J.-M. Mariotti

Improvement in the rejection rate of a nulling interferometer by spatial filtering.

We propose applying the techniques of spatial filtering to the concept of interferometric coronography. In such a system, provided that the object being studied is not resolved by the individual apertures of the interferometric array, the beams can be considered as coherent or, more exactly, single mode. Hence spatial filtering allows one to cleanse the beams of imperfections generated by defects on the optical components of the interferometer and thus to obtain very high rejection rates in the destructive output of the interferometer (coronographic output) for an on-axis star. Numerical simulations show that the very stringent constraints on the optical quality of a space IR interferometer aimed at detecting extrasolar planets can be relaxed to values achievable with current technology. In particular, we show that the difficulties induced by dust scattering, small micrometeorite impacts on the primary mirror, and high-frequency ripples of polishing residuals can be eliminated by simple pinhole spatial filtering. The effects, however, will be less dramatic on large-scale defects such as coating defects and pointing errors in the telescopes.

The Darwin Project

Angel and co-workers have proposed to detect exoplanets around nearby stars in the infrared (6–17 μm) and to analyze their spectra, searching for H2O, CO2, CH4, NH3, and O3 spectral features. The presence or absence of CO2 would indicate either a strong similarity or difference with Solar telluric planet atmospheres. Water would indicate a habitable planet, and O3 would reveal significant photosynthesis activity, due to the presence of carbon chemistry based life. Like these authors, we suggest an infrared nulling interferometer pointing to the star and working as a coronograph. Our main contribution is to propose an observatory made of four to five 1-meter class telescopes observing from about 4 to 5 AU to avoid the Solar Zodiacal Light (ZL) background at 10μm instead of four 8-meter ones observing from the Earth vicinity. This allows the mission to be feasible in thenear future. The concept, named DARWIN, is under consideration by the European Space Agency for its Horizon 2000 Plus program.

Interferometric coronography for the DARWIN space mission { Laboratory demonstration experiment

The DARWIN mission aims at directly detecting exoplanets, particularly telluric ones, around nearby stars and at performing a spectral analysis of their potential atmosphere. The major diculty in achieving this goal is the huge luminosity contrast between the star and its possible planet(s). The mission concept is based on the star light rejection by nulling interferometry. This concept has never been experimentally demonstrated with high values of rejection (10 5 ) in the thermal infrared where DARWIN will observe. In this paper, we describe a laboratory test-bed that uses a CO2 laser at 10.6 m to achieve this goal in a monochromatic case. We describe its principle, its sub-systems, their realizations and the rst results we obtained with this interferometer.

VLTI program: a status report

The VLTI (Very Large Telescope Interferometer) is one of the operating modes of the VLT, presently being built on Cerro Paranal, Chile. It aims at providing access to an observing mode at very high angular resolution and very high sensitivity (with respect to the currently operating astronomical interferometers). After a long period of conceptual, then detailed, studies, ESO is starting to build and to procure the main components of the interferometer in order to open this unpaired observing facility by the turn of the century.

Multistage fiber optic delay line for astronomical interferometry

In fiber optic Fourier transform spectrometry (FTS), fiber optic double Fourier interferometry (DFI) and, probably, in single-mode fiber-linked telescope array, fiber delay lines are needed to make the system all-fiber optic, which will results in many advantages. Fiber-stretching operation has been demonstrated to be one method in which a piece of fiber is stretched mechanically to generate optical path difference (OPD). Nevertheless, this fiber-stretching operation has two drawbacks. First, it introduces additional polarization and dispersion effects, both of which are stretching state and wavelength dependent. Second, in practive, its maximum delay is limited. In this paper, we describe a new type of fiber delay line in which the fiber-stretching device (we call it OPD generator) is used to generate continuous OPD, and, OPD stores made of optical switch and two segments fibers having different lengths are employed to save and release the OPD that the OPD generator produces. The main advantages of this kind of fiber delay line are: 1) dispersion in the OPD stores may be minimized because the fiber in an OPD store is free of external strain, and polarization effect can be eliminated; 2) fiber-stretching operation will introduce less dispersion and polarization effects; hence 3) large optical delay may be obtained with shorter fiber. At the same time, performance of this kind of delay line is analyzed and the characteristics are discussed.

