FINITO (the VLTI three beam fringe-tracker) has been offered in September 2007 to the astronomical community for observations with the scientific instruments AMBER and MIDI. In this paper, we describe the last improvements of the fringe-tracking loop and its actual performance when operating with the 1.8m Auxiliary Telescopes. We demonstrate the gain provided to the scientific observations. Finally, we discuss how FINITO real-time data could be used in post-processing to enhance the scientific return of the facility.

The CHARA Michigan Phase-tracker (CHAMP) is a real-time fringe tracker for the CHARA Array, a six-telescope long baseline optical interferometer on Mount Wilson, California. CHAMP has been optimized for tracking sensitivity at J, H, or K bands and is not meant as a science instrument itself. This ultimately results in maximum sensitivity for all the science beam combiners that benefit from stabilized fringes. CHAMP was designed, built, and tested in the laboratory at the University of Michigan and will be delivered to the CHARA Array in 2008. We present the final design of CHAMP, highlighting some its key characteristics, including a novel post-combination transport and imaging system. We also discuss testing and validation studies and present first closed-loop operation in the laboratory.

The Fringe Sensor Unit (FSU) is the central element of the dual-feed facility PRIMA at the VLT Interferometer (VLTI). Two identical FSU fringe detectors deliver real-time estimates of phase delay, group delay and signal-to-noise ratio for the two observed targets. They serve both as the scientific instrument for astrometry with PRIMA and as sensor for the fringe tracking system of the interferometer. Prior to its installation at the VLTI scheduled for mid-2008, the FSU is going through an extensive laboratory test phase. It is therefore embedded in a semi-realistic environment, involving a VLTI-like control system and a laser metrology. This allows us to probe the system response to atmospheric piston jitter, tip-tilt disturbances and higher order aberrations, as they are expected at the observatory. We report on the system test results, outline the optimisation of the calibration procedure and we evaluate the FSU fringe tracking performance under realistic conditions. Finally, we compare the obtained performances to the scientific and technical requirements.

GRAVITY, a VLTI second generation instrument, requires a fringe tracker combining four beams. Its design is driven by the observation of the Galactic Center and implies stringent fringe sensor specifications. We present the simulations of the fringe tracking closed-loop performance with an optical path difference (OPD) turbulence spectrum using a Kolmogorov model of the atmosphere for typical seeing conditions at VLTI (r₀ = 0.95 m, t₀ = 47 ms at 2.2 μm). We show that the total residual OPD standard deviation can be as low as λ/12 at a sampling frequency of 350 Hz on a guide star with a magnitude of mK = 10. To obtain this performance, we compared several 4-beam pairwise co-axial combination conceptual architectures and show that the optimal 4-beam combination is the one measuring the OPD on the six baselines.

We report on the opto-mechanical design of the MROI fringe tracker. This instrument, currently under development in collaboration with the University of Cambridge, will be a dedicated fringe tracking beam combiner and spectrographs. I will utilize the "Y" geometry of the array to stabilize fringes on shorter "nearest neighbor" baselines, and thus allow for increased integration times on the longer baselines and the buildup of signal to noise. The beam combiner has been designed to accommodate light from a maximum of ten telescopes (three in each array arm, one at the "Y" vertex), but can operate with fewer without having to change the overall layout. A single spectrograph will multiplex up to five nearest neighbor combinations onto a single detector. Identical spectrographs are located at opposite sides of the combiner outputs to simultaneously sample combination pairs that are Π radians out of phase with respect to one another.

Fringe tracking in interferometers is typically analyzed with the implicit assumption that there is a single phase associated with each telescope in the array. If the telescopes have apertures significantly larger than r0 and only partial adaptive optics correction, then the phase measured by a fringe sensor may differ significantly from the "piston" component of the aperture phase. In some cases, speckle noise will cause "branch points" in the measured phase as a function of time, causing large and sudden jumps in the phase. We present simulations showing these effects in order to understand their implications for the design of fringe tracking algorithms.

In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer (NPOI).¹ Coherent integration relies on being able to phase reference interferometric measurements, which in turn relies on making measurements at multiple wavelengths.We first discuss the generalized group-delay approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration can be used to perform very precision measurement of stellar properties. For example, we demonstrate how we can measure the diameter of a star to a precision of one part in 350, and measure properties of binary stars. The complex phase is particularly attractive as a data product because it is not biased in the same way as visibility amplitudes.

One of the aims of next generation optical interferometric instrumentation is to be able to make use of information contained in the visibility phase to construct high dynamic range images. Radio and optical interferometry are at the two extremes of phase corruption by the atmosphere. While in radio it is possible to obtain calibrated phases for the science objects, in the optical this is currently not possible. Instead, optical interferometry has relied on closure phase techniques to produce images. Such techniques allow only to achieve modest dynamic ranges. However, with high contrast objects, for faint targets or when structure detail is needed, phase referencing techniques as used in radio interferometry, should theoretically achieve higher dynamic ranges for the same number of telescopes. Our approach is not to provide evidence either for or against the hypothesis that phase referenced imaging gives better dynamic range than closure phase imaging. Instead we wish to explore the potential of this technique for future optical interferometry and also because image reconstruction in the optical using phase referencing techniques has only been performed with limited success. We have generated simulated, noisy, complex visibility data, analogous to the signal produced in radio interferometers, using the VLTI as a template. We proceeded with image reconstruction using the radio image reconstruction algorithms contained in aips imagr (clean algorithm). Our results show that image reconstruction is successful in most of our science cases, yielding images with a 4 milliarcsecond resolution in K band. We have also investigated the number of target candidates for optical phase referencing. Using the 2MASS point source catalog, we show that there are several hundred objects with phase reference sources less than 30 arcseconds away, allowing to apply this technique.

The first astrophysical results of the VLTI focal instrument AMBER have shown the importance of the differential and closure phase measures, which are supposed to be much less sensitive to atmospheric and instrumental biases than the absolute visibility. However there are artifacts limiting the accuracy of these measures which can be substantially overcome by a specific calibration technique called Beam Commutation. This paper reports the observed accuracies on AMBER/VLTI phases in different modes, discusses some of the instrumental biases and shows the accuracy gain provided by Beam Commutation on the Differential Phase as well as on the Closure Phase.

Recovering images from optical interferometric observations is one of the major challenges in the field. Unlike the case of observations at radio wavelengths, in the optical the atmospheric turbulence changes the phases on a very short time scale, which results in corrupted phase measurements. In order to overcome these limitations, several groups developed image reconstruction techniques based only on squared visibility and closure phase information, which are unaffected by atmospheric turbulence. We present the results of two techniques used by our group, which employed coherently integrated data from the Navy Prototype Optical Interferometer. Based on these techniques we were able to recover complex visibilities for several sources and image them using standard radio imaging software. We describe these techniques, the corrections applied to the data, present the images of a few sources, and discuss the implications of these results.

This paper presents MIRA, a Multi-aperture Image Reconstruction Algorithm, which has been specifically developed for image restoration from optical interferometric data. The sought image satisfies agreement with the input interferometric data and with some a priori image properties (positivity, normalization and regularization). The algorithm can cope with very limited amount of data; as an extreme case, MIRA is able to restore images without any Fourier phase information. This leads to the possibility to perform imaging with only 2 telescopes or when the phase closures are corrupted.

LITpro is a software for fitting models on data obtained from various stellar optical interferometers, like the VLTI. As a baseline, for modeling the object, it provides a set of elementary geometrical and center-to-limb darkening functions, all combinable together. But it is also designed to make very easy the implementation of more specific models with their own parameters, to be able to use models closer to astrophysical considerations. So LITpro only requires the modeling functions to compute the Fourier transform of the object at given spatial frequencies, and wavelengths and time if needed. From this, LITpro computes all the necessary quantities as needed (e.g. visibilities, spectral energy distribution, partial derivatives of the model, map of the object model). The fitting engine, especially designed for this kind of optimization, is based on a modified Levenberg-Marquardt algorithm and has been successfully tested on real data in a prototype version. It includes a Trust Region Method, minimizing a heterogeneous non-linear and non-convex criterion and allows the user to set boundaries on free parameters. From a robust local minimization algorithm and a starting points strategy, a global optimization solution is effectively achieved. Tools have been developped to help users to find the global minimum. LITpro is also designed for performing fitting on heterogeneous data. It will be shown, on an example, how it fits simultaneously interferometric data and spectral energy distribution, with some benefits on the reliability of the solution and a better estimation of errors and correlations on the parameters. That is indeed necessary since present interferometric data are generally multi-wavelengths.

