial velocity method and the transit method, one can study the atmosphere of exoplanets and learn more about their internal structure. We started a large program of observations made of 40 stars hosting exoplanets and observable by VEGA/CHARA. We will measure their limb darkened diameters and derive their parameters. We also aim at better understanding stellar noise sources like spots, and study surface brightness relationships.

In this paper, we review the current performance of the VEGA/CHARA visible spectrograph and make a review of the most recent astrophysical results. The science programs take benefit of the exceptional angular resolution, the unique spectral resolution and one of the main features of CHARA: Infrared and Visible parallel operation. We also discuss recent developments concerning the tools for the preparation of observations and important features of the data reduction software. A short discussion of the future developments will complete the presentation, directed towards new detectors and possible new beam combination scheme for improved sensitivity and imaging capabilities.

Instrumentation developments at the Navy Precision Optical Interferometer (NPOI, formerly “Navy Optical Interferometer,” and “Navy Prototype Optical Interferometer”) since the last SPIE meeting in 2010 are summarized. The commissioning of new siderostat stations, progress towards the installation of four 1.8m telescopes, and other instrumentation and control systems upgrades, are reviewed.

VISION is the next generation science camera for the Navy Optical Interferometer (NOI). In comparison to the current beam combiner of NOI, VISION will deliver higher precision data products and better exibility by incorporating single mode bers for spatial ltering and by using low-noise detectors. VISION can coherently combine up to six telescope beams using an image-plane combination scheme. This results in simultaneous measurement of 15 visibility amplitudes and 10 independent closure phases that can be used to reconstruct multipixel images of stars.

This paper presents an overview of recent progress at the Sydney University Stellar Interferometer (SUSI). Development of the third-generation PAVO beam combiner has continued. The MUSCA beam combiner for high-precision differential astrometry using visible light phase referencing is under active development and will be the subject of a separate paper. Because SUSI was one of the pioneering interferometric instruments, some of its original systems are old and have become difficult to maintain. We are undertaking a campaign of modernization of systems: (1) an upgrade of the Optical Path Length Compensator IR laser metrology counter electronics from a custom system which uses an obsolete single-board computer to a modern one based on an FPGA interfaced to a Linux computer - in addition to improving maintainability, this upgrade should allow smoother motion and higher carriage speeds; (2) the replacement of the aged single-board computer local controllers for the siderostats and the longitudinal dispersion compensator has been completed; (3) the large beam reducing telescope has been replaced with a pair of smaller units with separate accessible foci. Examples of scientific results are also included.

In the course of our VLTI young stellar object PIONIER imaging program, we have identified a strong visibility chromatic dependency that appeared in certain sources. This effect, rising value of visibilities with decreasing wavelengths over one base, is also present in previous published and archival AMBER data. For Herbig AeBe stars, the H band is generally located at the transition between the star and the disk predominance in flux for Herbig AeBe stars. We believe that this phenomenon is responsible for the visibility rise effect. We present a method to correct the visibilities from this effect in order to allow "gray" image reconstruction software, like Mira, to be used. In parallel we probe the interest of carrying an image reconstruction in each spectral channel and then combine them to obtain the final broadband one. As an illustration we apply these imaging methods to MWC158, a (possibly Herbig) B[e] star intensively observed with PIONIER. Finally, we compare our result with a parametric model fitted onto the data.

The Magdalena Ridge Observatory Interferometer has been designed to be a 10 x 1.4 m aperture long-baseline optical/near-infrared interferometer in an equilateral "Y" configuration, and is being deployed west of Socorro, NM on the Magdalena Ridge. Unfortunately, first light for the facility has been delayed due to the current difficult funding regime, but during the past two years we have made substantial progress on many of the key subsystems for the array. The design of all these subsystems is largely complete, and laboratory assembly and testing, and the installation of many of its components on the Ridge are now underway. This paper serves as an overview and update on the facility's present status, and the plans for future funding and eventual operations of the facilities.

We report recent success to stabilize the VLTI/MIDI-10um beam-combiner with K-band fringe tracking, provided by PRIMA (FSU). After encouraging commissioning results, presented during the last SPIE, we accomplished in November 2011 a successful run of science demonstration. A broad range of targets were observed, from planet-hosting stars over YSO disks to AGN. We will present first results, focussing on the improvements on sensitivity and precision with respect to classical MIDI stand-alone observations. MIDI broad-band dispersion and group-delay can be reliably predicted by the fringe tracker to enable coherent integration beyond coherence time at the lambda/10 precision level.

MATISSE is a mid-infrared spectro-interferometer combining the beams of up to four Unit Telescopes or Auxiliary Telescopes of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. New characteristics present in MATISSE will give access to the mapping and the distribution of the material, the gas and essentially the dust, in the circumstellar environments by using the mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient uv-coverage: 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime. We give an overview of the instrument including the expected performances and a view of the Science Case. We present how the instrument would be operated. The project involves the collaborations of several agencies and institutes: the Observatoire de la Côte d’Azur of Nice and the INSU-CNRS in Paris, the Max Planck Institut für Astronomie of Heidelberg; the University of Leiden and the NOVA-ASTRON Institute of Dwingeloo, the Max Planck Institut für Radioastronomie of Bonn, the Institut für Theoretische Physik und Astrophysik of Kiel, the Vienna University and the Konkoly Observatory.

The Extrasolar Planet Search with PRIMA project (ESPRI) aims at characterising and detecting extrasolar planets by measuring the host star's reflex motion using the narrow-angle astrometry capability of the PRIMA facility at the Very Large Telescope Interferometer. A first functional demonstration of the astrometric mode was achieved in early 2011. This marked the start of the astrometric commissioning phase with the purpose of characterising the instrument's performance, which ultimately has to be sufficient for exoplanet detection. We show results obtained from the observation of bright visual binary stars, which serve as test objects to determine the instrument's astrometric precision, its accuracy, and the plate scale. Finally, we report on the current status of the ESPRI project, in view of starting its scientific programme.

We report the first phased images using adaptive optics correction from the Large Binocular Telescope Interferometer. LBTI achieved first fringes in late 2010, with seeing-limited operation. Initial tests verified the feasibility of the setup and allowed us to characterize the phase variations from both the atmosphere and mechanical vibrations. Integration of the secondary-base AO systems was carried out in spring 2011 and spring 2012 for the right and left side respectively. Single aperture, diffraction-limited, operation has been commissioned and is used as a productive mode of the LBTI with the LMIRCam subsystem. We describe the initial observation for dual aperture observations and coherent imaging results.

We present an update on the LINC-NIRVANA (LN) instrument, an innovative Fizeau-mode beam combiner for the Large Binocular Telescope (LBT). LN will deliver 10 mas spatial resolution in the near infrared over a 10 arcsec field of view. In addition to optical-path-difference control, the instrument must correct a wide field of view on the sky using multi-conjugated adaptive optics. This substantially increases sky coverage for fringe tracking reference stars. Subsystem delivery and testing is almost complete, and final Assembly, Integration, and Verification are well advanced. We report on closed-loop control of a number of subsystems, including fine-tuning and optimization of the delay line. Measurement and remediation of instrument flexure are key to the success of LN. Several laboratory performance experiments demonstrate that components are within specification. With several interacting subsystems, LN faces a complexity challenge. A Pathfinder experiment at LBT will verify multiple aspects of LINC-NIRVANA and the telescope starting in winter 2012-2013. Finally, we report on efforts to prepare for early science exploitation in "LINC" mode, which uses single-conjugate adaptive optics.

