Guy S. Perrin

A catalogue of calibrator stars for long baseline stellar interferometry

Long baseline stellar interferometry shares with other techniques the need for calibrator stars in order to correct for instrumental and atmospheric eects. We present a catalogue of 374 stars carefully selected to be used for that purpose in the near infrared. Owing to several convergent criteria with the work of Cohen et al. (1999), this catalogue is in essence a subset of their self-consistent all-sky network of spectro-photometric calibrator stars. For every star, we provide the angular limb-darkened diameter, uniform disc angular diameters in the J, H and K bands, the Johnson photometry and other useful parameters. Most stars are type III giants with spectral types K or M0, magnitudes V = 3-7 and K = 0-3. Their angular limb-darkened diameters range from 1 to 3 mas with a median uncertainty as low as 1.2%. The median distance from a given point on the sky to the closest reference is 5:2, whereas this distance never exceeds 16:4 for any celestial location.

FLUOR fibered instrument at the IOTA interferometer

The FLUOR project started in 1991 with a prototype fiber recombination unit that transformed a pair of independent 80 cm telescopes into a stellar interferometer. An improved version of this unit is now used as part of the instrumentation at the IOTA interferometer on Mt Hopkins (Arizona). The system is based on fluoride glass single-mode waveguides (non polarization-preserving) for observations at infrared wavelengths between 2 and 2.4 μm. A triple coupler performs the coherent recombination of the beams and extracts two calibration signals. A passive polarization control is sufficient to maintain the interferornetric efficiency above 80 %, with variations of the order of a few percents from one night to the next. The combination FLUOR/JOTA now routinely provides stellar interferograms on baselines ranging between 5 and 38 m, with an accuracy of 1 % or better in the fringe visibility measurements.

Ten-micron instrument MIDI: getting ready for observations on the VLTI

The mid-infrared interferometric instrument MIDI is currently undergoing testing in preparation for commissioning on the Very Large Telescope Interferometer VLTI at the end of this year 2002. It will perform interferometric observations over the 8 μm - 13 μm wavelength range, with a spatial resolution of 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 4 mag or 1 Jy for self-fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument and the performance during laboratory tests, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations.

FLUOR fibered beam combiner at the CHARA array

The fibered beam combiner FLUOR, which has provided high accuracy visibility measurements on the IOTA interferometer, is being moved to the CHARA array which provides five 1m telescopes on baselines ranging from 35 to 330m. The combination CHARA/FLUOR makes it possible for the first time to achieve sub-milliarcsecond resolution in the K band, with a dynamic range of 100 or more. We explore the scientific potential of CHARA/FLUOR, most notably in the domains of high contrast binaries and the characterization of Cepheid pulsations, and present some of the anticipated developements.

Advanced Imaging Methods for Long-Baseline Optical Interferometry

We address the data processing methods needed for imaging with a long baseline optical interferometer. We first describe parametric reconstruction approaches and adopt a general formulation of nonparametric image reconstruction as the solution of a constrained optimization problem. Within this framework, we present two recent reconstruction methods, Mira and Wisard, representative of the two generic approaches for dealing with the missing phase information. Mira is based on an implicit approach and a direct optimization of a Bayesian criterion while Wisard adopts a self-calibration approach and an alternate minimization scheme inspired from radio-astronomy. Both methods can handle various regularization criteria. We review commonly used regularization terms and introduce an original quadratic regularization called ldquosoft support constraintrdquo that favors the object compactness. It yields images of quality comparable to nonquadratic regularizations on the synthetic data we have processed. We then perform image reconstructions, both parametric and nonparametric, on astronomical data from the IOTA interferometer, and discuss the respective roles of parametric and nonparametric approaches for optical interferometric imaging.

10-μm interferometry on the VLTI with the MIDI instrument : a preview

We describe principles, design and present status of MIDI, the mid-infrared interferometric instrument for the VLTI, which is planned to come into operation at the ESO Very Large Telescope Interferometer during the second half of 2001.

Fibered large interferometer on top of Mauna Kea: OHANA, the optical Hawaiian array for nanoradian astronomy

The Mauna Kea site houses two eight-meter and two ten-meter class telescopes which will soon by fully operational. In addition to other existing large telescopes already available, the Mauna Kea summit offers a unique opportunity to build a large optical and infrared interferometer in the northern hemisphere with both the highest angular resolution and the highest sensitivity. We discuss the possibility to recombine with single-mode fibers this array whose large telescopes will all be equipped with adaptive optics facilities. We show the tremendous potential of this instrument for astrophysics and how complementary it is to other large arrays now under construction.

Squared Visibility Estimators: Calibrating Biases to Reach Very High Dynamic Range

In the near-infrared, where detectors are limited by readout noise, most interferometers have been operated in wide band in order to benefit from larger photon rates. We analyze in this paper the biases caused by instrumental and turbulent effects to V2 estimators for both narrowband and wideband cases. Visibilities are estimated from samples of the interferogram using two different estimators, V, which is the classical sum of the squared modulus of Fourier components, and a new estimator V, for which complex Fourier components are summed prior to taking the square. We present an approach for systematically evaluating the performance and limits of each estimator and for optimizing observing parameters for each. We include the effects of spectral bandwidth, chromatic dispersion, scan length, and differential piston. We also establish the expression of the signal-to-noise ratio of the two estimators with respect to detector and photon noise. The V estimator is insensitive to dispersion and is always more sensitive than the V estimator. However, the latter allows us to reach better accuracies when detection is differential piston noise limited. Biases and noise directly impact the dynamic range of reconstructed images. Very high dynamic ranges are required for direct exoplanet detection by interferometric techniques, thus requiring estimators to be bias-free or biases to be accurately calibrated. We discuss which estimator and which conditions are optimum for astronomical applications, especially when high-accuracy visibilities are required. We show that there is no theoretical limit to measuring visibilities with accuracies as good as 10-5, which is important in the prospect of detecting faint exoplanets with interferometers.

The final design of the GRAVITY acquisition camera and associated VLTI beam monitoring strategy

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

The GRAVITY acquisition and guiding system

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