Abstract
We show by numerical simulations a fundamental gain in contrast when combining coherently monochromatic light from two adaptive optics (AO) telescopes instead of using a single stand-alone ...AO telescope, assuming efficient control and acquisition systems at high speed. A contrast gain map is defined as the normalized point spread functions (PSFs) ratio of a single Large Binocular Telescope (LBT) aperture over the dual Large Binocular Telescope Interferometer (LBTI) aperture in Fizeau mode. The global gain averaged across the AO-corrected field of view is improved by a factor of 2 in contrast in long exposures and by a factor of 10 in contrast in short exposures (i.e. in exposures, respectively, longer or shorter than the coherence time). The fringed speckle halo in short exposures contains not only high-angular resolution information, as stated by speckle imaging and speckle interferometry, but also high-contrast imaging information. A high-gain zone is further produced in the valleys of the PSF formed by the dark Airy rings and/or the dark fringes. Earth rotation allows us to exploit various areas in the contrast gain map. A huge-contrast gain in narrow zones can be achieved when both a dark fringe and a dark ring overlap on to an exoplanet. Compared to a single 8-m LBT aperture, the 23-m LBTI Fizeau imager can provide a gain in sensitivity (by a factor of 4), a gain in angular resolution (by a factor of 3) and, as well, a gain in raw contrast (by a factor of 2–1000 varying over the AO-corrected field of view).
Abstract
We show numerical simulations with monochromatic light in the visible for the LBTI Fizeau imager, including opto-dynamical aberrations due here to adaptive optics (AO) errors and to ...differential piston fluctuations, while other errors have been neglected. The achievable Strehl by the LBTI using two AO is close to the Strehl provided by a single standalone AO system, as long as other differential wavefront errors are mitigated. The LBTI Fizeau imager is primarily limited by the AO performance and by the differential piston/tip–tilt errors. Snapshots retain high-angular resolution and high-contrast imaging information by freezing the fringes against piston errors. Several merit functions have been critically evaluated in order to characterize point spread functions and the modulation transfer functions for high-contrast imaging applications. The LBTI Fizeau mode can provide an image quality suitable for standard science cases (i.e. a Strehl above 70 per cent) by performing both at a time: an AO correction better than ≈λ/18 RMS for both short and long exposures, and a piston correction better than ≈λ/8 RMS for long exposures or simply below the coherence length for short exposures. Such results, which can be applied to any observing wavelength, suggest that AO and piston control at the LBTI would already improve the contrast at near- and mid-infrared wavelengths. Therefore, the LBTI Fizeau imager can be used for high-contrast imaging, providing a high-Strehl regime (by both AO systems), a cophasing mode (by a fringe tracker) and a burst mode (by a fast camera) to record fringed speckles in short exposures.
Future optical interferometric instrumentations mainly rely on the availability of an efficient cophasing system: once available, what has so far postponed the relevance of direct imaging to an ...interferometer will vanish. This paper focuses on the actual limits of snapshot imaging, inherent to the use of a sparse aperture: the number of telescopes and the geometry of the array impose the maximum extent of the field of view (FOV) and the complexity of the sources. A second limitation may arise from the beam combination scheme. Comparing already available solutions, we show that the so-called hypertelescope mode (or densified pupil) is ideal. By adjusting the direct imaging FOV to the useful FOV offered by the array, the hypertelescope makes an optimal use of the collected photons. It optimizes signal-to-noise ratio, drastically improves the luminosity of images and makes the interferometer compatible with coronagraphy, without inducing any loss of useful FOV.
In the future, optical stellar interferometers will provide true images thanks to larger number of telescopes and to advanced cophasing subsystems. These conditions are required to have sufficient ...resolution elements (resel) in the image and to provide direct images in the hypertelescope mode. It has already been shown that hypertelescopes provide snapshot images with a significant gain in sensitivity without inducing any loss of the useful field of view for direct imaging applications. This paper aims at studying the properties of the point spread functions of future large arrays using the hypertelescope mode. Numerical simulations have been performed and criteria have been defined to study the image properties. It is shown that the choice of the configuration of the array is a trade-off between the resolution, the halo level and the field of view. A regular pattern of the array of telescopes optimizes the image quality (low halo level and maximum encircled energy in the central peak), but decreases the useful field of view. Moreover, a non-redundant array is less sensitive to the space aliasing effect than a redundant array.