Although direct direction of light from hot Jupiters has recently been achieved by measurements from the Spitzer Space Telescope and the Hubble Space Telescope, information on those hot Jupiters are still not enough to break all the model degeneracies and provide detailed conclusions. More detections that can measure the astrometric orbits and flux variations of hot Jupiters, especially in the near-IR, are necessary. One promising way to reach this goal is to use precision closure phase measurements obtained with ground-based long baseline optical interferometers. Here we present our preliminary closure phase studies on the nearby hot Jupiter system υ And b using CHARA-MIRC. Our data analysis shows our closure phase precisions are at ~ 0.4σ and ~ 0.6σ level of the required signal for detections for the short and long triangles of CHARA respectively. In order to make real detections, we have several improvements in the future to increase the signal-to-noise of the data. Once these improvements are realized, our goal of directly detecting light from υ And b will be feasible to achieve.

The study of protoplanetary disks, where the planets are believed to form, will certainly allow the formation of our Solar System to be understood. To conduct observations of these objects at the milli-arcsecond scale, infrared interferometry provides the right performances for T Tauri, FU Ori or Herbig Ae/Be stars. However, the only information obtained so far are scarce visibility measurements which are directly tested with models. With the outcome of recent interferometers, one can foresee obtaining images reconstructed independently of the models. In fact, several interferometers including IOTA and AMBER on the VLTI already provide the possibility to recombine three telescopes at once and thus to obtain the data necessary to reconstruct images. In this paper, we describe the use of MIRA, an image reconstruction algorithm developed for optical interferometry data (squared visibilities and closure phases) by E. Thiébaut. We foresee also to use the spectral information given by AMBER data to constrain even better the reconstructed images. We describe the use of MIRA to reconstruct images of young stellar objects out of actual data, in particular the multiple system GW Orionis (IOTA, 2004), and discuss the encountered difficulties.

The Wide-Field Imaging Interferometry Testbed (WIIT) is a wide-field spectral imaging Michelson interferometer designed and developed at the NASA/Goddard Space Flight Center. WIIT is now operational and is being used to demonstrate imaging and spectroscopy over fields-of-view larger than the typically 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 hyperspectral image cube. Herein is developed and discussed the algorithm that constructs the image cube. We show our preliminary results using observed laboratory WIIT data and discuss our ongoing work for image deconvolution.

We present the results of the third Optical/IR Interferometry Imaging Beauty Contest. A formal comparison is presented of the performance of algorithms used for imaging data from optical/infrared long-baseline interferometers. The contest consists of blind imaging of test data sets derived from model sources and distributed in the OI-FITS format. The test data consisted of datasets on two objects each "observed" in J, H, and K bands. The majority of the entries produced accurate reconstructions of the initial models. Each of the methods presented is discussed.

We present a new lunar scintillometer, LuSci. A simple and accurate way to determine the Ground Layer (GL) turbulence profile is through measuring lunar and solar scintillation. The contribution of the first 10-100 m to the total seeing is usually significant. Measuring the seeing in this GL is important to evaluate sites, especially to set the height of future domes and to translate existing seeing data to higher domes. This holds in particular to Antarctic sites where the GL seeing is dominant, with obvious implications for AO and interferometry. We develop robust methods for turbulence profile restoration from LuSci data, incorporating the effect of lunar phases. We present restored profiles from initial campaigns. We also extract a simple model for the wind profile from the rich information present in the scintillation spectrum.

The optical properties of materials are wavelength-dependent. This property, called dispersion, affects the performance of a wide-band nulling interferometer by inducing wavelength-dependent phase differences between the arms of the interferometer. In this paper, we analyze the influence of dispersion in nulling interferometers for exoplanet detection.

Nulling interferometry has been suggested as the underlying principle for an instrument which could provide direct detection and spectroscopy of Earth-like exo-planets, including searches for potential bio-signatures. This paper documents the potential of optical path difference (OPD) stabilisation with dithering methods for improving the mean nulling ratio and its stability. The basic dithering algorithm, its refined versions and parameter tuning, are reviewed. This paper takes up the recently presented results¹ and provides an update on OPD-stabilisation at significantly higher levels of nulling performance.

We recently presented a new concept for designing an achromatic phase shifter. An APS is required in nulling interferometry, a technique that aims at directly detecting and characterizing planets around a star in the thermal infrared. Our solution is based on two cellular mirrors (alternatively, transparent plates can be used) where cells have thickness which introduce OPD that are respectively odd and even multiples of half the central wavelength, on the fraction of the wave it reflects. A destructive interference is thus produced on axis for the central wavelength when recombining the two beams. We have shown that if the thicknesses are distributed according to the Pascal triangle, a fair quasi-achromatism is also reached on typically one octave in wavelength, provided there is a suffcient number of cells. The major interest of this solution is that it allows a compact, simple and fully symmetric design, without complex sub-systems to adjust. In this paper, after reminding the basic concept, we first present the theoretical estimations for the expected performances in the two possible regimes of recombination: on axis and multi-axial (Fizeau). We then describe the laboratory setup of the demonstration bench we are developing, as well as the first results obtained.

Nulling interferometry requires, among other things, a symmetric recombination module and an optical path difference control system. The symmetric recombination stage has been particularly studied over the last ten years and several concepts are now well known. One of them is the "Modified Mach Zehnder" (MMZ) interferometer, proposed by Serabyn and Colavita (2001) [1]. In this paper, we describe a new version of the MMZ beam combiner which provides a deep null signal in the science channel and, at the same time, phase-sensitive signals in the so-called co-phasing channel. From the latter, accurate optical path difference measurements can be derived. This beam combiner works in the 0.8 to 3.3 μm spectral range (0.8 to 1.5 μm for the co-phasing channel and 1.65 to 3.3 μm for the science channel). Both optical functions can be implemented in the same device thanks to an original optical design involving dedicated phase shifts. In this paper, we describe its principle and detail the optical and mechanical design.

Deep, stable starlight suppression is needed for the direct interferometric detection of Earth-like planets and requires careful control of the intensity and phase of the beams that are being combined. We have developed a novel compensator for the Terrestrial Planet Finder Interferometer based on a deformable mirror to correct the intensity and phase at each wavelength across the bandwidth of 8 to 12 microns wavelength. This paper will discuss the results of using the adaptive nuller to achieve deep broadband nulling in the mid-IR.

Spectral characterization of exo-planets can be made by nulling interferometers. In this context, several projects have been proposed such as DARWIN, FKSI, PEGASE and TPF, space-based, and ALADDIN, ground-based. To stabilize the beams with the required nanometric accuracy, a cophasing system is required, made of piston/tip/tilt actuators on each arm and piston/tip-tilt sensors. The demonstration of the feasibility of such a cophasing system is a central issue. In this goal, a laboratory breadboard named PERSEE is under integration. Main goals of PERSEE are the demonstration of a polychromatic null from 1.65 μm to 3.3 μm with a 10^-4 rejection rate and a 10^-5 stability despite the introduction of realistic perturbations, the study of the interfaces with formation-flying spacecrafts and the joint operation of the cophasing system with the nuller. We describe the principle of the cophasing system made by Onera, operating in the [0.8 - 1] μm (tip/tilt) and [0.8 - 1.5] μm (piston) spectral bands. Emphasis is put on the piston sensor and its close integration with the nuller.

Spaceborne nulling interferometers are often characterized by means of their nulling ratio, which is defined as the deepest possible extinction of one target star supposed to harbor an extra-solar system. Herein is shown that another parameter, which is the transmitting efficiency of nearby bright fringes, is also of prime importance. More generally, "nulling maps" formed by the whole destructive and constructive fringe pattern projected on-sky, are found to be very sensitive on the design of some subsystems constituting the interferometer. In particular, we consider Spatial Filtering (SF) and Achromatic Phase Shifter (APS) devices, both required achieving planet detection and characterization. Consequences of the SF choice (pinhole or single-mode optical fiber) and APS properties (with or without induced pupil-flip) are discussed, for both monochromatic and polychromatic cases. Examples of numerical simulations are provided for single Bracewell interferometer, Angel cross and X-array configurations, demonstrating noticeable differences in the aspect of resulting nulling maps. It is concluded that both FS and APS designs exhibit variable capacities for serendipitous planet discovery.

A rotating nulling coronagraph has been built for use on ground-based telescopes. The system is based on the concept of sub-aperturing the pupil of the telescope with two elliptical apertures and combining the resulting two input beams on a single-mode fiber. By a relative π phase shift of the beams, the starlight can be nulled and a relatively faint companion star can be detected. Rotation of the aperture mask on the telescope pupil results in a signal similar to that expected from a space-borne telescope system such as the proposed TPF/Darwin interferometer. The design of the nulling coronagraph and the ancillary systems that are needed, such as the fringe tracker, are described and the potential for observations on telescopes such as the Palomar 200" is discussed. Results of a nulling experiment using a single mode fiber as a beam combiner for broadband light between 1.50 μm and 1.80 μm are shown.