Unveiling the structure of the Broad Line Region (BLR) of AGNs is critical to understand the quasar phenomenon. Resolving a few BLRs by optical interferometry will bring decisive information to confront, complement and calibrate the reverberation mapping technique, seed of the mass-luminosity relation in quasars. BLRs are much smaller than the angular resolution of the VLT and Keck interferometers and they can be resolved only by differential interferometry very accurate measurements of differential visibility and phase. The latest yields the photocenter variation with λ, and constrains the size, position and velocity law of various regions of the BLR. AGNs are below the magnitude limit for spectrally resolved interferometry set by currently available fringe trackers. A new “blind” observation method and a data processing based on the accumulation of 2D Fourier power and cross spectra permitted us the first spectrally resolved interferometric observation of a BLR, on the K=10 quasar 3C273. A careful bias analysis is still in progress, but we report strong evidence that, as the baseline increases, the differential visibility decreases in the Pa_α line. Combined with a differential phase certainly smaller than 3°, this yields an angular radius of the BLR larger than 0.4 milliarcseconds, or 1000 light days at the distance of 3C273, much larger than the reverberation mapping radius of 300 light days. Explaining the coexistence of these two different scales, and possibly structures and mechanisms, implies very new insights about the BLR of 3C273.

Since our first detection of a resolved near-infrared emission around the main sequence star Vega, which we identified as the signature of hot dust grains close to the sublimation limit, we have been systematically searching for similar signatures around a magnitude-limited sample of nearby main sequence stars with the FLUOR instrument at the CHARA array. About 40 targets with spectral types ranging from A to K have been observed within the last 6 years, leading to first statistical trends on the occurence of the bright exozodi phenomenon as a function of spectral type. Our target sample is balanced between stars known to harbour cold dust populations from space-based missions (e.g., Spitzer, Herschel) and stars without cold dust, so that the occurence of abundant hot dust can also be correlated with the presence of large reservoirs of cold planetesimals. In this paper, we present preliminary conclusions from the CHARA/FLUOR survey. We also discuss the first results obtained in 2011/2012 with the new PIONIER visiting instrument at the VLTI, which is now used to extend our survey sample to the Southern hemisphere and to fainter targets. A first measurement of the exozodi/star flux ratio as a function of wavelength within the H band is presented, thanks to the low spectral resolution capability of PIONIER. Finally, we also briefly discuss our plans for extending the survey to fainter targets in the Northern hemisphere with an upgraded version of the FLUOR beam combiner.

The CHARA Array possesses the longest baselines in the world for infrared and visible interferometry, while the Michigan Infrared Combiner (MIRC) is the most advanced beam combiner for imaging. CHARA+MIRC has allowed imaging the surfaces of rapid rotators, interacting binary stars, and magnetically-active stars all for the first time. In this presentation, I will give an overview of the discoveries made by MIRC over the past five years and discuss technical and scientific lessons learned.

Michigan InfraRed Combiner (MIRC) is a near-infrared image-plane combiner at the CHARA array which consists of six 1-m diameter telescopes with the longest baseline of 330m. MIRC was upgraded from a 4-beam to a full 6-beam combiner in July 2011, which now records interferometry data of 15 baselines and 20 triangles simultaneously. The improved snapshot UV coverage has greatly boosted the ability for imaging complicated targets such as the asymmetry of circumstellar disks, interacting binaries and the surfaces of spotted stars. In addition, the Photometric Channels subsystem, which directly measures the real time flux of individual beams, has been upgraded to increase the light throughput to improve the visibility and closure phase calibration. The system sensitivity has been improved as well to allow fainter objects such as Young Stellar Objects (YSOs) to be observable with MIRC for the first time. Our presentation will conclude with first preliminary results of imaging two Be binaries observed by the upgraded MIRC.

For information-rich direct images at high resolution, hypertelescopes combine light from a sparse array of many subapertures, using pupil densification. Among the possible architectures, the Arecibo-like spherical class has fixed mirrors arrayed as elements of a common spherical locus, matching approximately the natural curvature of a crater or valley. A focal gondola suspended on the focal sphere, is tracking the primary star image, and several more can be added for independent observations of di.erent sources. Since no delay lines are needed, hundred of mirrors can be used for reaching the theoretical information gain with respect to fewer apertures. The aperture size of such instruments may range from 50 to perhaps 1200m at available terrestrial sites. As an example of their broad science capabilities, we have simulated the resolved and spectro-imaging- of an exoplanet transiting across the disk of its parent star, achievable with adaptive optics. Faint cosmological sources may also become observable if a Laser Guide Star can be fitted. We describe the current construction and in situ opto-mechanical testing of a 57m hypertelescope, later expandable to 200 with 100 or more sub-apertures. The preliminary operating experience gained in a year, without stellar fringes yet, indicates the likely feasibility of larger versions at suitable sites. Labeyrie et al., (this conference) discuss an "Extremely Large Hypertelescope" (ELHyT) having 1200m sparse aperture and, at similar cost, a larger collection area and higher limiting magnitude than a 40m ELT.

The hypertelescope construction initiated in the Southern Alps (Labeyrie et al., this conference) has provided some preliminary operating experience indicating that larger versions, up to perhaps 1200m, are probably feasible at suitable sites. The Arecibo-like architecture of such instruments does not require the large mount and dome which dominate the cost of a 40m ELT. For the same cost, an "Extremely Large Hyper Telescope” ( ELHyT) may therefore have a larger collecting area. It may thus in principle reach higher limiting magnitudes, both for seeing-limited and, if equipped with a Laser Guide Star and adaptive phasing, for high-resolution imaging with gain as the size ratio, i.e. about 30 with respect to a 40m ELT. Like the radio arrays of antennas, such instruments can be grown progressively. Also, they can be up-graded with several focal gondolas, independently tracking different sources. Candidate sites have been identified in the Himalaya and the Andes. We describe several design options and compare the science achievable for both instruments, ELTs and ELHyTs. The broad science addressed by an ELHyT covers stellar chromospheres, transiting exoplanets and those requiring a high dynamic range, achieved by array apodization or coronagraphy. With a Laser Guide Star, it extends to faint compact sources beyond the limits of telescopes having a smaller collecting area, supernovae, active galactic nuclei, gamma ray bursts. The sparse content of remote galaxies seen in the Hubble Deep Field appears compatible with the crowding limitations of an ELHyT having 1000 apertures.

GAME is a recent concept for a small/medium class mission aimed at Fundamental Physics tests in the Solar system, by means of an optimised instrument in the visible, based on smart combination of coronagraphy and Fizeau interferometry. The targeted precision on the γ and β parameters of the Parametrised Post-Newtonian formulation of General Relativity are respectively in the 10^-7-10^-8 and 10^-5-10^-6 range, improving by one or two orders of magnitude with respect to the expectations on current or near future experiments. Such precision is suitable to detect possible deviations from the unity value, associated to generalised Einstein models for gravitation, with potentially huge impacts on the cosmological distribution of dark matter and dark energy from a Solar system scale experiment. The measurement principle is based on the differential astrometric signature on the stellar positions, i.e. based on the spatial component of the effect rather than the temporal component as in the most recent experiments using radio link delay timing variation (Cassini). The instrument concept is based on multiple field, multiple aperture Fizeau interferometry, observing simultaneously regions close to the Solar limb (requiring the adoption of coronagraphic techniques), and others in opposition to the Sun. The diluted optics approach is selected for achieving an efficient rejection of the scattered solar radiation, while retaining an acceptable angular resolution on the science targets. The multiple field observation is aimed at cost-effective control of systematic effects through simultaneous calibration. We describe the science motivation, the proposed mission profile, the instrument concept and the expected performance.

We have built at the Haute-Provence observatory (France) the rst diluted telescope in the world. We describe this prototype called Carlina, made of three 25 cm mirrors separated by a maximum baseline of 10.5 m. The three mirrors in place are already coherenced and rst light is scheduled for June-July 2012. In this article, we will mainly describe the focal gondola. We propose to build in the near future a 100 m aperture Large Diluted Telescope. This diluted telescope will be more sensitive than regular interferometers (Keck, VLTI, etc.), with higher imaging capabilities. A LDT will open new elds of research in astrophysics thanks to very high angular resolution imaging of the surface of supergiant stars, AGN, gravitational micro-lens systems, exo-planets, etc.

This article reports the advances on the development of mid-infrared integrated optics for stellar interferometry. The devices are fabricated by laser writing techniques on chalcogenide glasses. Laboratory characterizaton is reported and analyzed.