Long baseline interferometry is now a mature technique in the optical domain. Current interferometers are, highly limited in, number of subapertures and concepts are being developed for future ...generations of very large optical arrays and especially with the goal of direct imaging. In this paper, we study the effects of introducing single-mode fibres in direct imaging optical interferometers. We show how the flexibility of optical fibres is well adapted to the pupil densification scheme. We study the effects of the truncation of the Gaussian beams in the imaging process, either in the Fizeau mode or in the densified pupil mode or in the densified image mode. Finally, in the pupil densification configuration, we identify an optimum of the diaphragm width. This optimum maximizes the on-axis irradiance and corresponds to a trade-off between the loss of transmission and the efficiency of the densification.
In addition to site characterization, measurements of critical atmospheric parameters are required to design and to optimize future adaptive optic systems and long-baseline interferometers. It is ...possible to estimate seeing conditions by processing data obtained with existing High Angular Resolution instruments. We report the results of joint observations with the GI2T interferometer and the GSM site-testing monitor performed over a period of several nights. We compared estimates of the wavefront outer scale done at various baselines as well as the seeing (Fried's parameter). We processed interferometric data by calculating power spectra of dispersed fringe images. Deduced measurements of the optical path difference lead to the estimates of the outer scale. We found that the outer scale values obtained from the GI2T data are mostly in the 5-30 m range, in good agreement with GSM measurements.
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Single-mode waveguides for GRAVITY Perraut, K.; Jocou, L.; Berger, J. P. ...
Astronomy and astrophysics (Berlin),
06/2018, Volume:
614
Journal Article
Peer reviewed
Open access
Context.
Within the framework of the second-generation instrumentation of the Very Large Telescope Interferometer of the European Southern Observatory we have developed the four-telescope beam ...combiner in integrated optics.
Aims.
We optimized the performance of such beam combiners, for the first time in the near-infrared
K
band, for the GRAVITY instrument dedicated to the study of the close environment of the galactic centre black hole by precision narrow-angle astrometry and interferometric imaging.
Methods.
We optimized the design of the integrated optics chip and the manufacturing technology as well, to fulfil the very demanding throughput specification. We also designed an integrated optics assembly able to operate at 200 K in the GRAVITY cryostat to reduce thermal emission.
Results.
We manufactured about 50 beam combiners by silica-on-silicon etching technology. We glued the best combiners to single-mode fluoride fibre arrays that inject the VLTI light into the integrated optics beam combiners. The final integrated optics assemblies have been fully characterized in the laboratory and through on-site calibrations: their global throughput over the
K
band is higher than 55% and the instrumental contrast reaches more than 95% in polarized light, which is well within the GRAVITY specifications.
Conclusions.
While integrated optics technology is known to be mature enough to provide efficient and reliable beam combiners for astronomical interferometry in the
H
band, we managed to successfully extend it to the longest wavelengths of the
K
band and to manufacture the most complex integrated optics beam combiner in this specific spectral band.
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Context. The mass loss from massive stars is not understood well. η Carinae is a unique object for studying the massive stellar wind during the luminous blue variable phase. It is also an eccentric ...binary with a period of 5.54 yr. The nature of both stars is uncertain, although we know from X-ray studies that there is a wind-wind collision whose properties change with orbital phase. Aims. We want to investigate the structure and kinematics of η Car’s primary star wind and wind-wind collision zone with a high spatial resolution of ~6 mas (~14 au) and high spectral resolution of R = 12 000. Methods. Observations of η Car were carried out with the ESO Very Large Telescope Interferometer (VLTI) and the AMBER instrument between approximately five and seven months before the August 2014 periastron passage. Velocity-resolved aperture-synthesis images were reconstructed from the spectrally dispersed interferograms. Interferometric studies can provide information on the binary orbit, the primary wind, and the wind collision. Results. We present velocity-resolved aperture-synthesis images reconstructed in more than 100 different spectral channels distributed across the Brγ 2.166 μm emission line. The intensity distribution of the images strongly depends on wavelength. At wavelengths corresponding to radial velocities of approximately −140 to − 376 km s-1 measured relative to line center, the intensity distribution has a fan-shaped structure. At the velocity of − 277 km s-1, the position angle of the symmetry axis of the fan is ~126°. The fan-shaped structure extends approximately 8.0 mas (~18.8 au) to the southeast and 5.8 mas (~13.6 au) to the northwest, measured along the symmetry axis at the 16% intensity contour. The shape of the intensity distributions suggests that the obtained images are the first direct images of the innermost wind-wind collision zone. Therefore, the observations provide velocity-dependent image structures that can be used to test three-dimensional hydrodynamical, radiative transfer models of the massive interacting winds of η Car.
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