Nulling interferometry is one of the most promising methods to study habitable extrasolar systems. Several projects, such as Darwin, TPF, Pegase, FKSI or Aladdin, are currently considered and supported by R&D programs. One of the main issues of nulling interferometry is the feasibility of a stable polychromatic null despite the presence of significant disturbances, induced by vibrations, atmospheric turbulence on the ground or satellite drift for spaceborne missions. To reduce cost and complexity of the whole system, it is necessary to optimize not only the control loop performance at platform and payload levels, but also their interaction. In this goal, it was decided in 2006 to build a laboratory demonstrator named Persee. Persee is mostly funded by CNES and built by a consortium including CNES, IAS, LESIA, OCA, ONERA and TAS. After a definition phase in 2006, the implementation of the sub-systems has now begun and the integration near Paris by GIS-PHASE (LESIA, ONERA and GEPI) is planned in 2009. This paper details the main objectives of PERSEE, describes the definition of the bench, presents the current status and reports results obtained with the first sub-systems.

The Planet Detection Test-bed is a lab based simulation of the optics and control systems for an interferometer based Terrestrial Exoplanet characterization mission. The test-bed supports starlight nulling at 10um infrared wavelengths, with fringe tracking at 2um wavelengths and angle and shear tracking at visible wavelengths. It further allows injection of simulated planet light in the presence of the nulled star light, to allow testing of planet detection methods. We will describe the detailed construction and operation of the test-bed from an optical and control system perspective. We will also report the latest results for narrow band nulls, and the detection of broad band planet light in the presence of nulled starlight.

With the launch of planet-transit missions such as CoRoT and Kepler, it is expected that Earth-sized planets orbiting distant stars will be detected soon. This milestone will open the path towards the definition of missions able to study the atmosphere of Earth-sized extrasolar planets, with the identification of bio-signatures as one of the main objectives. In that respect, both the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) have identified nulling interferometry as one of the most promising techniques. Trying to minimize the cost and the technological risks while maximizing the scientific return, ESA and NASA recently converged towards a single mission architecture, the Emma X-array. In this paper, we present the expected science performance of this concept computed with two independent mission simulators. The impact of different observational parameters such as planet radius and exozodiacal cloud density is specifically addressed.

The current results of our ongoing Galactic Center (GC) observations with optical long baseline interferometry (OLB-IF) are presented. We achieved first IR-IF fringes in both available IR science regimes of the VLTI (MIDI: 10 μm) and (AMBER: 2 μm), demonstrating the new capabilities provided by large aperture telescope arrays to the Galactic center research. We show that the highest angular resolution only available through interferometric techniques is necessary to observe the GC ISM production in the making and distinguish individual sources from its dusty surroundings. An overview over the currently available IF-technology is given, biased towards the GC science case. The feasibility of phase-referencing to the supergiant GCIRS 7, located only 5" away from SgrA*, to increase the sensitivity and spectral resolution of the observations, is discussed, and supported by the first real data. The presentation will conclude with an outlook to the near future about how the upcoming astrometric and off-axis phase-referencing capabilities of the Keck and VLT Interferometers, nicknamed ASTRA and PRIMA, will greatly extend the currently existing capabilities to observe astrophysical phenomena in the Galactic center at the borderline to General relativity in a yet uninvestigated regime.

The VEGA spectrograph and polarimeter has been recently integrated on the visible beams of the CHARA Array. With a spectral resolution up to 35000 and thanks to operation at visible wavelengths, VEGA brings unique capabilities in terms of spatial and spectral resolution to the CHARA Array. We will present the main characteristics of VEGA on CHARA, some results concerning the performance and a preliminary analysis of the first science run.

The Precision Astronomical Visible Observations (PAVO) beam combiner is a new concept in visible beam combination, recently commissioned at the CHARA array. By creating spatially-modulated fringes in a pupil plane and then dispersing with an integral field unit, PAVO utilizes the full multi-r0 aperture of the CHARA array over a standard 50% (630-950nm) bandwidth. In addition, minimal optimized spatial filtering ensures calibration that is in principle as good as using single-mode fibers. We describe the design of and initial results from the PAVO instrument.

We report on the design and performance of a 6-way multi-wavelength beam combining instrument for the MRO Interferometer, allowing for fringe measurements at any of the J/H/K near-infrared bands at switchable spectral resolution with high sensitivity. Three preliminary designs for the instrument are presented and compared. The results of an ongoing evaluation performed on the performance, costs, and risks of each designs are analysed. Signal-to-noise analyses confirm in particular the utility of one of the design at magnitudes as faint as K=13.

LINC-NIRVANA is an innovative imaging interferometer fed by dedicated multi-conjugated adaptive optics systems. The instrument combines the light of the two, 8.4-meter primary mirrors of the Large Binocular Telescope (LBT) on a single focal plane, providing panoramic imagery with 23-meter spatial resolution. LINC-NIRVANA is entering its final integration phase, with the large adaptive-optics and imaging subsystems coming together in the clean room in Heidelberg. Here, we report on progress, including insights gained on instrument assembly and vibration control.

LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA. It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable interference pattern in the focal plane of the science camera during the integration. Two independent control loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and instrumental differential piston between the two arms of the interferometer. A second loop controls common and differential image motion resulting from changing orientations of the two optical axes of the interferometer. Such changes are caused by flexure but also by atmospheric dispersion. Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston is determined by fitting a parameterized analytical model to the observed point spread function of a reference target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for each optical axis individually in out-of-focus images of the same reference target. In this contribution we present the principles of operation, the latest changes in the opto-mechanical design, the current status of the hardware development.

The Large Binocular Telescope Interferometer, a thermal infrared imager and nulling interferometer for the LBT, is currently being integrated and tested at Steward Observatory. The system consists of a general purpose or universal beamcombiner (UBC) and three camera ports, one of which is populated currently by the Nulling and Imaging Camera (NIC). Wavefront sensing is carried out using pyramid-based "W" units developed at Arcetri Observatory. The system is designed for high spatial resolution, high dynamic range imaging in the thermal infrared. A key project for the program is to survey nearby stars for debris disks down to levels which may obscure detection of Earth-like planets. During 2007-2008 the UBC portion of the LBTI was assembled and tested at Steward Observatory. Initial integration of the system with the LBT is currently in progress as the W units and NIC are being completed in parallel.

The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions down to 1.1 milliarcsecond and spectral resolutions up to R = 12000. Targets as faint as K = 13 will be imaged without requiring a brighter nearby reference object; fainter targets can be accessed if a suitable reference is available. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral resolution enables a scientific program which serves a broad user community and at the same time provides the opportunity for breakthroughs in many areas of astrophysics. The high level specifications of the instrument are derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution images of a wide range of targets including: probing the initial conditions for planet formation in the AU-scale environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities using technologies which have been extensively tested in the past and VSI requires little in terms of new infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize the scientific return.

We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources.

MATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study demonstrates the enormous capability of a new generation mid-infrared beam combiner. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar environments by using a wide 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.

The design of a space-based interferometer with a large number of apertures has been a subject of much discussion. In this paper, we argue in favor of using a Michelson beam combiner for the case where a small field of view is acceptable. We address the normal criticism that a Michelson combiner is too complicated by presenting an optical design for a 90-aperture, laboratory prototype. Coupling this device to a large, ground-based telescope with a coherent fiber array forms an imaging system that is a significant improvement over aperture masking.

Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m² of light gathering area, and distributed over ~1km². These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.

There are two different types of beam combination: Fizeau interferometer and Michelson interferometer. Pupil plane beam combination is referred as Fizeau interferometer. On the other hand, image plane beam combination is referred as Michelson interferometer. In general, working principles of Michelson interferometers are based on double Fourier interferometry. It is possible to acquire two-dimensional spatial and one-dimensional spectral information of the sky by applying a Fourier transform spectrometer algorithm and the Van Cittert-Zernike theorem. This imaging scheme is referred to as the double Fourier interferometry. On the other hand, it is so far thought to be difficult to perform the imaging with a Fizeau interferometer, because Fizeau interferometers basically don't have a delay line that is equipped with Michelson interferometers. Here, Matsuo et al.¹ presented a new spectral imaging method for Fizeau interferometers, based on double Fourier interferometry. They noticed that a delay axis in Michelson interferometers is equal to the axis of a fringe pattern on an image plane in Fizeau interferometers. Therefore, this new approach can acquire three-dimensional information of the sky using a linear array detector placed on the image plane. In this paper, we compare the new spectral imaging method for Fizeau interferometer with the conventional one used for Michelson interferometer and discuss spectral resolutions and field of views of these imaging methods.

We have proposed an original Michelson type bolometric interferometer dedicated to CMB B mode observations in which a large format detector array is able to mounted on the focal plane since the diameter of the primary beam do not have to be smaller than the baseline length of the interferometer and is able to be taken as large as one want. This instrument is named as MuFTPol. The MuFTPol dramatically improves the sensitivity of the B mode experiments based on Michelson type bolometric interferometer otherwise the primary beam size could not be larger than 10cm in the CMB B mode experiment targetting the gravity wave origin B mode.