We present the experimental results of a 3D photonic component designed for the determination of coherence properties of astronomical targets in optical interferometry. The component is based on the properties of two dimensional arrays of evanescently coupled waveguides and has the potential of being scalable to arbitrary large arrays of telescopes. Simulations and rst experimental results will be presented together with perspectives for implementation of future instruments combining multiple telescopes.

In this communication an extraction procedure that takes into account the spectral dispersion function (the spectral analog of the PSF) is presented. The method is named least-squares deconvolution. It allows the recovery of the relative line-to-continuum visibility amplitude ratio and the relative line to continuum visibility phase difference. The method only uses as input the AMBER data making the sole hypotheses that the spectral broadening of the spectra in the photometric channel is the same as that of the interferometric data. A subset of this hypothesis is the case of unresolved lines. It is extremely robust being able to recover line to continuum visibility and phase at very low signal-to-noise ratio. It is shown that it is superior to other differential visibility and phase methods presented in the literature, which in certain conditions are biased. The method can be trivially generalized to similar instruments as those available at CHARA and Keck-I. Least squares deconvolution opens the possibility of delivering legacy quality measurements from the AMBER archive without relying on visibility calibration or environmental effects such as vibrations. It is a key tool for the astrophysical exploitation of this instrument.

Optical Interferometry has long been limited by low SNR making it nearly impossible to measure the small visibilities required to make resolved images. Although the SNR exists in the raw data, much SNR is lost in the conventional squared-visibility processing. In modern interferometers fringes are recorded simultaneously at many wavelengths and baselines. This makes phase-referencing possible, which is the key to coherent integration, which in turns can greatly improve the SNR of measurements, making small-amplitude resolving measurements possible. In this paper we will detail the theory of coherent integration. We will also explain why coherent integration should, in most cases, be carried out during post-processing in software rather than in real-time in hardware. We will then compare it to conventional processing approaches for some data from the Navy Optical Interferometer. We will demonstrate how coherent integration can improve the accuracy of observations.

We developed the tool GEM-FIND that allows to constrain the morphology and brightness distribution of ob- jects. The software fits geometrical models to spectrally dispersed interferometric visibility measurements in the N-band using the Levenberg-Marquardt minimization method. Each geometrical model describes the bright- ness distribution of the object in the Fourier space using a set of wavelength-independent and/or wavelength- dependent parameters. In this contribution we numerically analyze the stability of our nonlinear fitting approach by applying it to sets of synthetic visibilities with statistically applied errors, answering the following questions: How stable is the parameter determination with respect to (i) the number of uv-points, (ii) the distribution of points in the uv-plane, (iii) the noise level of the observations?

We present here three recipes for getting better images with optical interferometers. Two of them, Low- Frequencies Filling and Brute-Force Monte Carlo were used in our participation to the Interferometry Beauty Contest this year and can be applied to classical imaging using V² and closure phases. These two addition to image reconstruction provide a way of having more reliable images. The last recipe is similar in its principle as the self-calibration technique used in radio-interferometry. We call it also self-calibration, but it uses the wavelength-differential phase as a proxy of the object phase to build-up a full-featured complex visibility set of the observed object. This technique needs a first image-reconstruction run with an available software, using closure-phases and squared visibilities only. We used it for two scientific papers with great success. We discuss here the pros and cons of such imaging technique.

Optical interferometers provide multiple wavelength measurements. In order to fully exploit the spectral and spatial resolution of these instruments, new algorithms for image reconstruction have to be developed. Early attempts to deal with multi-chromatic interferometric data have consisted in recovering a gray image of the object or independent monochromatic images in some spectral bandwidths. The main challenge is now to recover the full 3-D (spatio-spectral) brightness distribution of the astronomical target given all the available data. We describe a new approach to implement multi-wavelength image reconstruction in the case where the observed scene is a collection of point-like sources. We show the gain in image quality (both spatially and spectrally) achieved by globally taking into account all the data instead of dealing with independent spectral slices. This is achieved thanks to a regularization which favors spatially sparsity and spectral grouping of the sources. Since the objective function is not dierentiable, we had to develop a specialized optimization algorithm which also takes into account the non-negativity of the brightness distribution.

We report on our progress toward a flexible image reconstruction software for optical interferometry capable of "5D imaging" of stellar surfaces. 5D imaging is here defined as the capability to image directly one or several stars in three dimensions, with both the time and wavelength dependencies taken into account during the reconstruction process. Our algorithm makes use of the Healpix (Gorski et al., 2005) sphere partition scheme to tesselate the stellar surface, 3D Open Graphics Language (OpenGL) to model the spheroid geometry, and the Open Compute Language (OpenCL) framework for all other computations. We use the Monte Carlo Markov Chain software SQUEEZE to solve the image reconstruction problem on the surfaces of these stars. Finally, the Compressed Sensing and Bayesian Evidence paradigms are employed to determine the best regularization for spotted stars. Our algorithm makes use of the Healpix (reference needed) sphere partition scheme to tesselate the stellar surface, 3D Open Graphics Language (OpenGL) to model the spheroid, and the Open Compute Language (OpenCL) framework to model the Roche gravitational potential equation.

We present the results of the fifth Interferometric Imaging Beauty Contest. The contest consists in blind imaging of test data sets derived from model sources and distributed in the OIFITS format. Two scenarios of imaging with CHARA/MIRC-6T were offered for reconstruction: imaging a T Tauri disc and imaging a spotted red supergiant. There were eight different teams competing this time: Monnier with the software package MACIM; Hofmann, Schertl and Weigelt with IRS; Thiebaut and Soulez with MiRA ; Young with BSMEM; Mary and Vannier with MIROIRS; Millour and Vannier with independent BSMEM and MiRA entries; Rengaswamy with an original method; and Elias with the radio-astronomy package CASA. The contest model images, 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 super-massive 4 million solar mass black hole (SMBH) SgrA* shows variable emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flux density excursions are dominated by a single state power law, with the low states of SgrA* are limited by confusion through the unresolved stellar background. We show that for 8-10m class telescopes blending effects along the line of sight will result in artificial compact star-like objects of 0.5-1 mJy that last for about 3-4 years. We discuss how the imaging capabilities of GRAVITY at the VLTI, LINC-NIRVANA at the LBT and METIS at the E-ELT will contribute to the investigation of the low variability states of SgrA*.

The VLTI/MIDI AGN Large Programme comprises 14 AGNs for which torus properties are being determined. Observations and data reduction are finished and show resolved dust emission in almost all of the sources. Questions addressed with this programme range from unification to the physics of accreting galactic nuclei. In particular, the observations allow us to dissect the parsec-scale infrared emission and to determine the sizes and fluxes of the various components. These AGNs are among the faintest objects ever studied with MIDI and require the data reduction software to be adjusted.

Latest-generation spectro-interferometric instruments combine a milliarcsecond angular resolution with spectral capabilities, resulting in an immensely increased information content. Here, I present methodological work and results that illustrate the fundamentally new scientific insights provided by spectro-interferometry with very high spectral dispersion or in multiple line transitions (Brackett and Pfund lines). In addition, I discuss some pitfalls in the interpretation of spectro-interferometric data. In the context of our recent studies on the classical Be stars β CMi and ζ Tau, I present the first position-velocity diagram obtained with optical interferometry and provide a physical interpretation for a phase inversion, which has in the meantime been observed for several classical Be-stars. In the course of our study on the Herbig B[e] star V921 Sco, we combined, for the first time, spectro-interferometry and spectro-astrometry, providing a powerful and resource-efficient way to constrain the spatial distribution as well as the kinematics of the circumstellar gas with an unprecedented velocity resolution up to R = λ/Δλ = 100,000. Finally, I discuss our phase sign calibration procedure, which has allowed us to calibrate AMBER differential phases and closure phases for all spectral modes, and derive from the gained experience science-driven requirements for future instrumentation projects.

