Proceedings Volume 7013

Optical and Infrared Interferometry

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Proceedings Volume 7013

Optical and Infrared Interferometry

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Volume Details

Date Published: 14 July 2008
Contents: 40 Sessions, 166 Papers, 0 Presentations
Conference: SPIE Astronomical Telescopes + Instrumentation 2008
Volume Number: 7013

Table of Contents

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Table of Contents

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  • Front Matter: Volume 7013
  • Highlights
  • Status I
  • Science I
  • Status II
  • Systems
  • Subsystems
  • Theses
  • New Projects
  • IO and Fibers
  • Fringe Tracking
  • Phases
  • Imaging
  • Atmosphere
  • Nulling
  • Science IV
  • Instruments
  • Single Apertures
  • New Concepts
  • Astrometry
  • Interferometry from Space: Joint Session with Conference 7010
  • Poster Session: Ground Interferometers
  • Poster Session: Projects
  • Poster Session: Instruments
  • Poster Session: Systems
  • Poster Session: Subsystems
  • Poster Session: New Concepts
  • Poster Session: Optical Technology, Integrated Optics and Fibers
  • Poster Session: Single Telescopes
  • Poster Session: Astrometry
  • Poster Session: Imaging
  • Poster Session: Coherent Integration and Phase Referencing
  • Poster Session: Fringe Tracking
  • Poster Session: Control Software, Data Reduction and Model Fitting
  • Poster Session: Calibration
  • Poster Session: Community
  • Poster Session: Site Testing
  • Poster Session: Science
  • Poster Session: Nulling
  • Poster Session: Space
Front Matter: Volume 7013
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Front Matter: Volume 7013
This PDF file contains the front matter associated with SPIE Proceedings Volume 7013, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Highlights
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Imaging the surface of Altair and a MIRC update
We report the first scientific results from the Michigan Infrared Combiner (MIRC), including the first resolved image of a main-sequence star besides the Sun. Using the CHARA Array, MIRC was able to clearly resolve the well-known elongation of Altair's photosphere due to centrifugal distortion, and was also able to unambiguously image the effect of gravity darkening. In this report, we also show preliminary images of the interacting binary β Lyr and give an update of MIRC performance.
VLTI-AMBER observations of Eta Carinae with the FINITO fringe tracker and spectral resolution 12000
We present interferometric near-infrared observations of the Luminous Blue Variable (LBV) η Car using the Very Large Telescope Interferometer (VLTI) and the AMBER instrument of the European Southern Observatory (ESO). A high spatial resolution of 5 mas (~11.5 AU) and a high spectral resolution R = λ/Δλ=1500 and 12000 were obtained. Some of the data was recorded using the fringe tracker FINITO. The observations were obtained in the wavelength range around both the He I 2.059 μm and the Brγ 2.166 μm emission lines. The AMBER interferograms allow the investigation of the wavelength dependence of η Car's visibility, wavelength-differential phase, and closure phase. If we fit Hillier et al. model visibilities to the observations, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17 μm continuum, the He I, and the Brγ emission lines, respectively. In the continuum, an elongation along a position angle of 120° ± 15° was derived from the visibilities. The VLTI observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the pole.
Status I
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The SUSI instrument: new science and technology
The Sydney University Stellar Interferometer (SUSI) has now been operating with a significanly more sensitive beam combiner system (the Red Table) for several years. A number of novel results from the realms of single, binary and pulsating star astrophysics are presented. Simultaneous dual beam-combiner operation with the Red Table and an upgraded Blue Table has been demonstrated, enabling a high spectral dispersion oberving mode for stellar studies. Plans for a major upgrade are briefly described. One component of this will be remote operation of the array for routine observational data taking, with first steps on this path reported here with the inception of the Sydney Remote Operations Center. The backbone of future plans with SUSI center around the installation of the southern of the twin PAVO instruments. PAVO is also discussed in more detail in Ireland et al, this proceedings.
Recent progress at the MIRA: development of fringe tracking system
The MIRA-I.2 is a 30m baseline optical interferometer located at the Mitaka campus of the National Astronomical Observatory of Japan. After the detection of the first fringes with Vega in 2002, we have continued improvement of system performance and have demonstrated stellar diameter measurement in wide band (600nm-1000nm). Recently, we begin on development of two scientific detectors: spectrometer with separate fringe tracking system and interfeometric polarimetry. Recent progress and performance of our two system is reported.
NPOI: recent progress and future prospects
Donald J. Hutter, James A. Benson, Tim Buschmann, et al.
The instrumental status of the Navy Prototype Optical Interferometer (NPOI) since the last SPIE meeting in 2006 is summarized, along with the results of the current science programs. The commissioning of new stations and plans for greatly increased telescope apertures are discussed, along with other instrumentation upgrades. Recent results in the areas of wide-angle astrometry, binary stars, physical modeling of the circumstellar disks of early-type stars, improvements in coherent averaging, and phase-reference imaging are also reviewed.
Science and technical progress at the Palomar Testbed Interferometer
The Palomar Testbed Interferometer (PTI) is a long-baseline, near-infrared interferometer located on Palomar Mountain. PTI has been in operation since 1997 and is used for extensive science observing as well as technical development. I will review the recent progress at PTI covering both science and instrumentation work. The science topics include fundamental stellar properties, novae, and an evaluation of all calibrator observations. The technical work includes the narrow-angle phase scanning mode and installation of the integrated optics combiner IONIC.
An update on the CHARA array
The CHARA Array is a six telescope optical/IR interferometer run by the Center for High Angular Resolution Astronomy of Georgia State University (GSU) and is located at Mount Wilson Observatory just to the north of Los Angeles, California. The CHARA Array has the largest operational baselines in the world and has been in regular use for scientific observations since 2004. In this paper we give an update of instrumentation improvements, primarily focused on the beam combiner activity. The CHARA Array supports seven beam combiners: CHARA CLASSIC, a two way high sensitivity K/H band system; CLIMB, an upgrade to CLASSIC that includes closure phase measurements; FLUOR, a two way K band high precision system; MIRC, a six way H/K band imaging system; CHAMP, a six way K band fringe tracker; VEGA, a 4 way visible light high spectral resolution system; and PAVO, a 3 way visible light high sensitivity system. The paper will conclude with a brief review of some science results obtained over the last few years.
Science I
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The asymmetric mass-loss of evolved stars
The VLTI is a breakthrough for its capabilities to hunt and study asymmetrical circumstillar environments encountered in many targets experiencing large mass-loss rate. These environements provide important information on the perturbations of the wind carrying the mass and momentum lost by the evolved source. These perturbations are often due to companions, the short or long term interactions of the dying star with a companion encompassing a large diversity of situations, including giant planets. The VLTI is also well-suited to investigate the spatial and spectral developments of transient, explosive phenomena like Novae and some recent results on that subject will be presented.
Status II
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Keck Interferometer nuller update
M. M. Colavita, E. Serabyn, A. J. Booth, et al.
The Keck Interferometer combines the two 10 m Keck telescopes as a long baseline interferometer, funded by NASA, as a joint development among the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Since 2004, it has offered an H- and K-band fringe visibility mode through the Keck TAC process. Recently this mode has been upgraded with the addition of a grism for higher spectral resolution. The 10 um nulling mode, for which first nulling data were collected in 2005, completed the bulk of its engineering development in 2007. At the end of 2007, three teams were chosen in response to a nuller key science call to perform a survey of nearby stars for exozodiacal dust. This key science observation program began in Feb. 2008. Under NSF funding, Keck Observatory is leading development of ASTRA, a project to add dual-star capability for high sensitivity observations and dual-star astrometry. We review recent activity at the Keck Interferometer, with an emphasis on the nuller development.
Recent progress at the Keck Interferometer: operations and V2 science
The Keck Interferometer combines the two 10m diameter Keck telescopes for near-infrared fringe visibility, and mid-infrared nulling observations. We report on recent progress with an emphasis on new visibility observing capabilities, operations improvements for visibility and nulling, and on recent visibility science. New visibility observing capabilities include a grism spectrometer for higher spectral resolution. Recent improvements include a new AO output dichroic for increased infrared light throughput, and the installation of new wave-front controllers on both Keck telescopes. We also report on recent visibility results in several areas including (1) young stars and their circumstellar disks, (2) pre-main sequence star masses, and (3) Circumstellar environment of evolved stars. Details on nuller instrument and nuller science results, and the ASTRA phase referencing and astrometry upgrade, are presented in more detail elsewhere in this conference.
