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Very Large Telescope Inerferometer (VLTI) schools have nearly a 20-year history and have trained a significant fraction of today���s optical interferometrists who use high-angular-resolution techniques on a regular basis. Very early in the development of the VLTI, training was identified by the community as a necessary tool, as the expertise in optical long-baseline interferometry was limited to a few groups in France and Germany (in those early years the UK was not an ESO member state). The first VLTI school took place in Les Houches, France, in 2002 and since then VLTI schools have been organised in several locations (France, Germany, Hungary, Poland, Portugal) roughly every two years, the previous one being held in 2018 in Lisbon. The VLTI schools are funded and coordinated through the European Interferometry Initiative (Eii)., Published in The Messenger vol. 185, pp. 28-30, December 2021.

DAM (Discretized Aperture Mapping) is an original filtering device able to improve the performance in high-angular resolution and high-contrast imaging by the present class of large telescopes equipped with adaptive optics (Patru et al. 2011, 2014, 2015). DAM is a high-spatial frequency filter able to remove the problematic phase errors produced by the small scale defects in the wavefront. Various effects are related to high-order aberrations (ie the high-spatial frequency content) which are neither seen by any wavefront sensor (WFS) nor corrected by any adaptive optics (AO) and is thus transmitted up to the final detector. In particular, any wavefront sensor, due to its finite sub-apertures size, is fundamentally limited by the well-known aliasing effect, where high-spatial frequencies are seen as spurious low frequencies. DAM can be used as an anti-aliasing filter in order to improve both the accuracy of the WFS measurements and the stability of the AO compensation.

In the next future, the optical stellar interferometers are going to provide real images, by increasing the number of telescopes and by cophasing the beams. These conditions are requiered to have sufficient resolution elements (resel) in the image and to observe the low bright objects. If both conditions are achieved, direct imaging becomes more interesting than Fourier synthesis imaging. From then on, it is time to study the future large array using the hypertelescope mode, which optimizes the image properties. An hypertelescope provides snapshot images with a significant gain in sensitivity, without inducing any loss of the useful field of view. Indeed, it has been shown that the direct imaging capabilities of a sparse aperture are determined by the geometry of the array only, whatever the beam combination scheme. The pupil densification allows to equalize the Direct Imaging Field (DIF) with the real usable field of view offered by the baselines of the interferometer.Numerical simulations (HYPERTEL) have been performed to study the direct imaging properties. For that, criteria are defined to characterize the image pattern. It is shown that the choice of the geometry of the array is a trade-off between the resolution, the dynamic, the field of view and the astrophysical objectives. A regular and non-redondant pattern of the input pupil optimizes the dynamic, the contrast and the fidelity of the densified image, but decreases the useful field of view. A spotted star, with a low contrast, requiere dynamic, whereas a large field is suitable for the multiple stars.A pupil densifier using monomode optical fibres in the visible wavelength (SIRIUS) has been developed at the Observatoire de la Côte d'Azur. The effects of introducing single-mode fibres in direct imaging optical interferometers have been studied with numerical simulations. We identify an optimum to define properly the output densified pupil. First densified images have been obtained in laboratory. Spatial filtering greatly enhances the quality and the stability of the densified image, but mainly decrease partially the sensitivity of the signal. Atmospheric perturbations are converted into differential photometric fluctuations, which are easier to calibrate. These photometric fluctuations have few influence on the image quality, which simplify the image deconvolution and the beams cophasing. Finally, the flexibility of the optical fibres is well adapted to carry the beams from the entrance to the exit pupil with the appropriate rearrangement of the sub-apertures, which is convenient for next generation of interferometers, such as VLTI, CHARA, NPOI, MROI or OHANA., Les interféromètres stellaires optiques sont en passe de devenir de véritables imageurs. Pour cela, il faut disposer d'un grand nombre de télescopes pour augmenter le nombre d'éléments de résolution (resel) dans l'image, et il faut cophaser activement les faisceaux pour observer des objets peu brillants en pose longue. Si ces deux conditions sont remplies, il devient plus intéressant de travailler en mode Imagerie Directe qu'en mode Synthèse de Fourier. Dès lors, une prospective est menée sur les futurs grands réseaux en mode hypertélescope, qui optimise les propriétés de l'image. En effet, un hypertélescope fournit une image directe instantanée, avec un fort gain en sensibilité sans perte de champ utile. Il a été démontré que le champ utile d'un interféromètre dilué est imposé par la géométrie du réseau, indépendamment du mode de recombinaison. Le fait de densifier la pupille optimise l'image en ajustant le champ d'imagerie direct avec le champ réellement exploitable par l'interféromètre.Un programme de simulation (HYPERTEL) étudie les propriétés d'une image directe à partir d'un ensemble de critères d'imagerie qualitatifs. Il est montré que le choix de la configuration du réseau est un compromis entre la résolution, la dynamique, le champ et l'objectif astrophysique. Un pavage régulier et non redondant des ouvertures améliore à la fois la dynamique, le contraste et la fidélité de l'image, mais minimise le champ d'imagerie. Les étoiles multiples requièrent un champ d'imagerie suffisant, tandis que les surfaces stellaires faiblement contrastées exigent de la dynamique.Un nouveau concept de densifieur de pupille à fibres optiques monomodes dans le visible (SIRIUS) a été développé au laboratoire optique de Grasse de l'Observatoire de la Côte d'Azur. Des études préliminaires sur l'influence des fibres dans le processus d'imagerie ont mis en évidence un optimum pour redéfinir la pupille de sortie du densifieur. Les premières images de SIRIUS ont montré que la densification monomodale améliore la qualité et la stabilité de l'image d'un hypertélescope, moyennant une perte de flux global. Le filtrage spatial des fibres monomodes convertit les perturbations atmosphériques en fluctuations photométriques plus faciles à étalonner. Ces fluctuations photométriques affectent peu la qualité de l'image densifiée, ce qui permet de simplifier la déconvolution de l'image et le cophasage des faisceaux. Enfin, la flexibilité des fibres permet une reconfiguration entrée/sortie plus aisée de la pupille, ce qui convient bien aux nouveaux interféromètres comme le VLTI, CHARA, NPOI, ou encore MROI et OHANA.