The CHARA-Michigan Phasetracker (CHAMP) successfully tracks fringes in 4-telescope and 6-telescope modes when observing high-visibility targets. We have found that our primary targets (Young Stellar Objects) have unexpectedly low visibility fringes (<20%) for most baselines at CHARA, generally below our tracking thresholds. We have undertaken an upgrade cycle in 2011-2012 to re-optimize CHAMP to allow group-delay tracking on the faintest fringes possible. We describe our multi-pronged strategy using special dicroics, new piezo scanners, and our first attempts to explore CHARA J-band made possible by using special metrology-blocking laser filters. CHAMP can now be used with all the combiners at CHARA.

Since April 2011, realtime fringe tracking data are recorded simultaneously with data from the VLTI/AMBER interferometric beam combiner. Not only this offers possibilities to post-process AMBER reduced data to obtain more accurate interferometric quantities, it also allows to estimate the performance of the fringe tracking a function of the conditions of seeing, coherence time, flux, etc. First we propose to define fringe tracking performance metrics in the AMBER context, in particular as a function of AMBER’s integration time. The main idea is to determine the optimal exposure time for AMBER: short exposures are dominated by readout noise and fringes in long exposures are completely smeared out. Then we present this performance metrics correlated with Paranal local ASM (Ambient Site Monitor) measurements, such as seeing, coherence time or wind speed for example. Finally, we also present some preliminary results of attempts to model and predict fringe tracking performances, using Artificial Neural Networks.

The NOVA Fringe Tracker (NFT) is a proposed solution to the call by ESO for a second generation fringe tracking facility. This instrument at the VLTI will enable the cophasing of up to 6 telescopes simultaneously. Using broad band optics with detection from 1.2 to 2.4 microns, a unique configuration is employed that eliminates so-called “photometric crosstalk.” This refers to imbalance in the beam combiner which results in fluctuations of the incoming wavefronts and the proportion of power accepted by a spatial filter masquerading as a visibility, a common problem afflicting previous interferometric instruments and fringe trackers. Also proposed for use in “science instruments” (for the measurement of visibility), the “Polarization-Based Collimated Beam Combiner,” with its achievement of photometric symmetry in hardware, is particularly suited for combined use of the smaller AT (1.8 meter) telescopes with the UT (8 meter) telescopes involving a 20:1 intensity ratio of the interfering beams, and also for fringe tracking using highly resolved sources having a very small visibility. Recent enhancements to the proposed fringe tracker include selectable modes which detect only a single quadrature phase, both quadrature phases, or an uneven combination of the two. Optimization of partial spatial filtering using pinholes has been performed using a wavefront simulator and simulated tracking loop. Aiming for an instrument achieving the best limiting sensitivity, analysis and simulations predict that reliable cophasing will be obtained using the 1.8 meter AT telescopes tracking on an unresolved reference star with a K magnitude of 10

This paper reviews the recent laboratory results we have obtained on the demonstration of a cophasing algorithm based on the chromatic phase diversity method. The SIRIUS testbed was initially dedicated to the demonstration of the direct imaging capabilities of arrays of telescope. We have developed and numerically modeled a piston sensor based on the chromatic dependance of the spectral density phase. This method allows a global cophasing of the array over a capture range of many wavelengths aiming at improving the robustness of the method.

The characterization of ICoNN, the Magdalena Ridge Observatory Interferometer's fringe tracker, through labor­ tory simulations is presented. The performance limits of an interferometer are set by its ability to keep the optical path difference between combination partners minimized. This is the job of the fringe tracker. Understanding the behavior and limits of the fringe tracker in a controlled environment is key to maximize the science output. This is being done with laboratory simulations of on-sky fringe tracking, termed the closed-loop fringe experi­ ment. The closed-loop fringe experiment includes synthesizing a white light source and atmospheric piston with estimation of the tracking error being fed back to mock delay lines in real-time. We report here on the progress of the closed-loop fringe experiment detailing its design, layout, controls and software.

GRAVITY is a second generation VLTI instrument, combining the light of four telescopes and two objects simultaneously. The main goal is to obtain astrometrically accurate information. Besides correctly measured stellar phases this requires the knowledge of the instrumental differential phase, which has to be measured optically during the astronomical observations. This is the purpose of a dedicated metrology system. The GRAVITY metrology covers the full optical path, from the beam combiners up to the reference points in the beam of the primary telescope mirror, minimizing the systematic uncertainties and providing a proper baseline in astrometric terms. Two laser beams with a fixed phase relation travel backward the whole optical chain, creating a fringe pattern in any plane close to a pupil. By temporal encoding the phase information can be extracted at any point by means of flux measurements with photo diodes. The reference points chosen sample the pupil at typical radii, eliminating potential systematics due differential focus. We present the final design and the performance estimate, which is in accordance with the overall requirements for GRAVITY.

NEAT is an astrometric mission proposed to ESA with the objectives of detecting Earth-like exoplanets in the habitable zone of nearby solar-type stars. In NEAT, one fundamental aspect is the capability to measure stellar centroids at the precision of 5 x 10^-6 pixel. Current state-of-the-art methods for centroid estimation have reached a precision of about 4 x 10^-5 pixel at Nyquist sampling. Simulations showed that a precision of 2 μ-pixels can be reached, if intra and inter pixel quantum efficiency variations are calibrated and corrected for by a metrology system. The European part of the NEAT consortium is designing and building a testbed in vacuum in order to achieve 5 x 10^-6 pixel precision for the centroid estimation. The goal is to provide a proof of concept for the precision requirement of the NEAT spacecraft. In this paper we give the basic relations and trade-offs that come into play for the design of a centroid testbed and its metrology system. We detail the different conditions necessary to reach the targeted precision, present the characteristics of our current design and describe the present status of the demonstration.

Enhancing the limiting sensitivity of optical/infrared interferometry is one of the "holy grails" of interferometric research. While the use of adaptive optics is in principle attractive, a number of issues suggest that its ability to enhance the sensitivity of ground-based arrays is less clear. Indeed, the ultimate sensitivity of an array may be limited by any of the multiple active and photon-hungry subsystems that comprise its whole. In this paper we investigate the limiting sensitivity of interferometer arrays using unit telescopes of moderate size (i.e. with D ≤ 4 m) equipped with natural guide star adaptive optics systems. We focus on how to realise the best limiting sensitivity for observations in the near-infrared. We nd that for Vega-type targets, i.e. those that have similar magnitudes at all wavelengths, the use of an adaptive optics system can provide enchancements in limiting sensitivity of up to 1.5 magnitudes. However, for redder targets this improvement can decrease dramatically, and very similar sensitivity (Δm_limiting ≤ 0.5) can be obtained with arrays using 1.5m-class apertures and tip-tilt correction alone.

The mass-loss process is a key ingredient for our understanding in many fields of astrophysics, including stellar evolution and the enrichment of the interstellar medium (ISM) via stellar yields. We combined the capability of the VLTI/MIDI and VLT/VISIR instruments with very recent Herschel/PACS observations to characterize the geometry of mass loss from evolved red giants on the Asymptotic Giant Branch (AGB) at various scales. This paper describes the sample of objects, the observing strategy, the tool for the interpretation, and preliminary MIDI results for two targets: U Ant and θ Aps.

We present the design of a fringe tracking system for the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). BETTII is a balloon- borne, far-infrared, 8 m-baseline interferometer with two 50 cm siderostats. Beams from the two arms are combined in the pupil plane to enable double-Fourier, spatio-spectral interferometry. To maintain the phase stability of the system, we need to actively correct of the optical path difference (OPD) between the two arms. The fringe-tracking system will work in the near-infrared and will use a reference star within the field of view to achieve two goals: overlap the beams coming from the two siderostats, and track the location of the central fringe packet, which is a measure of the OPD. The fringe tracker will share most of the optical train with the science instrument. This system is part of the overall control architecture that feeds fast steering tip/ tilt mirrors and a warm delay line to ensure proper beam combination and OPD control for the science instrument. This paper investigates the different sources of perturbations that are expected at float altitude, and derives the sensitivity of the fringe-tracking system. We show progress on validating our design using a visible light, broadband Mach-Zehnder interferometer that was developed at NASA/GSFC. This system demonstrates the viability of our OPD determination approach and provides a means of testing and characterizing several OPD determination and control algorithms.

