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. (abridged)Comment: 11 pages, 36 figure

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.Comment: 11 pages, 26 figure

La méthode "speckles dispersés" (mise en phase pour faire de l'imagerie directe avec des interféromètres et des hypertélescopes)

La course aux résolutions angulaires toujours plus élevées et inaccessibles aux télescopes a dopé le développement de l interférométrie au cours de ces dernières années. L espace étant un domaine coûteux de l astronomie, l observation de la Terre n est pas prête à disparaître mais elle s assortit du besoin de s affranchir de la turbulence, toujours plus problématique sur de grandes ouvertures. Les méthodes de cophasage actuelles come celle de Shack-Hartmann concernent les télescopes monolithiques. Afin d analyser les différences de chemins optiques des interféromètres à ouvertures multiples, nous étudions, dans le cadre de ce travail de thèse, une nouvelle méthode originale basée sur l utilisation de transformées de Fourier à trois dimensions.The need of higher and higher angular resolution, which is inaccessible to telescopes, has led to the development of interferometry during these last years. Space is an expensive way of doing astronomy and observing from Earth is still a topical field, but for that reason turbulence has to be avoid. It is a worse problem with big apertures. The existing cophasing methods such as the Shack-Hartmann one concern monolithic telescopes. In order to analyse the optical path differences in interferometers with multiple apertures, we propose in this thesis work a new original method based on using three dimensional Fourier transforms.AIX-MARSEILLE1.OHP.Saint-Michel (041922301) / SudocNICE-BU Sciences (060882101) / SudocSudocFranceF

Adaptive Optics for Ground-based Hypertelescopes

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Adaptive Optics for Ground-based Hypertelescopes

International audienc

Adaptive Optics for Ground-based Hypertelescopes

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Hypertelescope architectures for direct imaging at high angular resolution

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Hypertelescope architectures for direct imaging at high angular resolution

International audienc

Hypertelescope architectures for direct imaging at high angular resolution

International audienc