SCDU (Spectral Calibration Development Unit) is a vacuum test bed that was built and operated for the SIM-Planetquest Mission and has successfully demonstrated the calibration of spectral instrument error to an accuracy of better than 20 picometers. This performance is consistent with the 1 micro-arc second goal of SIM. The calibration procedure demonstrated in the test bed is traceable to the SIM flight instrument. This article is a review of all aspects of the design and operation of the hardware as well as the methodology for spectral calibration. Spectral calibration to better than 20 picometers and implications for flight are discussed.

PRIMA, the instrument for Phase-Referenced Imaging and Micro-arcsecond Astrometry at the VLTI, is currently being developed at ESO. PRIMA will implement the dual-feed capability, at first for two UTs or ATs, to enable simultaneous interferometric observations of two objects that are separated by up to 1 arcmin. PRIMA is designed to perform narrow-angle astrometry in K-band with two ATs as well as phase-referenced aperture synthesis imaging with instruments like Amber and Midi. In order to speed up the full implementation of the 10 microarcsec astrometric capability of the VLTI and to carry out a large astrometric planet search program, a consortium lead by the Observatoire de Genève, Max Planck Institute for Astronomy, and Landessternwarte Heidelberg, has built Differential Delay Lines for PRIMA and is developing the astrometric observation preparation and data reduction software. When the facility becomes fully operational in 2009, we will use PRIMA to carry out a systematic astrometric Exoplanet Search program, called ESPRI. In this paper, we describe the narrow-angle astrometry measurement principle, give an overview of the ongoing hardand software developments, and outline our anticipated astrometric exoplanet search program.

Astrometry from space is capable of making extremely precise measurements of the positions of stars, at angular precision of well below 1 micro-arcsecond (uas) at each visit. Hundreds of visits over a period of five years could achieve a relative astrometric precision for the mission of below 0.05 uas; this is well below the astrometric signature of 0.3 uas for a Sun-Earth system at a distance of 10 pc. The Sun's photometric fluctuations on time scales from days to years are dominated by the rotation and evolution of stellar surface features (sunspots and faculae). This flux variability is a source of astrophysical noise in astrometric as well as radial velocity (RV) measurements of the star. In this paper we describe a dynamic starspot model that produces flux variability which is consistent with the measured photometric power spectra of the Sun and several other stars. We use that model to predict the jitter in astrometric and RV measurements due to starspots. We also employ empirical stellar activity models to estimate the astrometric jitter of a much larger sample of stars. The conclusion of these simulations is that astrometric detection of planets in the habitable zones of solar-type stars is not severely impacted by the noise due to starspots/faculae, down to well below one Earth mass.

A flexibly-scheduled astrometric interferometer can be used to address a wide range of problems in astrophysics. We use NASA's Space Interferometry Mission (SIM) Lite with microarcsecond accuracy astrometry on targets as faint as V=19 to illustrate the opportunities. SIM Lite can be scheduled to efficiently detect Earth-mass planets around nearby stars, including multiple planet systems, seriously test models of the astrophysics of stars, probe dark matter in our galaxy, and to track changes in the parsec-scale structure of distant active galactic nuclei. A space-based optical interferometer enables microarcsecond precision astrometry of stars, for a wide range of interesting problems in Galactic and stellar astronomy, including planet detection and characterization. The Space Interferometry Mission Lite will be the first space-based Michelson optical interferometer for precision astrometry. In this paper, we briefly summarize the many science applications of this flexibly-scheduled instrument. Details of the design and operation of SIM Lite are covered in other papers in this conference. One of the most important science areas for SIM Lite is the detection and characterization of planets orbiting other stars via the well-known astrometric wobble. With a precision of smaller than one microarcsecond in a single observation, SIM Lite has the capability to detect Earth-like planets around at least 60 nearby stars. This ability to sensitively survey our local stellar neighborhood is a unique opportunity. SIM Lite will be able to characterize multiple-planet systems, which are now known to exist, studying their dynamical properties including long-term stability. Detailed follow-up of the most interesting (perhaps Sun-like) systems is an exciting prospect. Astrometry is complementary to other techniques such as radial velocity, which has already yielded many new planets, because it enables measurement of planetary masses rather than mass lower limits. It will detect small planets around young stars (up to 100 Myr old) to help understand the formation and evolution of planetary systems; these are hard to study other than by astrometry. Thus astrometry permits the study of the nature and evolution of planetary systems in their full diversity, including age, by including young (0.5-100 Myr) solar-type stars. Because it is a pointed instrument, SIM Lite maintains its full astrometric accuracy on targets as faint as V=19, which opens up a range of rare (and therefore distant) stellar types to be observed. Stellar masses and luminosities can be measured to accuracies better than 1%, which is currently hard to do, especially for rare types. Its reach extends to probing dark matter in our Galaxy, and tracking changes in the nuclei of distant active galaxies. SIM Lite will make astrometric measurements by observing a grid of reference stars covering the sky, and make inertial observations of distant quasars; in this frame SIM Lite will deliver positions and parallaxes to better than 4 microarcsecond. SIM Lite uses technologies developed during more than a decade of testbed work and will see application in many future astrophysics missions, so this mission paves the way to the future technically as well as scientifically. The mission is currently in NASA Phase B, and is being considered for full-scale development.

SIM, a micro-arcsecond astrometry space mission, has been impacted by significant changes in NASA priorities over the last two years, resulting in the mission being indefinitely delayed. The project team has responded by investigating alternative mission concepts based upon completed SIM technology. Several alternative mission concepts have been identified, ranging from a planets-only concept, to versions of SIM, called SIM-Lite, that still address the full breadth of science envisioned by two previous National Academy Astrophysics Decadal Surveys but with lower precision and reduced throughput. These mission concepts are significantly more affordable and may fit into a nearer-term future scenario than the full SIM PlanetQuest¹⁷ would. This paper describes the current state of the project, including its design and technology, and the alternative mission concepts for the use of these designs and technology.

This paper provides an overview of technology development for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. The overall technology roadmap is presented and progress with each of the testbeds is summarized.

The detection of terrestrial exo-planets in the habitable zone of Sun-like stars as well as the proof of biomarkers is one of the most exciting goals in Astrophysics today. A nulling interferometer operated in the mid-infrared wavelength regime allows for overcoming the obstacles of huge contrast ratio and small angular separation between star and planet. Dedicated missions, as ESA's DARWIN or NASA's TPF-I, are implemented as a closely controlled formation of free-flying spacecraft which carry the distributed payload. We discuss various implementation alternatives and present an optimized design of the DARWIN instrument including the science payload and the formation-flying subsystem. We analyze the achievable scientific performance of the DARWIN instrument by taking into account the target properties and the instrument performance. We show that the DARWIN mission is feasible and that the mission goals can be fulfilled.

Programmable Micro-Diffraction Gratings (PMDG) are a new type of micro-opto-electro-mechanical systems (MOEMS), opening new observational capabilities in future astronomical instrumentation. PMDG components are built with the mature micro-electronics technology, allowing a high number of moving elements as well as high performance uniformity over the whole device. Programmable gratings are based on a serial of parallel ribbons able to move out of the plane. Typical dimensions are 5μm-wide and 200μm-long ribbons. By using electrostatic force, ribbons are actuated and a grating could be formed. A few ribbons are efficient enough to diffract the light; then, locally, this grating acts as a ON-OFF switch. If the spectrum is focused on this type of device, by setting ON and OFF a selected number of "wavelengths", the spectral response of the spectrograph is programmable. The Darwin mission will search, detect and characterize exo-planets, using high-contrast nulling interferometry, coupled with spectroscopic observation. We propose a new observational concept for Darwin using a programmable spectrometer. By tailoring the spectral response, sensitivity as well as signal to noise ratio of the instrument will be increased. A demonstrator breadboard has been designed and built. This demonstrator permits the tailoring of spectral patterns by the PMDG component, and optical analysis of the obtained results both in spectral channel and imaging channel. Typical exo-planet spectra have been generated and set by the PMDG. Simulated signatures of exo-planets with life forms are clearly revealed and characterized. These new observational modes using PMDG devices would optimize and enhance exo-planet detection and characterization capabilities in Darwin-like missions.

The Fourier-Kelvin Stellar Interferometer (FKSI) mission is a two-telescope infrared space interferometer with a 12.5 meter baseline on a boom, operating in the spectral range 3 to 8 (or 10) microns, and passively cooled to about 60 K. The main goals for the mission are the measurement and characterization of the exozodiacal emission around nearby stars, debris disks, and the atmospheres of known exoplanets, and the search for Super Earths around nearby stars. We discuss progress on this mission in the context of the upcoming Decadal Survey, in particular how FKSI is ideally suited to be an Exoplanet Probe mission in terms of crucial observations which should be done before a flagship mission can be undertaken, as well as technical readiness, cost, and risk.