We present design results of the 2nd generation VLTI instrument GRAVITY beam stabilization and light injection subsystems. Designed to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the VLTI optical train. To meet the astrometric requirements, we have developed a dedicated 'laser guiding system', correcting the longitudinal and lateral pupil position as well as the image jitter. The actuators for the correction are provided by four 'fiber coupler' units located in the GRAVITY cryostat. Each fiber coupler picks the light of one telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization analysis as well as atmospheric piston correction. Using a novel roof-prism design offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical train. Finally the stabilized beam is injected with minimized losses into singlemode fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype regarding the closed loop performance and the optical quality. Based on the lab results we discuss the on-sky performance of the system and the implications concerning the sensitivity of GRAVITY.

The fast tip-tilt correction system for the Magdalena Ridge Observatory Interferometer (MROI) is being designed and fabricated by the University of Cambridge. The design of the system is currently at an advanced stage and the performance of its critical subsystems has been verified in the laboratory. The system has been designed to meet a demanding set of specifications including satisfying all performance requirements in ambient temperatures down to -5 °C, maintaining the stability of the tip-tilt fiducial over a 5 °C temperature change without recourse to an optical reference, and a target acquisition mode with a 60” field-of-view. We describe the important technical features of the system, which uses an Andor electron-multiplying CCD camera protected by a thermal enclosure, a transmissive optical system with mounts incorporating passive thermal compensation, and custom control software running under Xenomai real-time Linux. We also report results from laboratory tests that demonstrate (a) the high stability of the custom optic mounts and (b) the low readout and compute latencies that will allow us to achieve a 40 Hz closed-loop bandwidth on bright targets.

Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will deliver narrow angle astrometry with 10μas accuracy at K-band. We present the system design of the science and fringe tracking spectrometers of GRAVITY: The fringe tracking spectrometer is optimised for highest sensitivity, providing a fixed spectral resolution. The science spectrometer provides 3 different low - medium spectral resolutions. Both spectrometers provide detector focus stages and deployable Wollaston prisms. The two spectrometers also feed the beams of the metrology laser system of GRAVITY backwards into the integrated optics beam-combiner, propagating back to the M2 mirrors of the 4 telescopes.

Optical interferometry and polarimetry have separately provided new insights into stellar astronomy, especially in the fields of fundamental parameters and atmospheric models. Optical interferometers will eventually add full-Stokes polarization measuring capabilities, thus combining both techniques. In this paper, we: 1) list the observables, calibration quantities, and data acquisition strategies for both limited and full optical interferometric polarimetry (OIP); 2) describe the masking interferometer AMASING and its polarization measuring enhancement called AMASING-POL; 3) show how a radio interferometry imaging package, CASA, can be used for optical interferometry data reduction; and 4) present imaging simulations for Be stars.

Interferometric Closure Phase (CP) yields information on the asymmetries of the source brightness distribu­ tion. While accurate closure phases are the key for detecting, odeling and imaging low contrast features, their experimental accuracy is usually far from what it could be: in the case of the AMBER/VLTI instrument, the guaranteed accuracy calibration is between 3 and 5 degrees, while the theoretical limit is better than 0.01 deg for bright sources. Closure phase should first be corrected for detection artifacts (mainly drifts in the detector and optics), using in our case the AMBER Beam Commutation Device. We show that closure phase is nevertheless contaminated by the pistons drifts of each baseline. This effect is attributed to a cross-talk between the fringes peaks, which cannot be completely avoided in a multi-axial beam combiner with a limited readout window. We show that the variable bias on CP is a linear function of the external pistons. This relationship can be determined from the calibration source data and applied for correcting the science data. The global process both unbiases and stabilizes the average CP, yielding, with our measurements, an accuracy of 0.3 deg for 1 minute exposures with ATs, which is close to the fundamental limit for our K=4 source. It also allows to correct the chromatic OPD effect by comparison with a well chosen calibrator, displaying a CP vs. wavelength curve with aRMS error of 0.1 deg per spectral channel, about a factor 3 to 4 better than with a straight calibration.

The extremely low signal contrast between an Earth-like extra-solar planet (exoplanet) and a parent star is a difficult obstacle in their detection, imaging and spectroscopic analysis. We suggest a method of using selected parts of the Fourier interferogram of the combined light sources (both planet and sun) in order to increase the signal to noise ratio and identify the specific spectral features from the planet in the background of the parent star. A habitable exoplanet is expected to reflect and emit a luminosity which is many orders of magnitude less than that of the parent star. However, its spectral features are much different, being much narrower than its sun. Narrower lines are more coherent, so their Fourier spectrum extends to much larger delays. Thus they can be discriminated for by looking at an off-center part of a Fourier spectrogram. As the center (with the shorter delay) has all the power from the star's wider features, these will not affect the result. Now all the power will be distributed at the longer delays (where the exoplanets lines appear), improving the signal to noise ratio. We support this idea by realistic simulations which include photon and thermal noise, and show it to be feasible at a luminosity ratio of 10-6 in the infra-red for a Sun-like star and an Earth-like planet. We also carried out a laboratory experiment to illustrate the method. The results suggest that this method should be applicable to a very large number of candidate stars.

The Sydney University Stellar Interferometer (SUSI) is being fitted with a new beam combiner, called the Micro-arcsecond University of Sydney Companion Astrometry instrument (MUSCA), for the purpose of high precision astrometry of bright binary stars. Operating in the visible wavelength regime where photon-counting and post-processing fringe tracking is possible, MUSCA will be used in tandem with SUSI’s primary beam combiner, Precision Astronomical Visible Observations (PAVO), to record high spatial resolution fringes and thereby measure the separation of fringe packets of binary stars. With continued monitoring of stellar separation vectors at precisions in the tens of micro-arcseconds over timescales of years, it will be possible to search for the presence of gravitational perturbations in the orbital motion such as those expected from planetary mass objects in the system. This paper describes the first phase of the development, which includes the setup of the dual beam combiner system and the methodology applied to stabilize fringes of a star by means of self-phase-referencing.

The quality of the tracking performed by the fringe sensor units (FSUs) of PRIMA, the ESO’s dual feed facility for the VLTI, is affected by the angular separation between the two objects being observed simultaneously, the detector integration time (DIT) and the atmospheric observational conditions. We describe the algorithm we developed to compute visibilities from the FSU data and discuss their validity for the study on the angular anisoplanatism measured through the dependence of the visibility amplitudes on the angular separation.

The database of the relocatable VLTI Auxiliary Telescopes was used to study their stability and long-term trends in their pointing models. The model parameters are functions of mechanical properties of the telescopes and station pads. The index error in elevation and non-perpendicularity in elevation depend mainly on telescope characteristics, while the tilt parameters on the pads. After some trends present in 2007-2010 the mechanical characteristics of all four VLTI 1.8m telescopes are stabilized. Some tilt trends are still present in recently opened stations, but unlike in APEX, the tilts are not significantly affected by seasonal temperature variations.

The PERSEE breadboard, developed by a consortium including CNES, IAS, LESIA, OCA, ONERA and TAS since 2005, is a nulling demonstrator that couples an infrared nulling interferometer with a formation flying simulator able to introduce realistic disturbances in the set-up. The general idea is to prove that an adequate optical design can considerably relax the constraints applying at the spacecrafts level of a future interferometric space mission like Darwin/TPF or one of its precursors. The breadboard is now fully operational and the measurements sequences are managed from a remote control room using automatic procedures. A set of excellent results were obtained in 2011. The measured polychromatic nulling depth with non polarized light is 8.8 10^-6 stabilized at 9 10^-8 in the 1.65-2.45 μm spectral band (37 % bandwidth) during 100 s. This result was extended to a 7h duration thanks to an automatic calibration process. The various contributors are identified and the nulling budget is now well mastered. We also proved that harmonic disturbances in the 1-100 Hz up to several ten’s of nm rms can be very efficiently corrected by a Linear Quadratic Control (LQG) if a sufficient flux is available. These results are important contributions to the feasibility of a future space based nulling interferometer.