The Very Large Telescope Interferometer: an update
Pierre Haguenauer, Roberto Abuter, Jaime Alonso, et al.
The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8 m Unit Telescopes (UT) and the four 1.8 m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The fourth AT has been delivered to operation in December 2006, increasing the flexibility and simultaneous baselines access of the VLTI. Regular science operations are now carried on with the two VLTI instruments, AMBER and MIDI. The FINITO fringe tracker is now used for both visitor and service observations with ATs and will be offered on UTs in October 2008, bringing thus the fringe tracking facility to VLTI instruments. In parallel to science observations, technical periods are also dedicated to the characterization of the VLTI environment, upgrades of the existing systems, and development of new facilities. We will describe the current status of the VLTI and prospects on future evolution.
Interferometry at mid-infrared wavelengths: the ISI system
C. H. Townes, E. H. Wishnow
The ISI (Infrared Spatial Interferometer) involves three movable telescopes of Pfund design, each in trailers, each with a radiation collecting mirror 65 inches in diameter. The telescopes can be moved, and then set down on concrete pads for stability. They are located on Mt. Wilson, where pads have been arranged to give baselines varying from 4 to approximately 75 meters and oriented in various directions. Initial work, between 1988 and 2002, was done with two telescopes; the third telescope was added in 2003.
Systems
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Polarization fidelity in an optical interferometer
The optical trains of interferometers invariably contain oblique reflections that alter the polarization state of the light from the source. Even for arrays with symmetric optical paths, large systematic visibility errors can be introduced when observing sources with intrinsic polarization. We have identified the key metric for polarization fidelity in an optical interferometer - the diattenuation of the optical train - and we evaluate the visibility penalties incurred by an interferometer that is not optimized for polarimetric purity for a number of different types of polarized source.
Polar-interferometry: what can be learnt from the IOTA/IONIC experiment
Jean-Baptiste Le Bouquin, Karine Rousselet-Perraut, Jean-Philippe Berger, et al.
We report the first near-IR polar-interferometric observations, performed at the IOTA array using its integrated optics combiner IONIC. Fringes have been obtained on calibration stars and resolved late-type giants. Optical modeling of the array and dedicated laboratory measures allowed us to confirm the good accuracy obtained on the calibrated polarized visibilities and closure phases. However, no evidences for polarimetric features at high angular resolution have been detected. The simulations and the results presented here open several perspectives for polar-interferometry, especially in the context of fibered, single-mode combiners.
Adaptive vibration cancellation on large telescopes for stellar interferometry
In stellar interferometry fringe-tracking is a method of stabilizing the Optical Pathlength Difference (OPD) from the observed astronomical source to the instrument detector via different telescopes in an interferometric array. At the ESO Very Large Telescope Interferometer, which includes four 8.2 m class Unit Telescopes (UTs), stabilization to better than a tenth of the observing wavelength is required in order to improve the quality and sensitivity of fringe measurements on the interferometer's scientific instruments. Unfortunately, fast mechanical vibrations due to myriad sources in the observatory infrastructure propagate to various mirrors in the optical path and must be compensated for in real time. Due to its limited bandwidth the fringe tracking loop cannot be used for this purpose. Alternative approaches must therefore be adopted. Vibrations imparted to the primary, secondary and tertiary mirrors of the UTs are currently measured by a grid of suitably placed accelerometers, converted to optical pathlengths and cancelled by a wideband feedforward compensation algorithm to a downstream optical delay line. Although very effective, it is obvious that this system can not compensate for vibrations originating elsewhere on the optical path. We present here an adaptive narrow-band cancellation algorithm that can compensate remaining vibrations measured on the stellar signal on condition that they are sufficiently stable in amplitude and frequency.
Fringe detection laser metrology for differential astrometric stellar interferometers
S. Rabien, S. Gillessen, J. Ziegleder, et al.
Differential measurements with dual feed stellar interferometers using large baselines can deliver extremely accurate astrometric data. Separating the phase difference measured on the stars from the path length differences occurring within the interferometric instrument itself requires the use of laser interferometers. Usually heterodyne differential path techniques are used for nanometer precision measurements. With these methods it is usually difficult to track the full beam size and follow the light path up to the secondary mirror. We will report on the concept and first tests of a differential path metrology system, developed within the GRAVITY project, that allows one to measure the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern created on the telescopes' secondaries. This novel method is almost free from systematic errors since the stellar and laser light are traveling along a common optical path.
Status of the VLTI complementary characterization
The first phase of the VLTI is now almost completed with 8 available telescopes, 2 science instruments and complementary facilities (in particular a fringe tracker and angle tracker). The next step will be the integration of PRIMA and the development of the second generation instruments for which a thorough understanding of the performance that can provided by the VLTI facility is required. This is the basic purpose of this ongoing complementary characterization. The rationale of this activity is recalled and the methods used as well as the results obtained so far presented.
Can laser self-focusing in air replace interferometer siderostats and delay lines?
Recent experiments in high-power lasers show that they modulate of the density of air at long ranges, up to filamentation in a restricted volume. There are two effects: light concentration and plasma filamentation, depending on the laser power. Two such laser-heated volumes can scatter stellar light into a central station, where they are made to interfere in speckled fringes. Usually the density modulations deflect the light only slightly, so the maximum baseline is not extended. However, if either the modulation of the density of air is strong, or its spatial frequency is high, then the stellar beam deflection is significant. In such a case, the scattering volumes can be further off to the sides, and baselines of hundreds of meters can be envisaged.
Subsystems
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Design of the unit telescopes of the MROI
Olivier Pirnay, Pierre Gloesener, Eric Gabriel, et al.
AMOS is in charge of the development of the unit telescopes of the MRO interferometer. Beyond the image quality and the tracking performance, the interferometry necessitates maintaining the optical pathlength between two telescopes sufficiently stable during observation. This paper depicts the detailed design of the telescopes and how it suits to the high end requirements of the system. The distinctive features of the elevation over elevation configuration are also discussed.
The long-stroke MROI vacuum delay lines: from concept to production
Christopher A. Haniff, Roger C. Boysen, David F. Buscher, et al.
We report on test results on the delay line system for the MRO Interferometer, currently under construction in Cambridge, UK. The delay lines are designed to provide 380 metres of vacuum path delay in a single stage, offering rapid star-to-star slews, high throughput and high transmitted wavefront quality. Details of the final design adopted for these delay lines are presented, together with lessons learnt from successful performance tests of the full-scale prototype trolley in a 20-metre long vacuum test rig. Delivery of the first production trolley is expected in New Mexico in early 2009.
The ESPRI Project: differential delay lines for PRIMA
F. Pepe, Didier Queloz, Th. Henning, et al.
ESPRI is a project which aims at searching for and characterizing extra-solar planets by dual-beam astrometry with PRIMA@VLTI. Differential Delay Lines (DDL) are fundamental for achieving the micro-arcseconds accuracy required by the scientific objective. Our Consortium, consisting of the Geneva Observatory, the Max-Planck Institut for Astronomy Heidelberg, and the Landessternwarte Heidelberg, in collaboration with ESO, has built and tested these DDLs successfully and will install them in summer 2008 at the VLTI. These DDLs consist of high quality cat's eyes displaced on a parallel beam-mechanics and by means of a two-stage actuation with a precision of 5 nm over a stroke length of 70 mm. Over the full range, a bandwidth of about 400 Hz is achieved. The DDLs are operated in vacuum. We shall present, in this paper, their design and their exceptional performances.