In the far-infrared (FIR) / THz regime the angular (and often spectral) resolution of observing facilities is still very restricted despite the fact that this frequency range has become of prime importance for modern astrophysics. ALMA (Atacama Large Millimeter Array) with its superb sensitivity and angular resolution will only cover frequencies up to about 1 THz, while the HIFI instrument for ESA'a Herschel Space Observatory will provide limited angular resolution (10 to 30 arcsec) up to 2 THz. Observations of regions with star and planet formation require extremely high angular resolution as well as frequency resolution in the full THz regime. In order to open these regions for high-resolution astrophysics we present a study concept for a heterodyne space interferometer, ESPRIT (Exploratory Submm Space Radio-Interferometric Telescope). This mission will cover the Terahertz regime inaccessible from the ground and outside the operating range of the James Webb Space Telescope (JWST).

Research with the Wide-Field Imaging Interferometry Testbed (WIIT) is ongoing, and in the past year we have achieved several important milestones. We have moved WIIT into the Advanced Interferometry and Metrology (AIM) Laboratory at Goddard, and have characterized the testbed in this well-controlled environment. The system is now completely automated and we are in the process of acquiring large data sets for analysis. In this paper, we discuss these new developments and outline our future research directions. The WIIT testbed, combined with new data analysis techniques and algorithms, provides a demonstration of the technique of wide-field interferometric imaging, a powerful tool for future space-borne interferometers. Algorithm development is discussed in a separate paper within this conference.

DAVINCI is a dilute aperture nulling coronagraph that has the potential of directly detecting an Earth in the habitable zone around ~100 nearby stars. The novel feature of this mission concept is to replace a filled aperture 5-6 meter telescope with 4 by 1.1 meter telescopes in a phased array, dramatically reducing the cost by potentially by a factor of 5-10.

Four 1.8 m outrigger telescopes were procured by NASA for use as part of the Keck Observatory Interferometer. Due to changes in NASA planning they will not be installed on Mauna Kea. The Navy Prototype Optical Interferometer (NPOI), a joint project of The U.S. Naval Observatory (USNO), The Naval Research Laboratory (NRL), and Lowell Observatory, located outside Flagstaff, Arizona, would like to upgrade the current siderostats on the project to larger apertures. The preliminary plans for integrating the Keck outrigger telescopes (OT) into NPOI are presented.

The infrared optical telescope array (IOTA), one of the most productive interferometers in term of science and new technologies was decommissioned in summer 2006. We discuss the testing of a low-resolution spectrograph coupled with the IOTA-3T integrated-optics beam combiner and some of the scientific results obtained from this instrument.

An optical delay line system for NIAOT Prototype long baseline stellar optical interferometer is being developed. The delay line system consists of optics part, machine part and control part. The optics part is a cat's-eye system which includes a paraboloidal mirror and a flat mirror. The flat mirror is placed at the focus, and is driven by a piezoelectric actuator for real-time compensation of the tracking error. The defocus of the flat mirror caused by this compensating is considered in optical design; and that the aberration of the optical system design and the manufacture precision of optical components should not cause the decline in visibility of the fringe is analyzed, also. The machinel part includes precision rails and delay line carriage. The rails require high stability and parallelism. The cart should be quakeproof when it is moved continuously in the observation process, so the rolling friction drive mode is selected as the suitable link method between the carriage and the rails. The control part includes delay line carriage device control and laser metrology system device control. During an observation, an astrometric model provides a demand cart position and velocity to control computer, the control computer send them to the device controllers. The metrology system produces tracking error fed back to the cart device controller via the control system. This feedback servo loop controls the tracking error.

The VLTI Spectro Imager project aims to perform imaging with a temporal resolution of 1 night and with a maximum angular resolution of 1 milliarcsecond, making best use of the Very Large Telescope Interferometer capabilities. To fulfill the scientific goals (see Garcia et. al.), the system requirements are: a) combining 4 to 6 beams; b) working in spectral bands J, H and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe tracking on-axis, or off-axis when associated to the PRIMA dual-beam facility. The concept of VSI consists on 6 sub-systems: a common path distributing the light between the fringe tracker and the scientific instrument, the fringe tracker ensuring the co-phasing of the array, the scientific instrument delivering the interferometric observables and a calibration tool providing sources for internal alignment and interferometric calibrations. The two remaining sub-systems are the control system and the observation support software dedicated to the reduction of the interferometric data. This paper presents the global concept of VSI science path including the common path, the scientific instrument and the calibration tool. The scientific combination using a set of integrated optics multi-way beam combiners to provide high-stability visibility and closure phase measurements are also described. Finally we will address the performance budget of the global VSI instrument. The fringe tracker and scientific spectrograph will be shortly described.

In this paper, we study the feasibility of obtaining near-infrared spectra of bright extrasolar planets with the 2nd generation VLTI Spectro-Imager instrument (VSI), which has the required angular resolution to resolve nearby hot Extrasolar Giant Planets (EGPs) from their host stars. Taking into account fundamental noises, we simulate closure phase measurements of several extrasolar systems using four 8-m telescopes at the VLT and a low spectral resolution (R = 100). Synthetic planetary spectra from T. Barman are used as an input. Standard χ²-fitting methods are then used to reconstruct planetary spectra from the simulated data. These simulations show that low-resolution spectra in the H and K bands can be retrieved with a good fidelity for half a dozen targets in a reasonable observing time (about 10 hours, spread over a few nights). Such observations would strongly constrain the planetary temperature and albedo, the energy redistribution mechanisms, as well as the chemical composition of their atmospheres. Systematic errors, not included in our simulations, could be a serious limitation to these performance estimations. The use of integrated optics is however expected to provide the required instrumental stability (around 10^-4 on the closure phase) to enable the first thorough characterisation of extrasolar planetary emission spectra in the near-infrared.

We present the optical and cryo-mechanical solutions for the Spectrograph of VSI (VLTI Spectro-Imager), the second generation near-infrared (J, H and K bands) interferometric instrument for the VLTI. The peculiarity of this spectrograph is represented by the Integrated Optics (IO) beam-combiner, a small and delicate component which is located inside the cryostat and makes VSI capable to coherently combine 4, 6 or even 8 telescopes. The optics have been specifically designed to match the IO combiner output with the IR detector still preserving the needed spatial and spectral sampling, as well as the required fringe spacing. A compact device that allows us to interchange spectral resolutions (from R=200 to R=12000), is also presented.

GRAVITY is a 2nd generation VLTI instrument that operates in the K-band and uses up to 4 telescopes simultaneously. GRAVITY will provide interferometric astrometry of two objects in a 2 arcsecond field of view at an astrometric precision of 10 μas. Using all four UTs and six interferometric baselines, it will allow for phase-referenced imaging at mas resolution in combination with spectroscopic and polarimetric observing capabilities. The large field of view of the VLTI delay lines is worldwide unique on a 140 m baseline, and no other VLTI instrument is taking advantage of that outstanding capability so far. In this paper we present the optical and mechanical design of the two spectrometers of the instrument. The presented design resulted from the successful Phase A study of the system and provides low-resolution spectroscopy using grisms and Wollaston prisms for polarimetry.

MATISSE (Multi-AperTure mid-Infrared SpectroScopic Experiment) is a mid-infrared spectroscopic interferometer combining the beams of up to four UTs or ATs of the VLTI. MATISSE will be the successor to MIDI and will provide imaging capability in three spectral bands of the mid-infrared domain: L, M, and N. MATISSE will extend the astrophysical potential of the VLTI by overcoming the ambiguities that often exist in the interpretation of simple visibility measurements. The concept of MATISSE was driven by a signal-to-noise ratio analysis aiming at comparing two basic principles that we call the global combination and the pair-wise one. We detail this comparison and explain what has led to the selected MATISSE concept: a pair-wise 0-π multi-axial mode [1].

We present in this paper the status of the calibration unit for the interferometric infrared imager LINC-NIRVANA that will be installed on the Large Binocular Telescope, Arizona. LINC-NIRVANA will combine high angular resolution (~10 mas in J), and wide field-of-view (up to 2'×2') thanks to the conjunct use of interferometry and MCAO. The goal of the calibration unit is to provide calibration tools for the different sub-systems of the instrument. We give an overview of the different tasks that are foreseen as well as of the preliminary detailed design. We show some interferometric results obtained with specific fiber splitters optimized for LINC-NIRVANA. The different components of the calibration unit will be used either during the integration phase on site, or during the science exploitation phase of the instrument.