We present the conceptual design of a new interferometric instrument designed to combine a large number of apertures by means of 3D photonic beam combiners. We discuss 1) the general layout of the system, 2) the technical constraints for the instrument and 3) estimated performance of the instrument based on the modeling of the beam combiner based on real coupling data from fiber-fed instruments.

The Fringe and Flexure Tracking System (FFTS) is meant to monitor and correct atmospheric piston varia­ tion and instrumental vibrations and flexure during near-infrared interferometric image acquisition of LING­ NIRVANA. In close work with the adaptive optics system the FFTS enables homothetic imaging for the Large Binocular Telescope. One of the main problems we had to face is the connection between the cryogenic upper part of the instrument, e.g. detector head, and the lower ambient temperature part. In this ambient temperature part the moving stages are situated that move the detector head in the given field of view (FOV). We show how we solved this problem using the versatile material glass fiber reinforced plastics (GFRP's) and report in what way this material can be worked. We discuss in detail the exquisite characteristics of this material which we use to combine the cryogenic and ambient environments to a fully working system. The main characteristics that we focus on are the low temperature conduction and the tensile strength of the GFRP's. The low temperature conduction is needed to allow for a low heat-exchange between the cryogenic and ambient part whereas the tensile strength is needed to support heavy structures like the baffle motor and to allow for a minimum of flexure for the detector head. Additionally, we discuss the way we attached the GFRP to the remaining parts of the FFTS using a two component encapsulant.

This paper describes two designs for beam combiners which measures all of the Stokes parameters. The first is the combination of a optical polarimeter for a single telescope with a simple beam combiner. The second approach forms 2N beams by splitting the light from each of the N telescopes into orthogonal, linear polarizations. Then, those 2N beams are fed into a single, multi-beam combiner. We present optical designs for both systems. We claim the second design is simpler and has the advantage of being easier to calibrate.

A method based on a variant of genetic algorithm is proposed to obtain the wavefront aberrations of a real ronchigrams using only one ronchigram without using polynomial fit or trapezoidal integration. The recovery of the aberration coefficients of third order is achieved by assigning random values but controlled in the equation of the optical path difference (OPD) which is given for a lateral shear interferometer. The proposed method retrieves the coefficients of the polynomial of the analyzed Ronchigram in a reliable and accurate way.

Visible nulling coronagraphy is one of the few approaches to the direct detection and characterization of Jovian and Terrestrial exoplanets that works with segmented aperture telescopes. Jovian and Terrestrial planets require at least 10^-9 and 10^-10 image plane contrasts, respectively, within the spectral bandpass and thus require a nearly achromatic π-phase difference between the arms of the interferometer. An achromatic π-phase shift can be achieved by several techniques, including sequential angled thick glass plates of varying dispersive materials, distributed thin-film multilayer coatings, or techniques that leverage the polarization-dependent phase shift of total-internal reflections. Herein we describe two implementations of such techniques: sequential thick glass plates and Fresnel rhomb prisms. A viable technique must achieve the achromatic phase shift while simultaneously minimizing the intensity difference, chromatic beam spread and polarization variation between the interferometer arms. In this paper we describe the above implementations and report on the trades associated with each technique that will lead to an implementation of the most promising one in Goddard's Visible Nulling Coronagraph (VNC).

The Wide-field Imaging Interferometer testbed (WIIT) at NASA’s Goddard Space Flight Center uses a dual-Michelson interferometric technique. The WIIT combines stellar interferometry with Fourier-transform interferometry to produce high-resolution spatial-spectral data over a large field-of-view. This combined technique could be employed on future NASA missions such as the Space Infrared Interferometric Telescope (SPIRIT) and the Sub-millimeter Probe of the Evolution of Cosmic Structure (SPECS). While both SPIRIT and SPECS would operate at far-infrared wavelengths, the WIIT demonstrates the dual-interferometry technique at visible wavelengths. The WIIT will produce hyperspectral image data, so a true hyperspectral object is necessary. A calibrated hyperspectral image projector (CHIP) has been constructed to provide such an object. The CHIP uses Digital Light Processing (DLP) technology to produce customized, spectrally-diverse scenes. CHIP scenes will have approximately 1.6-micron spatial resolution and the capability of producing arbitrary spectra in the band between 380 nm and 1.6 microns, with approximately 5-nm spectral resolution. Each pixel in the scene can take on a unique spectrum. Spectral calibration is achieved with an onboard fiber-coupled spectrometer. In this paper we describe the operation of the CHIP. Results from the WIIT observations of CHIP scenes will also be presented.

In an unbalanced nulling interferometer (UNI) of our coronagraph system, the incidence light is divided into two, and they interfere by a reverse phase with different amplitude. Thereby, phase errors are magnified and we can correct a wavefront with higher precision. But phase errors of the incident wave will be magnified together with the wavefront errors inside UNI. Now, I am developing a control algorithm of the adaptive optics which removes the wavefront errors inside the interferometer by operating the phase of the light to a suitable value before dividing. In a simulation, wavefront accuracy improved by about 3 times with this technique, and also a comparable effect was acquired experimentally.

A non-redundant pupil mask placed in front of a low-resolution integral field spectrograph (IFS) adds a spectral dimension to high angular resolution imaging behind adaptive optics systems. We demonstrate the first application of this technique, using the spectroscopic binary star system β CrB as our target. The mask and IFS combination enabled us to measure the first low-resolution spectrum of the F3-F5 dwarf secondary component of β CrB, at an angular separation 141 mas from its A5-A7Vp primary star. To record multi-wavelength closure phases, we collected interferograms simultaneously in 23 spectral channels spanning the J and H bands (1.1 μm-1.8 μm), using the Project 1640 IFS behind the 249-channel PalAO adaptive optics system on the Hale telescope at Palomar Observatory. In addition to providing physical information about the source, spectrally resolved mask fringes have the potential to enhance detection limits over single filter observations. While the overall dynamic range of our observation suffers from large systematic calibration errors, the information gleaned from the full channel range improves the dynamic range by a factor of 3 to 4 over the best single channel.

Preliminary investigations for an Aperture Masking Experiment at the Large Binocular Telescope (LBT) and its application to stellar surface imaging are presented. An algorithm is implemented which generates non redundant aperture masks for the LBT. These masks are adapted to the special geometrical conditions at the LBT. At the same time, they are optimized to provide a uniform UV-coverage. It is also possible to favor certain baselines to adapt the UV-coverage to observational requirements. The optimization is done by selecting appropriate masks among a large number (order 10⁹) of randomized realizations of non-redundant (NR) masks. Using results of numerical simulations of the surface of red supergiants, interferometric data is generated as it would be available with these masks at the LBT while observing Betelgeuse. An image reconstruction algorithm is used to reconstruct images from Squared Visibility and Closure Phase data. It is shown that a number of about 15 holes per mask is sufficient to retrieve detailed images. Additionally, noise is added to the data in order to simulate the influence of measurement errors e.g. photon noise. Both the position and the shape of surface structures are hardly influenced by this noise. However, the flux of these details changes significantly.

FLUOR, which has been operational on CHARA since 2002, is an infrared fiber beam combiner. The telescope array will soon be fitted with an adaptive optics system, which will enhance the interferometer performance. In this framework, FLUOR has been entirely redeveloped and will be able to measure visibilities with higher accuracy and better sensitivity. The technical upgrades consist of improving some existing systems and developing new features. The bench, which is now remotely operable, primarily offers spectral dispersion (long fringes scanning), a more sensitive camera and a Fourier Transform Spectrometer mode. This paper presents the detailed opto-mechanical design of JouFLU (FLUOR rejuvenation), and the current instrument status.

MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment) is the future spectro-interferometer of the European Southern Observatory VLT operating in the spectral bands L, M and N, and combining four beams from the telescopes UTs or ATs. This paper describes the concept, the specifications and the expected performances of the instrument. The requirements have been established including transmission and contrast degradation budgets. An assessment of the performances is given in this paper taking into account the instrument and VLTI characteristics.