Theses
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First science with the Keck Interferometer Nuller: high spatial resolution N-band observations of the recurrent nova RS Ophiuchi
We report observations of the nova RS Ophiuchi using the Keck Interferometer Nuller (KIN) taken approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. The KIN operates in N-band from 8 to 12.5 μm in a nulling mode. In this mode the stellar light is suppressed by a destructive fringe, effectively enhancing the contrast of the circumstellar material located near the star. In a second, constructive-fringe mode, the instrument detects primarily the light from the central, bright source. These are the outer and inner spatial regimes, respectively. We will describe the capabilities of the KIN, including these unique modes, and outline how they were key in our discovery that dust was created between nova events. We also show how these first results from the KIN are consistent with Spitzer data. The KIN data show evidence of enhanced neutral atomic hydrogen emission and atomic metals including silicon located in the inner spatial regime (< 4 AU from theWD) relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer spatial region, centered at approximately ~ 17 AU from the WD, that are not found in the inner regime. The KIN and Spitzer data suggest that these emissions were excited in the outer spatial regime before the blast wave reached these regions. We describe the present results in terms of a new model for dust creation in recurrent novae that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star. These data show the power and potential of the nulling technique which has been developed for the detection of Earth-like planets around nearby stars for the Terrestrial Planet Finder Mission and Darwin missions.
Infrared spectro-interferometry of YSOs
The latest generation of infrared long-baseline interferometric instruments combines high spatial resolution with spectroscopic capabilities, enabling fascinating new studies of the AU-scale circumstellar environment around young stellar objects. Here, we present recent investigations, which we conducted using the VLTI instruments AMBER and MIDI and which demonstrate these new observational possibilities. In one study, we combine near- and mid-infrared interferometry (H-/K-/N-band) to constrain the geometry and radial temperature profile of the circumstellar accretion disk around the Herbig Be star MWC147. Using detailed radiative transfer modeling, we find strong evidence for the presence of an optically-thick inner gaseous disk. In another investigation, we used AMBER's medium spectral resolution mode (R = 1500) to study the spatial origin of the hydrogen Brγ line for five Herbig Ae/Be stars, associating the line emission with different physical mechanisms, such as disk winds and magnetospheric accretion. Finally, we present AMBER H- and K-band observations of the close binary star θ 1OrionisC and illustrate the benefits of fitting wavelength-differential visibilities and closure phases. Besides yielding a high observing efficiency, this approach is also insensitive to calibration errors, induced, for instance, by fast changing atmospheric conditions.
Wolf-Rayet stars probed by AMBER/VLTI
Massive stars deeply influence their surroundings by their luminosity and the injection of kinetic energy. So far, they have mostly been studied with spatially unresolved observations, although evidence of geometrical complexity of their wind are numerous. Interferometry can provide spatially resolved observations of massive stars and their immediate vicinity. Specific geometries (disks, jets, latitude-dependent winds) can be probed by this technique. The first observation of a Wolf-Rayet (WR) star (γ2 Vel) with the AMBER/VLTI instrument yielded to a re-evaluation of its distance and an improved characterization of the stellar components, from a very limited data-set. This motivated our team to increase the number of WR targets observed with AMBER. We present here new preliminary results that encompass several spectral types, ranging from early WN to evolved dusty WC. We present unpublished data on WR79a, a massive star probably at the boundary between the O and Wolf-Rayet type, evidencing some Wolf-Rayet broad emission lines from an optically thin wind. We also present new data obtained on γ2 Vel that can be compared to the up-to-date interferometry-based orbital parameters from North et al. (2007). We discuss the presence of a wind-wind collision zone in the system and provide preliminary analysis suggesting the presence of such a structure in the data. Then, we present data obtained on 2 dusty Wolf-Rayet stars: WR48a-b and WR118, the latter exhibiting some clues of a pinwheel-like structure from the visibility variations.
The gas-dust transition region in young stellar objects: a sub-milli-arcsecond view through the eyes of CHARA
Using the sub-milli-arcsecond resolution of the CHARA interferometer array and combining light with the 2-telescope combiner CHARA Classic, we have detected strong near-infrared (NIR) emission interior to the dust-sublimation radius of Herbig Ae stars MWC275 and AB Aur. The large contribution of this emission component, which we argue to be hot gas, to the total NIR spectral energy distribution (SED) is not predicted by current models of the dust evaporation front, indicating that the NIR disk is more complicated than expected. Furthermore, we demonstrate that the structure of the evaporation front in MWC275 is time variable, making single epoch, large uv coverage observations critical to decoding front geometry. With the commissioning of CHARA Michigan Phase Tracker in the summer of 2008, the Michigan Infrared Combiner (a 6 telescope combiner at CHARA) will become an ideal instrument for studying the evaporation front, achieving the required sensitivities to begin the first "true" interferometric imaging of the gas-dust transition region in young stellar objects (YSOs). Here, we summarize results on the evaporation front structure obtained with CHARA Classic and describe future prospects with CHARA MIRC in elucidating morphology of the gas-dust transition region.
New Projects
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Magdalena Ridge Observatory Interferometer: progress toward first light
The Magdalena Ridge Observatory Interferometer is a 10-element 1.4 meter aperture optical and near-infrared interferometer being built at 3,200 meters altitude on Magdalena Ridge, west of Socorro, NM. The interferometer layout is an equilateral "Y" configuration to complement our key science mission, which is centered around imaging faint and complex astrophysical targets. This paper serves as an overview and update on the status of the observatory and our progress towards first light and first fringes in the next few years.
Interest of a double field interferometer at Dome C
B. Valat, F. X. Schmider, B. Lopez, et al.
Long baseline interferometry now faces two main challenges. The first one is the image reconstruction from interferometric data. Indeed, the reduced information on the phase of the object Fourier transform during an observation makes image reconstruction quite critical. The second challenge is the improvements of the overall sensitivity. Strong improvement are expected from double field interferometry : For instance, double field interferometry enables phase referencing which is a way to obtain information on phases. In addition, double field interferometry increases the sensitivity of an interferometer. In this paper we present simulations of double field interferometry. Our simulations take into account; the turbulence conditions, the detection noise, the fringe tracking error. For simulated double field data, we perform an image reconstruction using the coaddition of fringes in the image plane. Since the performances of a double field interferometer are very closely related to and dependent on the site characteristics we studied the results for two different locations: Paranal and Dome C. The comparison shows that Dome C offers much better results, and that it is probably the best site on Earth to build a double-field interferometer.
IO and Fibers
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Silver halide single mode fibers for modal filtering in the middle infrared
Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can help achieve deeper interferometric nulls. Silver halide fibers of composition AgClxBr1-x(0
Integrated optics for nulling interferometry in the thermal infrared
Modal filtering is based on the capability of single-mode waveguides to transmit only one complex amplitude function to eliminate virtually any perturbation of the interfering wavefronts, thus making very high rejection ratios possible in a nulling interferometer. In the present paper we focus on the progress of Integrated Optics in the thermal infrared [6-20μm] range, one of the two candidate technologies for the fabrication of Modal Filters, together with fiber optics. In conclusion of the European Space Agency's (ESA) "Integrated Optics for Darwin" activity, etched layers of chalcogenide material deposited on chalcogenide glass substrates was selected among four candidates as the technology with the best potential to simultaneously meet the filtering efficiency, absolute and spectral transmission, and beam coupling requirements. ESA's new "Integrated Optics" activity started at mid-2007 with the purpose of improving the technology until compliant prototypes can be manufactured and validated, expectedly by the end of 2009. The present paper aims at introducing the project and the components requirements and functions. The selected materials and preliminary designs, as well as the experimental validation logic and test benches are presented. More details are provided on the progress of the main technology: vacuum deposition in the co-evaporation mode and subsequent etching of chalcogenide layers. In addition, preliminary investigations of an alternative technology based on burying a chalcogenide optical fiber core into a chalcogenide substrate are presented. Specific developments of anti-reflective solutions designed for the mitigation of Fresnel losses at the input and output surface of the components are also introduced.
Full integrated beam combiner instrument based on SWIFTS concept
We present how photonics associated with new arising detection technologies is able to provide fully integrated instrument for interferometric beam combination. The feasibility and operation of on-chip beam combiners have been demonstrated now according to various combination schemes. More recently we have proposed a novel detection principle that allows to directly sample and extract the spectral information of the incoming optical signal together with the flux level measurement. The so-called SWIFTS concept that stands for Stationary-Wave Integrated Fourier Transform Spectrometer, is able to provide the full spectral and spatial information recorded simultaneously thanks to a motionless detecting device. Throughout the newly available detection principle that could be considered for SWIFTS implementation, some technologies are even able to provide photo-counting operation that may bring a significant extension of the astrophysical domain of investigation of interferometry. The proposed new concept is applicable either to a fringe tracker instrument with fast and sensitive capabilities, or to a dispersive instrument with high spectral resolution capabilities.