Interferometric connection of large telescopes at Mauna Kea

We present and discuss the possibility to coherently couple several (up to seven) large telescopes located on the Mauna Kea summit, in order to obtain interferometric capabilities at visible and infrared wavelengths. The advent of adaptive optics and single mode optical fibers allows the use of telescopes which were not built for interferometric use. The diameter of the telescopes, the orientations and lengths of potential baselines can lead to impressive performances in terms of sensitivity and angular resolution, with a negligible site impact.

Fibered recombination unit for the Infrared-Optical Telescope Array

The Infrared Optical Telescope Array (IOTA) is an interferometric facility currently observing in the near infrared bands at the Smithsonian Institutions Fred Lawrence Whipple Observatory in Arizona. The 45 cm siderostats can be moved on an L-shaped track allowing discrete bases ranging between 5 and 38 m. The capability to combine beams with fiber optics in the K band (2 micrometers <EQ (lambda) <EQ 2.4 micrometers ) has been demonstrated on the Fiber Link Unit for Optical Recombination (FLUOR) at Kitt Peak National Observatory, in which two existing 0.8 m telescopes have been coherently coupled by means of optical fibers. FLUOR is now set as a focal instrument of IOTA. It uses single-mode fluoride glass waveguides and couplers as a substitute for mirrors and beamsplitters to perform beam transportation and recombination. Processing the light in single-mode waveguides offers the possibility to self-calibrate each interferogram against the loss of fringe visibility induced by atmospheric turbulence, thus improving the accuracy of the fringe visibility measurements. The FLUOR unit can be operated as a Mach-Zehnder interferometer to produce zero-baseline spectra used in double-Fourier interferometry to obtain the visibility as a function of wavelength. In the current status, a N-S baseline of 21.2 m is used to observe late-type starts and derive their angular diameters.

Direct Observation of Extrasolar Planets with an Infrared Interferometer

Direct observation of extrasolar planets with an infrared interferometer is a difficult task because of the very high dynamic range required (typically 104 – 105 in the best cases), but not impossible if a very careful spatial filtering and calibration is imposed upon the recombined beams.

Observing Stellar Surfaces with a High Precision Infrared Interferometer

In this paper we report the results of a fiber optics beam-combiner at the focus of the IOTA interferometer. The spatial filtering of the atmospheric turbulent optical modes by single-mode fiber optics enabled us to measure fringe contrasts with precisions superior to that of classical all-optics beam-combiners. Diameter measurements of late-type stars at 2.2 μm with accuracies better than 1% are presented. Preliminary effective temperatures are derived for a sample of giant stars. These results demonstrate the potential of single-mode fibers for light recombination in imaging interferometers with high-dynamic range. A similar instrument set on the VLTI would have a much higher sensitivity. Among all possible applications, study of stellar surfaces would benefit of high accuracy measurements provided by that instrument.

Nulling interferometry for the DARWIN mission: experimental demonstration of the concept in the thermal infrared with high levels of rejection

Present projects of space interferometers dedicated to the detection and analysis of extrasolar planets (DARWIN in Europe, TPF in the United States) are based on the nulling interferometry concept. This concept has been proposed by Bracewell in 1978 but has never been demonstrated with high values of rejection, in the thermal infrared range, where the planet detection should be performed (6 - 18 micrometers ). We have thus built a two-beam laboratory interferometer to validate this concept in a monochromatic case (at 10 micrometers ). The keypoint of our interferometer is the use of optical filtering by pinhole and optical fibers to clean the interfering beams. We present in this paper the principle of the experimental setup, its realization, and the first measurements of rejection it allowed. We also present the future developments of this interferometer.