Most subsystems of the Magdalena Ridge Observatory Interferometer (MROI) have progressed towards final mechanical design, construction and testing since the last SPIE meeting in San Diego - CA. The first 1.4-meter telescope has successfully passed factory acceptance test, and construction of telescopes #2 and #3 has started. The beam relay system has been prototyped on site, and full construction is awaiting funding. A complete 100-meter length delay line system, which includes its laser metrology unit, has been installed and tested on site, and the first delay line trolley has successfully passed factory acceptance testing. A fully operational fringe tracker is integrated with a prototyped version of the automated alignment system for a closed looping fringe tracking experiment. In this paper, we present details of the final mechanical and opto-mechanical design for these MROI subsystems and report their status on fabrication, assembly, integration and testing.

A limitation of the current generation of long baseline optical interferometers is the need to make the light interfere prior to detection. This is unlike the radio regime where signals can be recorded fast enough to use electronics to accomplish the same result. This paper describes a modern optical intensity interferometer based on electronics with picosecond timing resolution. The instrument will allow for portable optical interferometry with much larger baselines than currently possible by using existing large telescopes. With modern electronics, the limiting magnitude of the technique at a 4-m aperture size becomes competitive with some amplitude-based interferometers. The instrumentation will permit a wireless mode of operation with GPS clocking technology, extending the work to extremely large baselines. We discuss the basic observing strategy, a planned observational program at the Lowell Observatory 1.8-m and 1.0-m telescopes, and the science that can realistically be done with this instrumentation.

Geostationary satellites are generally too small to image at high resolution with conventional single-dish tele- scopes. An alternative to a 100+ m diameter telescope is to use an optical/infrared interferometer consisting of multiple smaller telescopes in an array configuration. In this paper we focus on what is required to achieve the required signal-to-noise ratio to image. We will look at the signal-to-noise ratio required to track fringes on satellites on multiple baselines, a pre-requisite to imaging.We will also look at how to achieve the required signal- to-noise required for image reconstruction. We compare these performance specifications to the performance of existing interferometers as well as that of a new interferometer concept optimized for satellite imaging.

Interferometry provides the only practicable way to image meter-scale structure in geosynchronous satellites. This capability represents a unique commercial opportunity for astronomical interferometry, but to date no interferometer has been able to make an image of such a satellite. We discuss the challenges of imaging these objects and present results of sensitivity calculations and imaging simulations which show that the Magdalena Ridge Observatory Interferometer is likely to be well-suited to this application. Our preliminary results suggest that a significant proportion of GEO targets may be accessible and that it may be possible to routinely extract key satellite diagnostics with an imaging capability that would be able to distinguish, for example, 70 cm features on a 5-meter satellite bus and payload, 30 cm features on a 2-meter satellite bus or similarly sized structure, as well as precise quantitative information on much larger structures such as 10 m long solar panels. Optimised observation and data reduction strategies are likely to allow these limits to be improved in due course.

We simulate the observations of a red supergiant star and an asteroid with an optical interferometer mounted on a boom. This instrument has an advantage over more traditional interferometers because it significantly reduces the number of reflections and surfaces, thus allowing one to combine a larger number of telescopes without a significant loss of sensitivity. We investigate two telescope arrays distributed on a hexagonal pattern, one that produces a non redundant coverage of the uv-plane and one that produces a redundant coverage of the uv-plane. These simulated observations are combined with traditional aperture synthesis techniques to reconstruct images and determine the accuracy of these images relative to the original ones.

We are presenting a low-cost near-infrared heterodyne interferometer based on 1.55ìm fiber-components, amateur telescopes and a 3-GSPS-ROACH-based correlator. While first performance is estimated to be sufficient for the brightest stars, we expect science-relevant astronomical performance given various improvements we are working on, as stabilization of fiber coupling, detectors near the quantum limit requiring lowest possible local oscillator power, and fiber line-length correction. These will enable the investigation of extremely long baselines, adaption of existing medium-class telescopes, and testing at the pointing-telescope slots of ALMA, and/or the extension to three of more baselines.

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, both being operated at cryo-vacuum. In this paper we present the phase-C final optical design of the two spectrometers as it got derived from the scientific and technical requirements and as it was presented and reviewed successfully at the Final Design Review (FDR) at the European Southern Observatory (ESO) in October 2011.

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 beam splitter and two long wave band-pass filters with the top level requirements of high transmission of the science light in the K-Band (1.95 - 2.45 μm) region and high blocking power optical density (OD) ≥ 8 for each filter at the metrology laser wavelength of 1.908 μm. The laser metrology blocking filters were identified as one critical optical component in the GRAVITY science and fringe tracker spectrometer design. During the Phase-C study of GRAVITY all the filters were procured and individually tested in terms of spectral response at K-band, transmission, blocking (OD) and reflection at the metrology laser wavelength. We present the measurements results of the full metrology blocking system in its final configuration as to be implemented in the GRAVITY spectrometers.

GRAVITY belongs to the 2nd generation of the Very Large Telescope Interferometer (VLTI) and will operate inK-band on 6 baselines using all 4 Unit Telescopes of the VLT. With an unprecedented astrometrical accuracy of l0μas it will be, amongst others, capable of detecting the highly relativistic motion of the photocenter of a flare surrounding the supermassive black hole at the Galactic Centre, and thus probe General Relativity. The contribution of the University of Cologne within the international GRAVITY-Consortium is the design, manufacturing, qualification and assembly of the Fringe Tracking Spectrometer and the Science Spectrometer in the Beam Combiner Instrument (BCI). The BCI will be located in the interferometric lab of the VLTI. The spectrometers will be operated at 85K in a 200K environment in the BCI. We present the design and qualification of a linear displacement mechanism, which will be used at the focus stages of the detectors in both spectrometers and at the zoom stage in the Fringe Tracking Spectrometer. The mechanism consists of 4 double-hinged compliant joints which support the stage and provide a linear motion along the optical axis. The stage characterization at room and cryogenic conditions are presented.

Because they will comprise more and more collecting telescopes, modern interferometric facilities such as the Very Large Telescope Interferometer, the Center for High Angular Resolution Astronomy or the future Magdalena Ridge Observatory should provide new and astounding visible and IR high angular resolution images in the near future. In this perspective, such interferometer arrays should be equipped with new generation fringe trackers, being able to combine all the beams originating from an increasing number of telescopes and to sense differential piston errors varying between them. Here is described a new method suitable for co-phasing many interferometer sub-apertures at the same time, by means of a multi-axial integrated optics beam combiner associated with a phase-shifting technique originally intended for wavefront sensing. We present the principle of the method and its achievable performance in the cases of four, six and eight telescope arrays as function of the magnitude of the guide star in presence of various noise sources. Numerical simulations are carried out assuming typical VLTI parameters for what concerns geometry and radiometry. The main results are discussed in the concluding sections, showing an increasing advantage in favour of the phase-shifting technique when more and more collecting telescopes are operating together.

GRAVITY is a second generation VLTI instrument for high-precision narrow-angle astrometry and phase-referenced interferometric imaging in the astronomical K-band. The cryostat of the beam combiner instrument provides the required temperatures for the various subunits ranging from 40K to 290K with a milli-Kelvin temperature stability for some selected units. The bath cryostat is cooled with liquid nitrogen and makes use of the exhaust gas to cool the main optical bench to an intermediate temperature of 240K. The fringe tracking detector will be cooled separately by a single-stage pulse tube cooler to a temperature of 40K. The pulse tube cooler is optimized for minimum vibrations. In particular its warm side is connected to the 80K reservoir of the LN2 cryostat to minimize the required input power. All temperature levels are actively stabilized by electric heaters. The cold bench is supported separately from the vacuum vessel and the liquid nitrogen reservoir to minimize the transfer of acoustic noise onto the instrument.