Characterization of integrated optics components for the second generation of VLTI instruments
S. Lacour, L. Jocou, T. Moulin, et al.
Two of the three instruments proposed to ESO for the second generation instrumentation of the VLTI would use integrated optics for beam combination. Several design are studied, including co-axial and multi-axial recombination. An extensive quantity of combiners are therefore under test in our laboratories. We will present the various components, and the method used to validate and compare the different combiners. Finally, we will discuss the performances and their implication for both VSI and Gravity VLTI instruments.
Rapid prototyping of integrated sol-gel devices for astronomical interferometry
A. Ghasempour, D. Alexandre, C. Brites, et al.
Integrated optics is a mature technology with standard applications to telecommunications. Since the pioneering work of Berger et al. 1999 beam combiners for optical interferometry have been built using this technology. Classical integrated optics device production is very expensive and time consuming. The rapid production of devices using hybrid sol-gel materials in conjunction with UV laser direct writing techniques allows overcoming these limitations. In this paper this technology is tested for astronomical applications. We report on the design, fabrication and characterization of multiaxial two beam combiners and a coaxial beam combiner for astronomical interferometry. Different multiaxial two beam combiner designs were tested and high contrast (better than 90%) was obtained with a 1.3 μm laser diode and with an SLD ( λ0 = 1.26 μm, FWHM of 60 nm). High contrast fringes were produced with 1.3 μm laser diode using the coaxial two beam combiner. These results show that hybrid sol-gel techniques produce devices with high quality, allowing the rapid prototyping of new designs and concepts for astronomy.
Fringe Tracking
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Fringe tracking at VLTI: status report
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.
CHARA Michigan phase-tracker (CHAMP): a preliminary performance report
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.
Results from the VLTI-PRIMA fringe tracking testbed
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.
Fringe tracking optimization with 4 beams: application to GRAVITY
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.
Fringe tracking at the MROI
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 and spatial filtering: phase jumps and dropouts
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.
Phases
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Practical coherent integration with the NPOI
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.
Phase referencing in optical interferometry
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.
AMBER closure and differential phases: accuracy and calibration with a beam commutation
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.
Imaging
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Solving the imaging problem with coherently integrated multiwavelength data
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.
MIRA: an effective imaging algorithm for optical interferometry
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: a model fitting software for optical interferometry
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.
Exoplanet studies with CHARA-MIRC
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.
Imaging reconstruction for infrared interferometry: first images of YSOs environment
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.
Wide-field imaging interferometry testbed (WIIT): image construction algorithms
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.
2008 imaging beauty contest
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.
Atmosphere
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LuSci: a lunar scintillometer to study ground layer turbulence
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.
Nulling
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Compensation and optimization of dispersion in nulling interferometry
J. F. P. Spronck, J. W. N. Los, S. F. Pereira
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.
Stabilising a nulling interferometer using optical path difference dithering: an update
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.
Laboratory characterization of the chessboard achromatic phase shifter
Daniel Rouan, D. Pelat, N. Meilard, et al.
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.
PERSEE: description of a new concept for nulling interferometry recombination and OPD measurement
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.
Progress in deep broadband interferometric nulling with the adaptive nuller
Robert D. Peters, Oliver P. Lay, Akiko Hirai, et al.
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.
PERSEE, the dynamic nulling demonstrator: recent progress on the cophasing system
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.
Fine art of computing nulling interferometer maps
F. Hénault
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.
The development and applications of a ground-based fiber nulling coronagraph
Stefan Martin, Eugene Serabyn, Kurt Liewer, et al.
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.
Persee: a nulling demonstrator with real-time correction of external disturbances
F. Cassaing, J. M. Le Duigou, J. P. Amans, et al.
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.
Exoplanet exploration program planet detection test-bed: latest results of planet light detection in the presence of starlight
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.
Earth-like planets: science performance predictions for future nulling interferometry missions
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.
Science IV
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First spectro-interferometry on galactic center sources in the infrared: results and science prospects at the sensitivity limit of current larger aperture arrays
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.
Instruments
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VEGA: a new visible spectrograph and polarimeter on the CHARA Array
D. Mourard, K. Perraut, D. Bonneau, et al.
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.
Sensitive visible interferometry with PAVO
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.
A high-sensitivity near-infrared science combiner for MROI
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: the Fizeau interferometer for the Large Binocular Telescope
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.
The LINC-NIRVANA fringe and flexure tracking system
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.
Status of the LBT interferometer
Philip M. Hinz, Teresa Bippert-Plymate, Andy Breuninger, et al.
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.
VSI: the VLTI spectro-imager
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.
GRAVITY: getting to the event horizon of Sgr A*
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: perspective of imaging in the mid-infrared at the VLTI
B. Lopez, P. Antonelli, S. Wolf, et al.
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.
Single Apertures
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Beam combination with a large number of apertures
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.
New Concepts
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Toward a revival of stellar intensity interferometry
Stephan LeBohec, Cesare Barbieri, Willem-Jan de Wit, et al.
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.
Novel spectral imaging method for Fizeau interferometer
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.
Application of Michelson type bolometric interferometer to CMB B mode polarization observations
Makoto Hattori, Izumi S. Ohta, Yuji Chinone, et al.
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.
Astrometry
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Spectral calibration at the picometer level on SCDU (spectral calibration development unit)
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.
The ESPRI project: astrometric exoplanet search with PRIMA
R. Launhardt, Didier Queloz, Th. Henning, et al.
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.
Astrometric detection of exo-Earths in the presence of stellar noise
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.
Interferometry from Space: Joint Session with Conference 7010
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Precision astrometry with a space-based interferometer
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-Lite: progress report
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.
Terrestrial Planet Finder Interferometer: 2007-2008 progress and plans
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.
Terrestrial exo-planet science by nulling interferometry: instrument design and scientific performance
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.
New observational concept for Darwin-like missions using a MOEMS-based programmable spectrometer
Frederic Zamkotsian, Patrick Lanzoni, Thierry Viard, et al.
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): a review, progress report, and update
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.
ESPRIT: a study concept for a far-infrared interferometer in space
W. Wild, Th. de Graauw, F. Helmich, et al.
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).
The wide-field imaging interferometry testbed (WIIT): recent progress and results
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, a diluter aperture visible nulling coronagraphic instrument
M. Shao, S. Bairstow, B. Martin Levine, et al.
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.
Poster Session: Ground Interferometers
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Plans for utilizing the Keck Outrigger Telescopes at NPOI
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.
Last technology and results from the IOTA interferometer
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.
Poster Session: Projects
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Optical delay line system for the NIAOT Prototype Stellar Interferometer
Zhen Wu, Yongtian Zhu, Yi Chen, et al.
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.
Poster Session: Instruments
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System overview of the VLTI Spectro-Imager
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.
Prospects for near-infrared characterisation of hot Jupiters with the VLTI Spectro-Imager (VSI)
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.
Near-IR spectrograph of VSI (VLTI Spectro Imager): dispersing the light from an integrated optics beam-combiner
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.
Opto-mechanical design of the spectrometers of GRAVITY: the 6-Baseline K-Band Interferometer for the VLTI
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: concept analysis
S. Lagarde, B. Lopez, R. G. Petrov, et al.
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].
Preparation of the calibration unit for LINC-NIRVANA
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.
Analysis of LBT LINC-NIRVANA simulated images of galaxies and young stellar objects
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 observation preparation software: a flexible approach
Alexey Pavlov, Jan Trowitzsch, Wolfgang Gässler, et al.
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.
The LINC-NIRVANA Fringe and Flexure Tracker: the testbed interferometer
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.
The LINC-NIRVANA Fringe and Flexure Tracker: testing piston control performance
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.
NIC: LBTI's nulling and imaging camera
Philip M. Hinz, Elliott Solheid, Oli Durney, et al.
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.
LMIRcam: an L/M-band imager for the LBT combined focus
John C. Wilson, Philip M. Hinz, M. F. Skrutskie, et al.
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.
Poster Session: Systems
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Adaptive optics for the CHARA Array
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.
General performance analysis of a Fizeau interferometer
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.
Innovative pupil topographies for sparse aperture telescopes and SNR
James Breckinridge, Nevin Bryant, John Lorre
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.