LINC-NIRVANA (LN) is a Fizeau interferometer that will provide for the first time coherent images in the near-IR combining the beams from the two Large Binocular Telescope (LBT)arms, by adopting a Multi-Coniugate Adaptive Optics system (MCAO) that allows for atmospheric turbulence compensation. We applied a software for the simulation and the reconstruction of LN images (AIRY-LN, see Desidera et al.¹ this Conference) in two specific scientific cases: a relatively distant galaxy at redshift about 1 and a collimated jet from a Young Stellar Object (YSO). These two cases have been chosen to test the capability of LN in the observations of faint and small (1-2 arcsec) extragalactic objects and objects with diffuse emission and high dynamical range, respectively. A total of six images at different hour angles have been obtained for both cases. Using these simulated images, we obtained the final reconstructed images using the software package AIRY-LN. These images have been analyzed with the standard data reduction software (IRAF and IDL). Our analysis show that the reconstruction algorithm is fundamental to obtain a good reproduction of the original flux and morphology while the optimal number of iterations strongly depends on the scientific goal.

LINC-NIRVANA (LN) is a near-infrared imaging interferometer for the Large Binocular Telescope (LBT). It is expected to have unprecedented imaging performance in the near-infrared both in terms of angular resolution and limiting magnitude, thanks to the large collecting area of the two LBT mirrors (8.4m) and by means of Multi-Conjugated Adaptive Optics (MCAO) and a Fringe and Flexure Tracker System (FFTS). For such a complex instrument the process of observations has to be carefully prepared, considering the constraints imposed by features of the instrument and scientific objectives. This paper addresses the design of the LN Observation Preparation Software (LOPS), the main goal of which is to provide the observer with a tool to create valid observation program for LINC-NIRVANA. The current status of LOPS with its key components is described and critical aspects are addressed.

LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe and Flexure Tracking System (FFTS) is mandatory for an effcient interferometric operation of LINC-NIRVANA: the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera. A testbed interferometer, set up as laboratory experiment, is used to develop the FFTS control loop and to test the robustness of the fringe tracking concept. The geometry of the resulting interferometric intensity distribution in the focal plane of the implemented CCD corresponds to that of the LBT PSF. The setup allows to produce monochromatic (He-Ne laser) and polychromatic (halogen lamp) PSFs and allows to actively introduce well defined low-order phase perturbations, namely OPD and differential tip/tilt. Furthermore, all components that are required in a fringe tracking servo loop are included: a sensor for fringe acquisition and an actuator to counteract measured OPD. With this setup it is intended to determine the performance with which a fringe tracking control loop is able to compensate defined OPD sequences, to test different control algorithms, and to optimize the control parameters of an existing servo system. In this contribution we present the design and the realization of the testbed interferometer. Key parameters describing the white light testbed interferometer, such as fringe contrast and thermal sensitivity are discussed. The effects of all controllable phase perturbations are demonstrated.

LINC-NIRVANA is the NIR homothetic imaging camera for the Large Binocular Telescope (LBT). Its Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA: the task of this cophasing system is to assure a time-stable interference pattern in the focal plane of the camera. Differential piston effects will be detected and corrected in a real-time closed loop by analyzing the PSF of a guide star at a frequency of 100Hz-200Hz. A dedicated piston mirror will then be moved in a corresponding manner by a piezo actuator. The long-term flexure tip/tilt variations will be compensated by the AO deformable mirrors. A testbed interferometer has been designed to simulate the control process of the movement of a scaled piston mirror under disturbances. Telescope vibration and atmospheric variations with arbitrary power spectra are induced into the optical path by a dedicated piezo actuator. Limiting factors of the control bandwith are the sampling frequency and delay of the detector and the resonance frequency of the piston mirror. In our setup we can test the control performance under realistic conditions by considering the real piston mirrors dynamics with an appropriate software filter and inducing a artificial delay of the PSF detector signal. Together with the expected atmospheric OPD variations and a realistic vibration spectrum we are able to quantify the piston control performance for typical observation conditions. A robust control approach is presented as result from in-system control design as provided by the testbed interferometer with simulated dynamics.

The Nulling and Imaging Camera is the main science camera being developed for use with the LBTI. The camera has two science channels: an 8-13 um wavelength Nulling Optimized Mid-Infrared Camera (NOMIC) and a 3-5 micron imaging camera, dubbed LMIRCam. The NIC cryostat also houses a K band fast readout camera (Phasecam) to sense phase variations between the LBT apertures and carry out closed loop correction. The design, comprising these three components, is housed in a single cryostat cooled by a mechanical pulse-tube coldhead. The optical design uses diamond-turned biconical mirrors to realize diffraction-limited performance in a compact space. A range of cryogenic actuators and alignment mechanisms have been developed to carry out fine alignment of the interferometer and to feed the several channels of NIC.

The L/M-band mid-InfraRed Camera (LMIRcam) will use a mid-wave (5.1 μm cut-off) Teledyne Imaging Systems HgCdTe HAWAII 1-RG array to image the coherently combined (Fizeau) focus of the Large Binocular Telescope's twin 8.4-meter primary mirrors generated by the University of Arizona's beam combiner - the Large Binocular Telescope Interferometer (LBTI). The 1024x1024 array will have a pixel scale of 10.9 milliarcsec (mas) per pixel and a field of view of 10"x10". The highest achievable angular resolution will be 26mas (34mas) for 3.6 μm (4.8 μm). LMIRcam will operate in parallel with the Nulling Infrared Camera (NIC), sharing the same Dewar. In addition to a suite of broad and narrow-band filters, LMIRcam will contain grisms for low-resolution spectroscopy, and serve as a test-bed for novel pupil masks to enable high-contrast imaging. The opto-mechanical design, anticipated performance, and a sample of potential science applications are presented. LMIRcam is funded by the National Science Foundation and the University of Virginia.

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

A Fizeau interferometer combines the light of several telescopes to obtain panoramic images with an angular resolution equivalent to the longest edge-to-edge separation in the system. The overall performance of a Fizeau interferometer depends critically on the performance of the (MC)AO system and the efficiency of atmospheric piston correction, but also on other effects like alignment accuracies, filter bandwidths, tracking errors, atmospheric dispersion and field rotation. Due to the mutual dependence, Strehl ratio or fringe contrast like in conventional Adaptive Optics systems or pupil plane interferometers are not sufficient for a consice assessment of the performance of such an instrument. As a measure for the actual performance, we propose to use the ratio R23, which is the actual high-spatial frequency information in the images, divided by what could be measured in principal with a 23m telescope (as the LBT). We present the theoretical concept of this method and show the results of various simulations of the abovementioned effects as an application to LINC-NIRVANA, a Fizeau interferometer currently being built for the LBT.

Since the first application of the telescope to astronomy in 1610, most new astronomical discoveries require larger and larger radiation collecting areas. Today, the twin 10-meter Keck telescopes are operational and several 30-meter-aperture class telescopes are being planned. Optical interferometers and sparse aperture ground telescopes for astronomy have been proposed and built. Fienup showed the dependence between exposure time and the dilution factor of the aperture needed to maintain image quality.¹ Carpenter suggests a sparse aperture telescope system for the purpose of imaging across the surfaces of stars.² This paper demonstrates that the ability to reconstruct images from white-light extended sources with different contrast levels also depends on the specific pupil topography that is applied to the telescope system. Signal-to-noise ratios for recorded images are calculated for scene contrast, pupil shape, detector full-well, detected photons, and exposure times.

ESO is building the Phase Referenced Imaging and Microarcsecond Astronomy (PRIMA) facility for the Very Large Telescope Interferometer (VLTI) in Chile. PRIMA will enable interferometric imaging of very faint objects and high precision astrometry with both Unit (UT) and Auxiliary (AT) telescopes. The PRIMA facility consists of four major sub systems: Star Separators, Differential Delay Lines, Metrology and Fringe Sensor Units. TNO has developed the PRIMA Star Separator (STS) subsystems for both the UT and AT telescopes. The STS separates the light of two astronomical objects and feeds it into the long stroke delay line. The STS compensates for field rotation, stabilizes the beam tip tilt and adjust the lateral and axial alignment of the pupil. Chopping and/or counter-chopping on the science object or the guide star has also been implemented.

The Magdalena Ridge Observatory Interferometer (MROI) will be a reconfigurable (7.5-345 meter baselines) 10 element optical/near-infrared imaging interferometer. Depending on the location of each unit telescope (UT), light can travel distances ranging from 460 to 660 meters via several reflections that redirect the beam's path through the beam relay trains, delay lines (DL), beam reducing telescope (BCR), switchyards and finally to the beam combiners (BC). All of these sub-systems comprise three major optical axes of the MROI to be coaligned on a nightly basis by the alignment system. One major obstacle in designing the automated alignment system (AAS) is the required simultaneous measurements from the visible through near-IR wavelengths. Another difficulty is making it fully automated, which has not been accomplished at other optical/near-IR interferometers. The conceptual design of this system has been completed and is currently in its preliminary design phase. Prototyping has also commenced with designs of some hardware near completion. Here is presented the current outline and progress of MROI's automated alignment system design and some results of the prototyping.