In an optical interferometer, the phase of astronomical signals is corrupted not only by the Earth’s atmosphere, but also by instrumental artifacts, among which birefringence, that affects the relative phase of two orthogonal polarizations. Instrumental birefringence occurs at each mirror reflection, but can also occur in the instrument where the signals are recombined., especially when optical fibers are used on the signal path. We present here a new technique to cancel instrumental birefringence, in the context of the PIONIER instrument at the VLTI (ESO, Paranal observatory). This is achieved by introducing in each of the beams a birefringent plate with an adjustable inclination, that allows to cancel the differential birefringence between the beams before recombination. We present our derivation of the amount of birefringence introduced by such an inclined plate. We show the actual performance achieved with the Pionier instrument. The initial alignment takes of order of one hour, and is stable for several days. Tweaking is performed at the beginning of each observing run and takes approximately fifteen minutes. We demonstrate a simple and effective technique to cancel differential birefringence in interferometric instruments. This technique should be applicable to second generation VLTI instruments.

Gravity aims at enhancing infrared imaging at VLTI to significantly improve our understanding of the physical processes related to gravitation and accretion within compact objects. With its fiber-fed integrated optics, infrared wavefront sensors, fringe tracker, beam stabilization and a novel metrology concept, GRAVITY will push the sensitivity and accuracy of astrometry and interferometric imaging far beyond what is offered today. Four telescopes will be combined in dual feed in the K band providing precision astrometry of order 10 micro-arcseconds, and imaging with 4- milliarcsecond resolution. The fringe tracker and the scientific instrument host an identical integrated optics beam combiner made by silica-on-silicon etching technology that is put inside a cryogenic vessel and cooled down to 200K to reduce thermal background and increase sensitivity. This paper gives the design of the integrated beam combiner and of its fibered array that allows feeding the combiner with stellar light. Lab measurement of spectral throughput and interferometric performance for beam combiners made by Flame Hydrolysis Deposition and by Plasma-Enhanced Chemical Vapor Deposition (PECVD) are given. The procedure to glue together the beam combiner and its fibered array is described as well as the tests to validate the performance and the ageing effects at low temperature. Finally the thermal analysis and the eigen-frequency study of the whole device are presented.

GRAVITY is a second generation instrument for the VLTI that will combine four telescopes in the K band. To achieve astrometric precision of 10μas and angular resolution of 4mas for objects with magnitude up to K=16 in the science beam, its fringe tracker will stabilize fringes to 350nm rms on a reference star as faint as K=10. To efficiently correct both atmospheric piston and longitudinal vibrations, we developed a controller based on Kalman filtering, which is a predictive algorithm providing optimized commands based on a model of the disturbances. In order to validate the Kalman algorithm as a fringe tacker for GRAVITY, we performed numerical simulations of 2-telescope fringe tracking with a Kalman controller with on-sky measurements from the PRIMA Fringe Sensor Unit (FSU). We find that tracking with the Kalman controller is more efficient than with the PRIMA FSU for all archive data provided by ESO. We deduced an average gain of 170nm rms over the residual OPD using the PRIMA FSU.

We have developed a balloon-borne, astronomical far-infrared interferometer (FITE). Because the interferometer is a Fizeau-type two beam interferometer consisting of two off-axis parabolic mirrors, it is important to establish a method by which the two beams can be adjusted simultaneously. A conventional Hartmann test was originally employed in our previous system, but it enabled the adjustment of only one beam at one time, thus quite inefficient. We developed a new optical adjustment system that can simultaneously measure and evaluate two beams by using a Shack - Hartmann wave front sensor. In the first stage, the field of view (FOV) of the wave front sensor was adapted to the full beam size of 40 cm (the beam diameter), and the mirror surface accuracy as well as the mirror alignment were measured and adjusted for each beam. After the adjustment of both beams, they are focused at the input aperture hole of the far-infrared sensor system by expanding the FOV of the wave front sensor so that it included both beams. With this new method, we can make real-time measurements and analyses of converging beams, and can also realize fast switching between the single beam mode and double beam mode. We demonstrated this new adjustment method by performing laboratory measurements, and designed and assembled the new optical adjustment system for FITE.

A new hardware system is being implemented at The Navy Precision Optical Interferometer (NPOI) to vastly increase the data recording capabilities of the observatory. NPOI has three spectrographs each with 32 channels ranging the visible spectrum. Siderostat path lengths are modulated at 500 Hz strokes to create fringe patterns. The new system will be able to record and use all of the data generated, unlike the existing system. Utilizing parallel architecture of FPGA‟s, all channels are processed simultaneously and piped to a host computer via DMA such that an entire stroke‟s data is made available before the start of the next stroke. This is expected to increase the amount of scientific data from the NPOI by an order of magnitude over the current system. Furthermore, this data can also be used to provide feedback to the delay lines to close the fringe tracking loop. In this paper we discuss the hardware and software components of the new system and current progress.

The GRAVITY instrument’s adaptive optics system consists of a novel cryogenic near-infrared wavefront sensor to be installed at each of the four unit telescopes of the VLT. Feeding the GRAVITY wavefront sensor with light in the 1.4 - 2.4 micrometer band, while suppressing laser light originating from the GRAVITY metrology system, custom-built optical components are required. Here we report on optical and near-infrared testing of the silicon entrance windows of the wavefront sensor cryostat and other reflective optics used in the warm feeding optics.

Coherent integration is central to extracting maximum signal-to-noise ratio (SNR) from optical interferometric data, with post-processing being the most effective method. More sophisticated algorithms produce better results, but also use more computing time, sometimes as much as several minutes of computing time per second of obser- vation. As data volumes continue to increase, it is becoming impractical to transfer the data to a supercomputer. To address this problem, we have explored using a General Purpose Graphics Processor (GPGPU) to perform these computations on a local machine, exploiting the fact that the problem is, in principle, massively parallel. In this paper, we discuss methods to optimize the fringe-tracking algorithm. In particular, we emphasize the parameter extraction process, and describe implementations using both genetic algorithms and Powell’s method. Using these methods, we were able to improve performance by a factor of 100.

LINC-NIRVANA is the near-infrared interferometric imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. To meet the tight requirements that result from long exposure interferometric imaging over a large field of view, active control beyond fringe tracking and adaptive optics has to be in place in the telescope and in the instrument domain. The incoming beams of the binocular telescope have to be controlled along the entire optical path, from the entrance pupil to the combined focal plane. The beams have to coincide in the focal plane of the science detector, their pointing origins, offsets, orientations, plate scales, and distortions have to match each other and must not change during the observation. Non-common path effects between AO and science channel, flexure and thermal effects have to be compensated and offioading requests from the adaptive optics and fringe tracking systems have to be arbitrated without introducing unwanted optical path length differences or changes in the geometry of the binocular entrance pupil. Beam Control aspects include pointing, co-pointing and field derotation, active optics and collimation control. In this presentation, the constraints for coherent imaging over a 1.5 arcminute field of view are discussed together with a concept for a distributed control scheme.

The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full telescope pupil, and the VLTI beam higher order aberrations. The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope. Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing them on the instrument database. The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed near to the detector implementing the different image modes.

Fringe tracking is a CPU intensive real time application. Rates of up to 1 KHz are needed to be able to freeze the atmosphere; in addition, each available baseline must be tracked independently. To be able to overcome the CPU performance limits, VLTI 1st generation trackers PRIMA fringe sensor unit (FSU) and FINITO were implemented in a distributed architecture separating the sensor from the controller. A side effect of this design was that the control loop delay increases as the system runs asynchronously causing an increase of the phase RMS value. This is in conflict with the objective of stabilizing the fringes with the smallest possible phase RMS. VLTI 2nd generation trackers (GRAVITY 6 and ESO 2GFT) currently being designed, will operate with up to six baselines and need to achieve phase RMS values smaller than 200 [nm]. This paper will present the computer architecture of the first generation trackers. Using simulations carried out in the PRIMA testbed, the induction of phase RMS by additional pure delays of the control loop will be precisely quantify. Afterwards, using the current architecture as reference, expected values of pure delays will be estimated for a six baselines tracker. Finally, an in order to overcome this problem, a new design that exploits the inherent parallelism of the multiples baselines and integrates the sensor and the controller in a single computer will be briefly proposed.

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 baffie. We give an overview of the current design and implementation stage of the FFTS opto-mechanical components. The optical components represent an update of the original design to assess slow image motion induced by the LN instrument separately.