Poster Session: Subsystems
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Simultaneous observation of two stars using the PRIMA Star Separator
J. Nijenhuis D.V.M., H. Visser, H. de Man, et al.
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.
MROI's automated alignment system
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.
Poster Session: New Concepts
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Steps toward hypertelescopes on Earth and in space
A. Labeyrie, H. Le Coroller, J. Dejonghe
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.
Poster Session: Optical Technology, Integrated Optics and Fibers
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Mount-induced deflections in 8-inch flat mirrors at the Navy Prototype Optical Interferometer
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.
Extracting the zero-gravity surface figure of a mirror
E. E. Bloemhof, J. C. Lam, V. A. Feria, et al.
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.
The Magdalena Ridge Observatory Interferometer: custom near-IR beamsplitter and AR coatings
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.
Developing high-performance reflective coatings for the tunable filter and the high-order interferometer of the 3D-NTT
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.
Far-Infrared Interferometeric Telescope Experiment (FITE): sensor optics
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.
Single-mode LiNbO3-based waveguides for L-band interferometry
Guillermo Martin, Eric Anselm, Thibaut Seure, et al.
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.
Characterization of silver halide fiber for modal filtering on mid-infrared: imaging, nulling, and spectral transmission
R. Grille, L. Labadie, P. Kern, et al.
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.
Poster Session: Single Telescopes
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Optical design of the Dragonfly interferometric imager
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.
Phase mask coronagraphy: use of a Mach-Zehnder interferometer for achromatic four-quadrant phase masks
Alexis Carlotti, Claude Aime, Gilbert Ricort
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.
IRAN: laboratory test bench for hypertelescope pupil-plane recombination
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.
Study of a new cophasing system for hypertelescopes
Nassima Tarmoul, Denis Mourard, François Hénault
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.
Poster Session: Astrometry
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The astrometric data-reduction software for exoplanet detection with PRIMA
Nicholas M. Elias II, Rainer Köhler, Ingo Stilz, et al.
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.
Extragalactic reference targets for PRIMA
Gerard T. van Belle, Roberto Abuter, Judith Ngoumou, et al.
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.
Poster Session: Imaging
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Image reconstruction at Cambridge University
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.
Aperture-synthesis imaging with the mid-infrared instrument MATISSE
Karl-Heinz Hofmann, Matthias Heininger, Walter Jaffe, et al.
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.
Phase closure image reconstruction for future VLTI instrumentation
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.
AIRY-LN: an ad-hoc numerical tool for deconvolution of images from the LBT instrument LINC-NIRVANA
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.
Poster Session: Coherent Integration and Phase Referencing
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First step to detect an extrasolar planet using simultaneous observations with the two VLTI instruments AMBER and MIDI
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.
Measurements of binary stars with coherent integration of NPOI data
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.
Poster Session: Fringe Tracking
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Multiple beam combination experiments for fringe tracking on next generation interferometers
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.
Simulations of imperfect PRIMA fringe sensing units and calibration strategies
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.
Fringe tracker for the VLTI spectro-imager
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.
Event-based simulation of fringe tracking in an optical interferometer
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.
Poster Session: Control Software, Data Reduction and Model Fitting
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AMDC: de-biasing AMBER visibilities for limiting magnitude sources
G. Li Causi, S. Antoniucci, E. Tatulli
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.
“Advanced” data reduction for the AMBER instrument
The amdlib AMBER data reduction software is meant to produce AMBER data products from the raw data files that are sent to the PIs of different proposals or that can be found in the ESO data archive. The way defined by ESO to calibrate the data is to calibrate one science data file with a calibration one, observed as close in time as possible. Therefore, this scheme does not take into account instrumental drifts, atmospheric variations or visibility-loss corrections, in the current AMBER data processing software, amdlib. In this article, we present our approach to complement this default calibration scheme, to perform the final steps of data reduction, and to produce fully calibrated AMBER data products. These additional steps include: an overnight view of the data structure and data quality, the production of night transfer functions from the calibration stars observed during the night, the correction of additional effects not taken into account in the standard AMBER data reduction software, and finally, the production of fully calibrated data products. All these new features are implemented in the modular pipeline script amdlibPipeline, written to complement the amdlib software.
The VLTI real-time reflective memory data streaming and recording system
The VLTI control architecture is based on a real time distributed system involving dozens of specialized computers. Several control loops are required to run the VLTI, e.g. for fringe tracking, angle tracking, injection optimization and vibration cancellation. These control systems rely on a low latency, deterministic shared memory mechanism. It communicates in the form of a close ring, which includes all devices involved in those loops. Through this ring, sensor data, intermediate filtered signals, final actuator set-points and feedbacks flow at rates up to 8 kHz. Data in this ring can be consumed by any node asynchronously. In many cases, those signals are also the astronomical observable (e.g. the beam combiner fluxes for astrometry) or are used offline, in order to improve the quality of the scientific data reduction and to debug the system. With the purpose of relieving the control applications of the simultaneous need to record their signals, a centralized generic recording device has been designed and implemented at the VLTI. In this paper, we describe its architecture and show that by over-sampling, streaming and posterior filtering on a separate computer it is possible to overcome the asynchronous nature of the system. We demonstrate that it is feasible to capture data in real time, verify time reference consistency and store on disk at rates up to ~50 Mbit/s, fulfilling the current VLTI requirements.
IVC: a simulation and model-fitting tool for optical-IR interferometry
I present a new software tool, called "Interferometry Visibility Computations" (IVC), for sparse uv-coverage optical/IR interferometry. It is aimed to compute interferometric observables from a large grid of geometrical toy models, or user supplied multi-wavelength radiative transfer simulations. It is designed to prevent aliases at high spatial frequencies when computing visibilities from imaging models and to perform multi-baseline model fitting taking into consideration various instrumental effects. The program can be called from any user's code to allow simultaneous model fitting of different types of observations, in order to compare e.g. interferometric and spectroscopic observations at the same time.
Software and control for the Magdalena Ridge Observatory interferometer delay lines
John Young, Roger Boysen, David Buscher, et al.
The delay lines for the Magdalena Ridge Observatory Interferometer (MROI) will provide remote control of optical delays of up to 380m with sub-wavelength precision in vacuum. The delay-line prototype is now fully functional, all features having been demonstrated in a 20m long evacuated test rig. We describe the architecture, design and performance of the delay line software: this features distributed real-time control and flexible remote logging of diagnostic data from the delay line hardware components at up to 5 kHz.
Poster Session: Calibration
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Pipeline reductions of AMBER calibrator data
Service (and visitor) mode operations by ESO involving the three-beam NIR combiner AMBER on the VLTI are now in their third year, and a large number of observations of calibrators have been accumulated. We present results on the stability of the transfer function, with and without the FINITO fringe tracker, as well as trends and correlations. The reductions were made using publicly available software for AMBER and therefore can be used as a reference for the data quality independent observers may expect from AMBER.
VLTI behind the scene: 1001 nights of data flow operations at ESO
Since the beginning of the Paranal Service Mode observations, the DFO group is dedicated in monitoring the VLT/VLTI instruments, in controlling the quality of the data and in providing the principal investigators (PI) with reduced calibrated data whenever possible. Each year, new instruments are coming into operations, bringing more complex challenges for our Data Flow Operations. We will briefly present the VLTI data flow operations implemented for the first VLTI instrument VINCI, then we will focus on the monitoring of the instruments MIDI and AMBER. For each of these instruments, basics parameters such as the detector, the instrument alignment are monitored. Calibrations are also processed either for to monitor the stability of the instrument or/and to be applied to the science data. We also developed more complex procedures to follow the behavior of the subsystems such as MACAO (adaptive optics), IRIS (fast guiding), FINITO (fringe tracker). Some procedures have been developed to monitor the instrumental Transfer Function for the different instrument setups and configurations will be shown. To understand the Transfer Function, it is important to have a good knowledge of the objects (calibrators) used for the measurement. We will discuss the issue of interferometric calibrators and present the work on this subject done specifically for VLTI. In 2008, PRIMA facility will start operations. Its astrometry mode will be commissioned. Like each new VLT/VLTI instrument this PRIMA astrometric mode will be supported by our group.