For imaging faint and complex sources at high angular resolution, hypertelescopes (direct-imaging many-aperture interferometers using a densified pupil) gain sensitivity with respect to few-aperture interferometers and to Fizeau interferometers. Steps are taken to expand the Carlina-Proto technical prototype built at Observatoire de Haute-Provence, 18m in aperture size, and to define a larger (100-200m) Carlina-Science version, incorporating 100 or more small apertures. Following initial observing by Speckle Interferometry, adaptive co-pistoning is expected to become available, using "Dispersed Speckle" piston sensing on bright stars, and a modified Laser Guide Star on faint (mv > 25) fields. "Extremely Large Hypertelescope" versions of such instruments, with aperture size beyond a kilometer, are considered for deep-field imaging on cosmological sources. These can be interferometrically coupled with ELTs, or arrays of telescopes, at sites such as the Macon range (Andes) considered by ESO for its E-ELT. Space versions are proposed to ESA and NASA.

The preservation of mirror surface quality and figure are of paramount importance at the Navy Prototype Optical Interferometer. There are on the order of 108 eight-inch optical flats mounted in the interferometer's optical train, 102 of which are permanently mounted inside the 9000 cubic foot vacuum feed system. The flats are specified for manufacture at λ/20 peak-to-valley surface variation (λ = 633 nm) over a 7.2 inch clear aperture. Silver coating with a dielectric overcoat is subsequently applied to the reflecting surface. The objective when mounting the mirror is to preserve the surface quality and figure of the coated flats as much as possible. Surface deflections occur due to pressure points inherent in the mount. The mount consists of a modified commercially available tangent-arm gimbaled-type structure. In order to minimize the mounting effects and allow for a wider thermal operational range, modifications were made to the primary mirror cell in the following areas: edge support region, front face tabs, rear face loaders, and diameter. In this paper we describe the detailed cell modifications, a finite element analysis (FEA) of the mounted flat, the free-standing and as-mounted surface figure of a typical eight-inch diameter flat as measured with a phase-shifting interferometer, the resulting mount-induced deflections, a comparison between the measured and FEA model, and conclusions.

Space-based astronomical instruments such as SIM, the Space Interferometry Mission [1, 2] require high-quality mirrors whose zero-gravity surface figure is specified to nanometer accuracy. Testing, however, necessarily proceeds in 1g, normal earth gravity. Extracting the zero-gravity surface has traditionally been done by combining measurements with the mirror in a number of different orientations, so that the effects of gravity cancel. Here we examine some refinements to that technique, showing that for precise results it is necessary that mount forces as well as gravity forces cancel over the different orientations. These ideas are illustrated with lab experience obtained with a 343 mm diameter spherical mirror (PT-M1), which is a prototype SIM compressor mirror.

This report focuses on the design, application, and testing of custom beamsplitter and anti-reflection coatings for use in the Magdalena Ridge Observatory Interferometer (MROI) beam combiners. The coatings were designed in collaboration with Optical Surface Technologies, and the University of Cambridge. The fringe tracker and science combiners will operate across the J, H, and K bands. The coatings were designed to achieve three optical characteristics critical to optical interferometry: 1) minimized stress of the substrate (leading to induced wavefront errors), 2) high throughput, and 3) high visibilities in broadband unpolarized light. The AR coating has mean reflection losses of less than 0.5%. Beamsplitter coatings experienced visibility losses less than 1% due to group delay dispersion and s and p phase differences.

The following article describes the coatings of both Fabry-Perot (FP) etalons to be installed in the integral field spectrometer 3D-NTT. This simultaneous use of two FP etalons of high and low resolution respectively is the new concept upon which the 3D-NTT is built. Design and fabrication of the coatings of those etalons is a critical step to be able to achieve the desired performances of the instrument. More precisely, these etalons will have to show less than a 10% variation of the finesse from 370 to 900nm and a better than lambda/100 cumulative optical uniformity over a Ø100mm surface. The aim is thus to design high-reflectivity coatings for each of the FP etalon. The design process of the two sets of coatings will be described first, then the expected performances of each etalon will be presented and finally the progresses in the making of these coatings will be discussed.

We have developed a sensor optical system for the Far Infrared Interferometric Telescope (FITE). The spatial resolution of FITE is expected to be 2.5 arcseconds. In order to derive the spatial extent of target objects, the visibility of interference fringes has to be measured precisely. For this purpose, we constructed the focal plane assembly of the FITE interferometer with the sensor optics. The focal plane is the entrance focus of the sensor optics. A far-infrared (FIR) array detector is installed on the final focal plane of the sensor optics. Its camera optics has F/106 beam for each beam of the interferometer. The PSF is dominated by diffraction, and its size corresponds approximately to the array size so that the fringe pattern can be measured by the array in real time. This system employs of two IR detectors and an optical CCD. The FIR detector has a format of 1.5mm ×15 pixels. In addition to the FIR array detector, we have a mid-IR detector and an optical CCD. They are also installed on the final focal plane of the sensor optics. These two detectors are used for the precise alignment of the interferometer optics.

We present three different techniques for single-mode waveguide realization in Lithium Niobate at the 3.39μm atmospheric transmission band, named L-band. These methods include Titanium diffusion, Ion Beam Implantation and Photo-inscription. After describing the fabrication process and waveguide characterization, we will present an integrated interferometer based on the Young's double slit experiment. From the recorded interferogram we recover information about the source, namely, its peak emission lines.

One of the critical issues for nulling interferometry breadboards for exo-earth detection relies on the possibility to manufacture and properly characterize optical waveguides that present suitable single mode behaviors in the foreseen spectral range (4-20 μm). We developed a dedicated bench for the characterization of available guides for the considered spectral range, in particulary silver halide fibers. This interferometric bench achieves modal characterization in nulling mode and spectral transmission in Fourier Transform spectrometer mode, defining the single mode domain of the components. We present the experimental setup and its performances. We present also some characterization of their single-mode behavior according to the injection conditions. These measurements are very useful for the conception of optimized components.

Dragonfly is designed to overcome the blurring effects of the Earth's atmosphere and achieve high dynamic range observations of bright objects close to the diffraction limit of a large ground-based telescope. Injection of the sub-pupil images into single-mode fibres will provide both spatial filtering and reformatting of the redundant pupil array into a non-redundant output suitable for interferometry. We describe the optical system which includes (i) interface with an Adaptive Optics system (ii) pupil fragmentation and injection into 36 single-mode fibres (iii) fibre path-length matching (iv) a beam combiner with spectral dispersion and output to a Low Light Level CCD detector.

We propose to use a Mach-Zehnder interferometer to greatly simplify and make achromatic the four quadrants phase mask coronagraph. The interferometer is used to get the achromatic π phase term. Two complementary binary masks of transmission 1 and 0 for quadrants of the same parity are set in the focal plane formed inside the interferometer. There is an equipartition of the energy in the exit aperture images, one being similar to the entrance aperture, the other being coronagraphied. We present preliminary results obtained with a coherent light. The current throughput of the experiment is 25 % only, but we give indications to improve it.

In 2004, our group proposed IRAN, an alternative beam-combination technique to the so-called hypertelescope imaging method introduced by Labeyrie in the 1990s. We have recently set up a laboratory experiment aiming at validating our image densification approach instead of the pupil densification scheme of Labeyrie. In our experiment, seven sub-apertures illuminated by laser sources are recombined using the IRAN scheme. The validation of the IRAN recombination consists basically in retrieving the point-spread intensity distribution (PSID), demonstrating the conservation of the object-image convolution relation. We will introduce IRAN, compare it to the hyper-telescope, and present the experimental results that we obtained.

As the next generation of giant optical interferometers, hypertelescopes will provide high resolution direct imaging of celestial sources by using the densification principle.¹ In order to determine the technical requirements of such an instrument, an interferometric testbench, called SIRIUS (Patru et al. 2004), has been developed at the Observatoire de la Cote d'Azur, France. The active cophasing of the beams remains the most significant hard point to preserve the quality of the image. It has been shown that this cophasing should be at the level of λ/10 so that more than 90% of the energy remains in the central peak of the point spread function.² In the current version of SIRIUS, the raw coherencing is done manually by adjusting air delay lines, whereas the cophasing is ensured by a fibered cophasing system. We present our study of an optimized cophasing system that we intend to develop on the SIRIUS testbench. One of the main goals is to be adaptable to any interferometer, whatever the configuration of the entrance pupil and the number of sub pupils. This new version will improve the cophasing system by using a derived version of the dispersed speckles method³ for fine cophasing. The observed images will then be stabilized during a longer period, allowing a more efficient analysis of the studied source.

