Context. In the future, giant optical interferometric arrays will be developed with kilometric baselines and a large number of telescopes. Such arrays could have direct imaging capabilities if optimized beam combiners are used. Aims. This paper aims at studying the performance of an interferometric beam combiner using single mode fibers and in the frame of a hypertelescope. Methods. A laboratory testbed called SIRIUS was developed. We describe the general concept, the technical specifications and the results obtained. These results are analyzed with the help of a numerical simulator. Results. Direct images were obtained at the densified focus of SIRIUS. We show that the fibers greatly ease the pupil rearrangement. They also greatly improve the quality and the stability of the direct image. The computed images allow us to reproduce the effects of differential photometry and the influence of optical path difference variations. Optical path difference errors less than λ/10 and differential photometries less than 60% are required to keep the quality of the direct image.Conclusions. These results demonstrate the great potential of direct imaging interferometric beam combiners for future optical large arrays. The excellent comparison between experience and simulation clearly shows the simplicity of the fibered pupil densifier. It also gives us a great confidence in the extrapolation of these results and specifications for future arrays with a very large number of apertures.
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.
We present a test bench designed to study the performances of interferometric recombination systems, mainly for direct imaging applications (hypertelescope principle). It aims at comparing the aperture synthesis, Fizeau and densified pupils beam combination schemes. It allows identification of the technical requirements like photometry and cophasing correction of the futur imaging recombiners for large arrays. A densified assembly has been designed in the visible wavelenghts, using a multi-apertures mask associated with a wavefront sensor. It allows pupil rearrangement and spatial filtering by using single mode fibers. The technical specifications and the conception of the fiber densifier are described here, with a particular attention to the correction of the differential chromatic dispersion.
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