Poster Session: Community
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Cost estimate for the Kilometric Optical Interferometer (KOI)
We present a parametric cost estimate for the Kilometric Optical Interferometer (KOI) in a classical array configuration: 24 telescopes, 4-meter primary mirror, up to 1 km baseline. The parametric cost estimate is based on available cost information from the Magdalena Ridge Observatory (MRO) Interferometer at New Mexico Tech. A Kilometric Optical Interferometer based on a classical array concept has an estimated construction cost between $1B and $3B if it would be built today (2008 dollars and technology). The implication of the estimated construction cost is that cost reductions are critical in the planning phase to bring the cost within a reasonable envelope. Hence we propose to set a budget ceiling that seems feasible given the support to be expected from the scientific community and funding agencies. Given a budget ceiling, a design-to-cost process should be followed. We propose to set a construction phase budget cap of $800M (2008 dollars) for KOI as an initial goal. Narrowing down of the science goals in combination with technology development to reduce cost and technological complexity are the main areas of activities for the next decade. We propose to establish a virtual project office to coordinate these activities.
A status report of the working group on calibrators of IAU Commission 54
Since its implementation by the IAU General Assembly in Prague, IAU Commission 54 (Optical and Infrared Interferometry) has spawned several working groups on scientific and technical issues common to all interferometers. Work on these topics is therefore of benefit to the community, and we will report on the status of the working group on interferometric calibrators. Active tasks include resources and tools necessary to make informed decisions on calibrator selections, as well as feedback mechanisms on suitability of calibrators for specific purposes.
Recent developments in optical interferometry data standards
A working group on interferometry data standards has been established within IAU Commission 54 (Optical/ Infrared Interferometry). The working group includes members representing the major optical interferometry projects worldwide, and aims to enhance existing standards and develop new ones to satisfy the broad interests of the optical interferometry community. We present the initial work of the group to enhance the OIFITS data exchange standard, and outline the software packages and libraries now available which implement the standard.
Poster Session: Site Testing
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Interferential seeing monitor
We present here a new approach to estimate the astronomical seeing which is a fundamental parameter in high angular resolution, in adaptive optics and site testing. Based on this approach, we developed seeing monitor, called Interferential Seeing Monitor (hereafter ISM). The principle of the ISM is based on the study of the diffraction-interference pattern produced by a Young's double-slit in a telescope focus. From the shape of that pattern, we determine the phase difference between the diffracted light rays that meet on the image plane. Then, the phase structure function is calculated which leads to the seeing value.
PropHAn: horizontal propagation sensor for interferometry at Dome C
Hervé Trinquet, Farrokh Vakili, Guillaume Petitjean, et al.
To prepare long baseline interferometric arrays with large telescopes, at Dome C on the Antarctic plateau, we have to know the effect of the strong turbulent surface layer on the wave front propagation as sensed by two telescopes. The main limit of long baseline interferometer is the phase fluctuations, induced by the optical turbulence above each telescope and towarsds the focal beam combiner. PropHAn (Horizontal Propagation in Antarctica) is an instrument to study the optical turbulence effect on the horizontal propagation. PropHAn is designed to retrieve the phase fluctuations between two different horizontal paths of a coherent laser beam. It is a Michelson periscopic interferometer with a variable baseline from 10 cm up to 1 m. The fringe pattern is recorded on a fast CCD camera to freeze the turbulent motions. The main goal of PropHAn is to test a simple interferometric table in Antarctic conditions, and to provide statistics on the turbulent coherence time and the differential pistonmode for a 1 m baseline. These results, in complement with the results provided by DIMM, C2N balloons profiles and Single Star Scidar measurements, would be required to design long baseline interferometers and fringe tracker at Dome C.
Poster Session: Science
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Study of the science capabilities of PRIMA in the Galactic Center
The Phase-Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) facility is scheduled for installation in the Very Large Telescope Interferometer observatory in Paranal, Chile, in the second half of 2008. Its goal is to provide an astrometric accuracy in the micro-arcsecond range. High precision astrometry can be applied to explore the dynamics of the dense stellar cluster. Especially models for the formation of stars near super massive black holes or the fast transfer of short-lived massive stars into the innermost parsec of our galaxy can be tested. By measuring the orbits of stars close the the massive black hole one can probe deviations from a Keplerian motion. Such deviations could be due to a swarm of dark, stellar mass objects that perturb the point mass solution. At the same time the orbits are affected by relativistic corrections which thus can be tested. The ultimate goal is to test the effects of general relativity in the strong gravitational field. The latter can be probed with the near infrared flares of SgrA* which are most likely due to accretion phenomena onto the black hole. We study the expected performance of PRIMA for astrometric measurements in the Galactic Center based on laboratory measurements and discuss possible observing strategies.
Prospects for observing the Galactic Center: combining LBT LINC-NIRVANA observations in the near-infrared with observations in the mm/sub-mm wavelength domain
Andreas Eckart, Gunther Witzel, Devaky Kunneriath, et al.
As a near-infrared (NIR) wide field interferometric imager offering an angular resolution of about 10 milliarcseconds LINC-NIRVANA at the Large Binocular Telescope will be an ideal instrument for imaging the center of the Milky Way especially in conjunction with mm/sub-mm interferometers like CARMA, ATCA or, in the near future, ALMA. Sagittarius A* (Sgr A*) is the electromagnetc manifestation of the ~4×106M super-massive black hole (SMBH) at the Galactic Center. First results from a mult-wavelength campaign focused on Sgr A*, based on the VLT and on CARMA, ATCA, and the IRAM 30m-telescope, in May 2007 show that the NIR data are consistent with partially depolarized non-thermal emission from confined hot spots in relativistic orbits around SgrA*. A 3mm flare following a May 2007 NIR flare is consistent with SSC emission from adiabatically expanding plasma in a wind or jet. With the LBT and ALMA we will be able to study the spectral evolution of NIR/sub-mm/mm flare emission in order to constrain the emission mechanism, the jet/wind physics, and possibly determine the angular momentum of the SMBH. LINC/NIRVANA will also serve to investigate the stellar population and dynamics in the cluster surrounding Sgr A*. A particular emphasis will lie on examining dust embedded and young stars and to unravel the star formation history in the cluster. For the 0.3 parsec core radius central star cluster the investigation of will be investigated.
Mid-infrared view of cool evolved stars with the Very Large Telescope Interferometer
Two interferometric instruments at ESO's Very Large Telescope Interferometer (VLTI) - MIDI and AMBER operating in the mid-infrared (8-13 μm) and the near-infrared (JHK), respectively - have proven to be very powerful to study the physical properties of the circumstellar material around evolved stars. With the "spectro-interferometric" capability of MIDI and AMBER, we can disentangle spectral and spatial information on the observed object. VLTI observations have confirmed our pictures on the circumstellar environment of cool evolved stars in some cases but brought about entirely unexpected pictures in other cases. Here, we present our recent results obtained with VLTI/MIDI.
Science case for 1 mas spectro-imagining in the near-infrared
We present the work developed within the science team of the Very Large Telescope Interferometer Spectro-Imager (VSI) during the Phase A studies. VSI aims at delivering ~ 1 milliarcsecond resolution data cubes in the near-infrared, with several spectral resolutions up to 12 000, by combining up to 8 VLTI telescopes. In the design of an instrument, the science case plays a central role by supporting the instrument construction decision, defining the top-level requirements and balancing design options. The overall science philosophy of VSI was that of a general user instrument serving a broad community. The science team addressed themes which included several areas of astrophysics and illustrated specific modes of operation of the instrument: a) YSO disks and winds; b) Multiplicity of young stars; c) Exoplanets; d) Debris disks; e) Stellar surface imaging; f) The environments of evolved stars; g) AGN tori; h) AGN's Broad Line Region; i) Supermassive black-holes; and j) Microlensing. The main conclusions can be summarized as follows: a) The accessible targets and related science are extremely sensitive to the instrument limiting magnitude; the instrument should be optimized for sensitivity and have its own fringe tracker. b) Most of the science cases are readily achievable with on-axis fringe tracking, off-axis fringe tracking enabling extra science. c) In most targets (YSOs, evolved stars and AGNs), the interpretation and analysis of circumstellar/nuclear dust morphology requires direct access to the gas via spectral resolved studies of emission lines, requiring at least a spectral resolution of 2 500. d) To routinely deliver images at the required sensitivity, the number of telescopes in determinant, with 6 telescopes being favored. e) The factorial increase in the number of closure phases and visibilities, gained in a single observation, makes massive surveys of parameters and related science for the first time possible. f) High dynamic range imaging and very high dynamic range differential closure phase are possible allowing the study of debris disks and characterization of pegasides. g) Spectro-imaging in the near-infrared is highly complementary to ALMA, adaptive optics and interferometric imaging in the thermal infrared.