PRIMA/PACMAN is scheduled for commissioning on Paranal in late 2008 as part of the VLTI. In this paper, we discuss the important aspects of its astrometric data-reduction software. For example, the top-level requirements, interfaces to existing ESO software, data types, data levels, and data flow among the recipes dictate the overall design of any software package. In addition, the complexity of the PACMAN instrument, the long-term nature of astrometric observations, and the need to improve algorithms as the understanding of the hardware improves, impose additional requirements on the astrometric data-reduction software.

The VLTI's PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument is designed to provide tens of microarcseond astrometry and faint-object imaging for the interferometer facility. Astrometry is to be enabled by PRIMA between object pairs that are separated on the sky by one isoplanatic patch (roughly 60 seconds of arc at Cerro Paranal in the K band), with at least one of the two objects being bright (K <10), and a second fainter object (ΔK < 7) that is nominally a 'stable' astrometric reference. The original expectation was for star-star pairs to be observed by PRIMA in its astrometric mode; however, we are exploring the possibility of also utilizing background galaxies in this role. Advantages of such source selection is eliminating the need to solve for dim object parallax and proper motion before obtaining similar values for the bright foreground star. Additionally, data from the galaxy may be of scientific interest as well, potentially leading to characterizations of object morphology at milliarcsecond scales. Towards that end, we have begun observations with ESO's NTT to explore the suitability of qualifying star-galaxy asterisms as potential PRIMA targets. Commissioning observations for PRIMA are slated to begin in Fall of 2008.

We present the recent results in image reconstruction obtained with the University of Cambridge's software package BSMEM (BiSpectrum Maximum Entropy Method). We also evaluate the performance of BSMEM reconstructions for several datasets susceptible to render the reconstruction process harder: with missing power-spectrum or bispectrum points, low Signal-to-Noise ratio on visibility phases or visibility amplitudes, or problematic source morphologies (important amount of resolved flux, centro-symmetry). Interferometer configurations with 4, 6, 8 and 15 telescopes are examined.

MATISSE is the second-generation mid-IR interferometry instrument proposed for ESO's Very Large Telescope Interferometer. MATISSE will combine the beams of up to four UTs or ATs of the VLTI and will allow aperture-synthesis imaging in the L, M, and N bands with a resolution of a few milli-arcseconds. We report on detailed image reconstruction experiments with simulated MATISSE interferograms. Using model images as input for many of our simulations, we study the dependence of the reconstructions on the brightness and size of the target, the uv coverage, and several other parameters.

Classically, optical and near-infrared interferometry have relied on closure phase techniques to produce images. Such techniques allow us to achieve modest dynamic ranges. In order to test the feasibility of next generation optical interferometers in the context of the VLTI-spectro-imager (VSI), we have embarked on a study of image reconstruction and analysis. Our main aim was to test the influence of the number of telescopes, observing nights and distribution of the visibility points on the quality of the reconstructed images. Our results show that observations using six Auxiliary Telescopes (ATs) during one complete night yield the best results in general and is critical in most science cases; the number of telescopes is the determining factor in the image reconstruction outcome. In terms of imaging capabilities, an optical, six telescope VLTI-type configuration and ~200 meter baseline will achieve 4 mas spatial resolution, which is comparable to ALMA and almost 50 times better than JWST will achieve at 2.2 microns. Our results show that such an instrument will be capable of imaging, with unprecedented detail, a plethora of sources, ranging from complex stellar surfaces to microlensing events.

LINC-NIRVANA (LN) is the German-Italian Fizeau beam combiner for the Large Binocular Telescope (LBT), composed of two 8.4-m apertures on a unique mount. It will provide multiple images of the same astrophysical target corresponding to different orientations of the 22.8-m maximum baseline. Starting from the already existing Sofware Package AIRY (a set of IDL-based modules developed within the CAOS "system" and dedicated to simulation and/or deconvolution of single or multiple images), an ad-hoc version has been especially designed for the data that will be obtained with LN. In this paper, we present the resulting Software Package AIRY-LN. Its capabilities, including quick-look methods, methods for specific classes of astronomical objects, PSF extraction, and a blind deconvolution algorithm are detailed. An IDL-licence-free (by means of the IDL Virtual Machine) and observer-oriented version of the whole package (with pre-setted LN image processing parameters) is also presented.

Performed in November 2007 as a part of the MIDI Guaranteed Time Observation exoplanet program, the observation of the hot Jupiter-like exoplanet Gliese 86b constituted the first attempt of exoplanet detection with the VLTI instrument MIDI. It is also a technical achievement as the first VLTI observation using AMBER and MIDI simultaneously. Fringes were obtained for both instruments with the aim to correct the phase in N-band from the dispersion using the fringes in K-band. In N-band, the parent star has an estimated magnitude of 3.8, and a flux ratio planet/star of about 10^-3 is expected. After simulating the effect of the data reduction process of MIDI (EWS), it appears that the theoretical interferometric phase spectrum is a curved-like function with an amplitude (that we call arrow) of about 0.05°. According to the phase spectra of the calibrator HD9362, taken during the first night of observation, we estimate that a precision on the curvature measurement of about 0.33° is currently reached. Consequently, we are at least at a factor 6 from a possible detection. The AMBER data, obtained in parallel, were too noisy to be used to extrapolate and remove the corresponding dispersion in N band at the required level of precision.

In this paper we use coherently integrated visibilities (see separate paper in these proceedings¹) to measure the properties of binary stars. We use only the phase of the complex visibility and not the amplitude. The reason for this is that amplitudes suffer from the calibration effect (the same for coherent and incoherent averages) and thus effectively provide lower accuracy measurements. We demonstrate that the baseline phase alone can be used to measure the separation, orientation and brightness ratio of a binary star, as a function of wavelength.

In this paper we present the status of different experiments set up at Turin Observatory on novel techniques for multiple beam combination, adopting mostly bulk optics. The goal of these experiments is to find the best scheme able to perform efficient fringe tracking operation on a densely populated (N>4) interferometer, while at the same time maximizing optical throughput and sensitivity on faint sources. One of these concepts has been proposed for the VSI fringe tracker (see Corcione et al, this conference). The schemes proposed have also the advantage of being in principle easily adapted to a large number of beams.

The PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument at ESO/VLTI is scheduled for commissioning in late 2008. It is designed for phased-referenced imaging and narrow-angle astrometry. The latter, which is the focus of this paper, may be used for exoplanet detection. A key PRIMA subsystem consists of two fringe sensing units. They employ polarized and dispersive optics to measure cross fluxes and differential phases in five narrow K band channels without the need of delay-line dithering. The differential phases are used to correct the differential delays, which are the primary observables used to determine relative proper motions, relative parallaxes, and planetary orbits. Real optical components are imperfect, which means that systematics will appear in the differential phases. In this paper, we 1) present a closed mathematical form for the differential phase, including small systematic offsets and random errors; 2) perform Monte Carlo simulations to understand how the small systematic offsets and random errors affect the differential phases; and 3) show that delay-line stepping can be used to eliminate the effects of small systematic offsets and random errors.

The implementation of the simultaneous combination of several telescopes (from four to eight) available at Very Large Telescope Interferometer (VLTI) will allow the new generation interferometric instrumentation to achieve interferometric image synthesis with unprecedented resolution and efficiency. The VLTI Spectro Imager (VSI) is the proposed second-generation near-infrared multi-beam instrument for the Very Large Telescope Interferometer, featuring three band operations (J, H and K), high angular resolutions (down to 1.1 milliarcsecond) and high spectral resolutions. VSI will be equipped with its own internal Fringe Tracker (FT), which will measure and compensate the atmospheric perturbations to the relative beam phase, and in turn will provide stable and prolonged observing conditions down to the magnitude K=13 for the scientific combiner. In its baseline configuration, VSI FT is designed to implement, from the very start, the minimum redundancy combination in a nearest neighbor scheme of six telescopes over six baselines, thus offering better options for rejection of large intensity or phase fluctuations over each beam, due to the symmetric set-up. The planar geometry solution of the FT beam combiner is devised to be easily scalable either to four or eight telescopes, in accordance to the three phase development considered for VSI. The proposed design, based on minimum redundancy combination and bulk optics solution, is described in terms of opto-mechanical concept, performance and key operational aspects.

We present the architecture of a software package for simulating an interferometer system. The different components of the interferometer operate in parallel with each other and communicate on regular or semi-regular time-scales. It is important to get this simultaneity correct while simulating the system on a sequential computer. The following paper describes a event-based simulation architecture which makes this possible.

The AMBER instrument, the three beams interferometric combiner of the VLTI, occasionally su.ers from a fringing artifact, called "correlated noise", likely induced by electromagnetic radio frequencies present in the lab. We analyze how this noise affects the extracted visibilities, becoming more important for fainter sources. This unwanted effect can cause an overestimate of the instrumental V² for low flux observations. We have developed a software tool, called "AMBER Detector Cleaner" (AMDC), which successfully removes this artifact and we present here the method on which it is based and some example results. Such software is made available to the community, so that AMBER users can perform optimal data reduction even for faint sources.