Poster Session: Nulling
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Tests of achromatic phase shifters performed on the SYNAPSE test bench: a progress report
The achromatic phase shifter (APS) is a component of the Bracewell nulling interferometer studied in preparation for future space missions (viz. Darwin/TPF-I) focusing on spectroscopic study of Earth-like exo-planets. Several possible designs of such an optical subsystem exist. Four approaches were selected for further study. Thales Alenia Space developed a dielectric prism APS. A focus crossing APS prototype was developed by the OCA, Nice, France. A field reversal APS prototype was prepared by the MPIA in Heidelberg, Germany. Centre Spatial de Liege develops a concept based on Fresnel's rhombs. This paper presents a progress report on the current work aiming at evaluating these prototypes on the Synapse test bench at the Institut d'Astrophysique Spatiale in Orsay, France.
Real-time estimation of exoplanetary systems astrometry and spectroscopy from nulling interferometry data
Adrian R. Belu, Céline Theys-Ferrari, Henri Lantéri
Several nulling signal processing techniques have been recently developed: CLEAN (Draper et al 2006) or Bayesian (Théibaut and Mugnier 2006, Marsh et al 2006). The days-long signal acquisition time required for exoplanet detection and characterization opens the possibility for "on the go" processing of the signal as it is being acquired. This permits dynamic observation strategy alteration, thus improved mission performance through optimal time assignment to targets. We have developed an implementation of the algorithm by Théibaut and Mugnier that is optimized for synchronous execution with the arrival of observation data. This will enable to investigate the behavior of the detection for planet layouts that may prove challenging, as well as to investigate the performance of a dynamic tuning of the regularization parameters. It is an additional step towards the end-to-end simulations required for validation of the overall performance of these observatories.
High dynamic range interferometric observations of exozodiacal discs: performance comparison between ground, space, and Antarctica
The possible presence of large amounts of exozodiacal dust around nearby main sequence stars represents a threat to the detection and characterisation of Earth-like extrasolar planets with future infrared space interferometers such as DARWIN or TPF. In this paper, we first review the current detection capabilities of ground-based infrared interferometers such as CHARA/FLUOR and the detections of hot dust that have been obtained so far around a few main sequence stars. With the help of realistic instrumental simulations, we then discuss the relative merits of various ground-based sites (temperate and Antarctic) versus space-based observatories for the detection of exozodiacal discs down to a few zodi by interferometric nulling as a preparation to future life-finding missions. In particular, we discuss the performance of four proposed nulling interferometers: GENIE, ALADDIN, PEGASE and FKSI. An optimised strategy for the characterisation of candidate DARWIN/TPF targets is finally proposed.
Breadboarding an achromatic phase shifter for mid-infrared nulling interferometry
Direct detections of Earth-like extrasolar planets are extremely challenging and require to overcome the huge brightness contrast between two sources that have a very small angular separation. One possible solution to this problem is nulling interferometry at mid-infrared wavelengths where the flux ratio between host star and planet is more favorable than in the visible. The beams of an array of telescopes are combined so that the light from the on-axis direction (the star) is canceled by destructive interference, while the light from an off-axis direction (the planet) is kept. The global performance of such a system depends strongly on the accuracy and stability of the achromatic phase shift and the beam combination. To assess the technological feasibility of this technique, the European Space Agency (ESA) and IAS Paris have initiated a study of different physical concepts and technical realizations of achromatic phase shifters (APS) that fulfill the following requirements: allowing a >10-6 rejection rate or better over a wavelength range 6-20μm and providing a transmission better than 95%. MPIA, in collaboration with the Kayser-Threde GmbH in Munich and the IOF Fraunhofer institute for applied optics in Jena has breadboarded and studied a phase shifter that is based on the geometric reversal of the electric field vectors (pupil flip) at two successive antisymmetric 90 degree reflections. In this paper we describe the bread-boarded phase-shifter device and the results of our characterization measurements in the Lab.
Poster Session: Space
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Overview of the SIM PlanetQuest Light mission concept
The Space Interferometry Mission PlanetQuest Light (or SIM-Lite) is a new concept for a space borne astrometric instrument, to be located in a solar Earth-trailing orbit. SIM-Lite utilizes technology developed over the past ten years for the SIM mission. The instrument consists of two Michelson stellar interferometers and a precision telescope. The first interferometer chops between the target star and a set of Reference stars. The second interferometer monitors the attitude of the instrument in the direction of the target star. The telescope monitors the attitude of the instrument in the other two directions. SIM-Lite will be capable of one micro-arc-second narrow angle astrometry on magnitude 6 or brighter stars, relative to magnitude 9 Reference stars in a two degree field. During the 5 year mission, SIM-Lite would search 65 nearby stars for planets of masses down to one Earth mass, in the Habitable Zone, which have orbit periods of less than 3 years. SIMLite will also perform global astrometry on a variety of astrophysics objects, reaching 4.5 micro-arc-seconds absolute position and parallax measurements. As a pointed instrument, SIM-Lite will be capable of achieving 8 micro-arc-second astrometric accuracy on 19th visual magnitude objects and 15 micro-arc-second astrometric accuracy on 20th visual magnitude objects after 100 hours of integration. This paper will describe the instrument, how it will do its astrometric measurements and the expected performance based on the current technology.
SIM-Lite: status of the engineering progress toward flight
We present an overview of the ongoing progress towards flight readiness of the SIM project. We summarize the engineering milestones that have been completed in the last two years, namely: the Brass-Board Internal and External Metrology Beam Launchers, the Brass-Board Metrology Source, and the Instrument Communication Hardware/Software Architecture Demonstration. We also show other progress such as: the life test of the bass-screw and PZT actuators, building the Metrology Fiducials and the Single Strut Test Article. We status the ongoing work on the Brass-Board Fast Steering Mirror and the Brass-Board Astrometric Beam Combiner. We end with a proposed path towards finishing the Brass-Board suite.
Metrology optical power budgeting in SIM using statistical analysis techniques
The Space Interferometry Mission (SIM) is a space-based stellar interferometry instrument, consisting of up to three interferometers, which will be capable of micro-arc second resolution. Alignment knowledge of the three interferometer baselines requires a three-dimensional, 14-leg truss with each leg being monitored by an external metrology gauge. In addition, each of the three interferometers requires an internal metrology gauge to monitor the optical path length differences between the two sides. Both external and internal metrology gauges are interferometry based, operating at a wavelength of 1319 nanometers. Each gauge has fiber inputs delivering measurement and local oscillator (LO) power, split into probe-LO and reference-LO beam pairs. These beams experience power loss due to a variety of mechanisms including, but not restricted to, design efficiency, material attenuation, element misalignment, diffraction, and coupling efficiency. Since the attenuation due to these sources may degrade over time, an accounting of the range of expected attenuation is needed so an optical power margin can be book kept. A method of statistical optical power analysis and budgeting, based on a technique developed for deep space RF telecommunications, is described in this paper and provides a numerical confidence level for having sufficient optical power relative to mission metrology performance requirements.
An introduction to the Guide-2 telescope testbed for the SIM Planet Quest Light mission
Inseob Hahn, J. Sandhu, M. Weilert, et al.
The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror (FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet) system.
Brassboard Astrometric Beam Combiner (ABC) development for the Space Interferometry Mission (SIM)
The Astrometric Beam Combiner (ABC) is a critical element of the Space Interferometry Mission (SIM) that performs three key functions: coherently combine starlight from two siderostats; individually detect starlight for angle tracking; and disperse and detect the interferometric fringes. In addition, the ABC contains: a stimulus, cornercubes and shutters for in-orbit calibration; several tip/tilt mirror mechanisms for in-orbit alignment; and internal metrology beam launcher for pathlength monitoring. The detailed design of the brassboard ABC (which has the form, fit and function of the flight unit) is complete, procurement of long-lead items is underway, and assembly and testing is expected to be completed in Spring 2009. In this paper, we present the key requirements for the ABC, details of the completed optical and mechanical design as well as plans for assembly and alignment.
SIM PlanetQuest spectral calibration development unit beam combiner
The beam combiner of an astronomical long-baseline interferometer combines the two beams of starlight to form white-light fringes. We describe beam combiner in the SIM PlanetQuest Spectral Calibration Development Unit (SCDU). In addition to forming white light fringes, the beam combiner provides other functions such as separating the light for guiding, fringe tracking, and science measurement. It is designed to function over the optical bandpass 450-950 nm. Coating design is critical to beam combiner as residual dispersion and mismatches affect the ability to accurately measure the position of stars of varying spectral types.
Picometer accuracy white light fringe modeling for SIM PlanetQuest spectral calibration development unit
C. Zhai, J. Yu, M. Shao, et al.
The SIM PlanetQuest Mission will perform astrometry to one microarcsecond accuracy using optical interferometers requiring optical path delay difference (OPD) measurements accurate to tens of picometers. Success relies on very precise calibration. Spectral Calibration Development Unit (SCDU) has been built to demonstrate the capability of calibrating spectral dependency of the white light fringe OPD to accuracy better than 20pm. In this article, we present the spectral calibration modeling work for SCDU to achieve the SIM PlanetQuest Engineering Milestone 4. SCDU data analysis shows that the wave front aberrations cause the instrument phase dispersions to vary by tens of nanometers over the bandwidth of a CCD pixel making the previous model inadequate. We include the effect of the wave front aberrations in the white light fringe model and develop a procedure for calibrating the corresponding model parameters using long stroke fringe data based on Discrete Fourier Transform. We make the calibration procedure flight traceable by dividing the whole calibration into the instrument calibration and the source spectral calibration. End-to-end simulations are used to quantify both the systematic and random errors in spectral calibration. The efficacy of the calibration scheme is demonstrated using the SCDU experimental data.
Achievements of picometer performance from interferometer spectral calibration development unit (SCDU)
This paper will present the analysis results taken from a well-designed interferometer SCDU. The objective is to deliver picometer performance to meet the allocated astrometric error budget from SIM PlanetQuest mission. It will describe the validation of optical designs and analysis procedures to achieve high accuracy of the tip-tilt and shear alignments. Then it will enumerate environmental factors essential to the SCDU performances. Finally it will report color-independent 3 picometer Narrow Angle (NA) performance and all-in-one 17 picometer NA performance. The all-in-one pico-performance will require spectral calibration modeling to remove delay differential induced by color.
Certification of the full size double corner cube fiducials for the Space Interferometer Mission-PlanetQuest test bed
Jan Burke, Katie L. Green, Nasrat Raouf, et al.
A full size Double Corner Cube (DCC) assembly was delivered recently to NASA's Space Interferometer Mission (SIM) PlanetQuest testbed at JPL. The DCC was developed at CSIRO's Australian Centre for Precision Optics (ACPO) to demonstrate the fabrication of the flight size DCC fiducials. The DCC was assembled from three 30°, high precision ULE glass wedges and a 132 mm diameter base plate. After alignment to sub arc-second angular tolerances, the three wedges were chemically bonded to the base-plate. Comprehensive testing was performed on the assembly to certify the compliance of several parameters including the dihedral angle errors, figure of all reflecting surfaces and the Non Common Vertex Error (NCVE) of the DCC. This paper elaborates on some of the metrology and the certification results of the delivered DCC assembly as well as the chemical bond strength tests.
The mask designs for Space Interferometer Mission (SIM)
The Space Interferometer Mission (SIM) consists of three interferometers (science, guide1, and guide2) and two optical paths (metrology and starlight). The system requirements for each interferometer/optical path combination are different and sometimes work against each other. A diffraction model is developed to design and optimize various masks to simultaneously meet the system requirements of three interferometers. In this paper, the details of this diffraction model will be described first. Later, the mask design for each interferometer will be presented to demonstrate the system performance compliance. In the end, a tolerance sensitivity study on the geometrical dimension, shape, and the alignment of these masks will be discussed.
Analysis of planet effect of reference star on searching for Earth-like planets
It is a challenging task to find exoplanets because of the huge contrast between star and planets in mass and in brightness. It is more challenging to determine the masses of exoplanets because it requires extremely high astrometric accuracy. In particular detection of Earth-like planets needs sub-microarcsecond (µas) precision which is possible only by narrow-angle astrometry via the SIM PlanetQuest mission. The narrow-angle observation mode of SIM PlanetQuest requires several distant reference stars, which are close enough in the sky around the target, typically within one degree. This paper provides statistical estimates of available reference stars for all candidate stars in searching for Earth-like planets. It is inevitable that some of reference stars will have binary components and planets. This paper describes the analysis techniques and various error estimates for binary jitters and planet effects of reference stars. Because of the limited number and duration of observations of SIM PlanetQuest, the Monte Carlo simulations indicate that certain long period planets around reference stars may not be detected. Earth-like planets around target stars, however, can be detected unambiguously. Finally, we demonstrate the current best estimates of instrument error, photon noise, reference stars planet disturbance, stellar jitters, etc., and conclude that the orbits of Earth-like planets with sub-μas astrometric signatures can be determined accurately by SIM PlanetQuest for nearby candidate stars.
High-precision narrow angle astrometry with a space-borne interferometer
This paper develops an observing and processing scheme for narrow angle astrometry using a single baseline interferometer without the aid of "grid" stars to characterize the interferometer baseline vector in inertial space. The basic concept derives from the recognition that over a narrow field the set of fundamental unknown instrument parameters that arise because the interferometer baseline vector has large uncertainties (since there are no grid star measurements) is indistinguishable from a particular set of unobservable errors in the determination of star positions within the field. Reference stars within the narrow field are used to circumvent these unobservable modes. Feasibility of the approach is demonstrated through analysis and example simulations.
Minimizing instrumental polarization in the Multiangle SpectroPolarmetric Imager (MSPI) using diattenuation balancing between the three mirror coatings
Special enhanced silver mirror coatings were designed and fabricated to minimize the polarization introduced by a three-mirror off-axis high-accuracy telescope. A system diattenuation of approximately 1% in the VIS-NIR was achieved by both reducing the diattenuation from each mirror individually and by balancing the diattenuations introduced by the three mirrors over the spectral range. This process of low-polarization engineering involves minimizing system polarization introduced by surface geometry, thin film coatings and birefringent elements, and measuring the system. In this report we will outline a methodology to minimize instrumental polarization aberrations, with an emphasis on achieving low diattenuation in the MSPI camera, given its off-axis geometry and coating design constraints imposed by the space-based application. This polarization balancing technique for mirror coatings can be applied to astrophysics applications.
Nonlinear sensitivity analysis for free-flying nulling interferometers
Klaus Ergenzinger, Oswald Wallner, Anthony Villien, et al.
Spectroscopy of exoplanets around near-by stars is one of the most fascinating but also most challenging science goals of our days. The ESA DARWIN mission as well as NASA TPF-I rely on nulling interferometry. The measurement principle underlying their nulling science mode is essentially nonlinear. On the one hand in terms of null depth as a function of amplitude and phase noise, and on the other hand in terms of fiber coupling as function of science beam pointing and lateral offset. We present a performance breakdown and an end-to-end performance simulation for DARWIN with focus on principal limitations, and with a clear distinction between static null depth contributors, dynamic error contributors, and so-called instability noise within the overall system. We additionally discuss the derived next-step development efforts for critical subsystems.
Novel reconfigurable wide-beam radio interferometer for space physics instrumentation
George Dekoulis, Farideh Honary
This paper describes the instrumentation design of a novel wide-beam interferometer system for radio astronomy studies. The system measures the Earth's or another planet's atmospheric layers attenuation of the highly energetic galactic electron emissions superimposed on the Cosmic Microwave Background (CMB) and other last scattering surface galactic and extragalactic radio astronomical background emissions. Right ascension coordinates are surveyed in a unique manner in terms of digital signal processing flexibility, compared to existing wide-beam instrumentations, allowing higher resolution analysis of the captured Space Physics events. The system provides a prototyping platform for other Space Physics projects, since a modular software and hardware design approach has been followed. The system is reconfigurable to meet a variety of testing scenarios.