Discretized Aperture Mapping (DAM) appears as an original filtering tech nique easy to play with existing adaptive optics (AO) systems. In its essential DAM operates as an optical passive filter removing part of the phase residuals in the wavefront without introducing any difficult-to-align component in the Fourier conjugate of the entrance pupil plane. DAM reveals as a new interferometric technique combined with spatial filtering allowing direct imaging over a narrow field of view (FOV). In fact, the entrance pupil of a single telescope is divided into many sub-pupils so that the residual phase in each sub-pupil is filtered up to the DAM cut-off frequency. DAM enables to smooth the small scale wavefront defects which correspond to high spatial frequencies in the pupil plane and to low angular frequencies in the image plane. Close to the AO Nyquist frequency, such pupil plane spatial frequencies are not well measured by the wavefront sensor (WFS) due to aliasing. Once bigger than the AO Nyquist frequency, they are no more measured by the WFS due to the fitting limit responsible for the narrow AOFOV. The corresponding image plane angular frequencies are not tran smitted by DAM and are useless to image small FOVs, as stated by interferometry. That is why AO and DAM are complementary assuming that the DAM cut-off frequency is equal to the AO Nyquist frequency. Here we describe the imaging capabilities when DAM is placed downstream an AO system, over a convenient pupil which precedes the scientific detector. We show firstly that the imaging properties are preserved on a narrow FOV allowing direct imaging throughout interferometry. Then we show how the residual pupil plane spatial frequencies bigger than the AO Nyquist one are filtered out, as well as the residual halo in the image is dimmed.

Dense Aperture Mapping (DAM) is a new interferometric technique allowing high-angular resolution over a narrow field of view (FOV) imaged by the present class of mono-pupil telescopes equipped with adaptive optics (AO). DAM is realized by a suited afocal double lenslet array (BIGRE), mapping the telescope pupil into coherent sub-pupils, and adopted as sub-pupils spatial filter and focal plane re-imager. Based on the densified pupil concept for diluted telescope arrays- hypertelescope - it is applied here on a segmented mirror telescope turned into a filtered direct imaging Fizeau combiner. We explain here as first why DAM is a pupil remapping technique.We define the Clean FOV (CLEAF) where an object-image convolution relation is preserved.We show how DAM and AO can match to reach high Strehl image quality, highlighting on the low spatial frequencies filtering property intrinsic to DAM.

In the next future, the optical stellar interferometers are going to provide real images, by increasing the number of telescopes and by cophasing the beams. These conditions are requiered to have sufficient resolution elements (resel) in the image and to observe the low bright objects. If both conditions are achieved, direct imaging becomes more interesting than Fourier synthesis imaging. From then on, it is time to study the future large array using the hypertelescope mode, which optimizes the image properties. An hypertelescope provides snapshot images with a significant gain in sensitivity, without inducing any loss of the useful field of view. Indeed, it has been shown that the direct imaging capabilities of a sparse aperture are determined by the geometry of the array only, whatever the beam combination scheme. The pupil densification allows to equalize the Direct Imaging Field (DIF) with the real usable field of view offered by the baselines of the interferometer.Numerical simulations (HYPERTEL) have been performed to study the direct imaging properties. For that, criteria are defined to characterize the image pattern. It is shown that the choice of the geometry of the array is a trade-off between the resolution, the dynamic, the field of view and the astrophysical objectives. A regular and non-redondant pattern of the input pupil optimizes the dynamic, the contrast and the fidelity of the densified image, but decreases the useful field of view. A spotted star, with a low contrast, requiere dynamic, whereas a large field is suitable for the multiple stars.A pupil densifier using monomode optical fibres in the visible wavelength (SIRIUS) has been developed at the Observatoire de la Côte d'Azur. The effects of introducing single-mode fibres in direct imaging optical interferometers have been studied with numerical simulations. We identify an optimum to define properly the output densified pupil. First densified images have been obtained in laboratory. Spatial filtering greatly enhances the quality and the stability of the densified image, but mainly decrease partially the sensitivity of the signal. Atmospheric perturbations are converted into differential photometric fluctuations, which are easier to calibrate. These photometric fluctuations have few influence on the image quality, which simplify the image deconvolution and the beams cophasing. Finally, the flexibility of the optical fibres is well adapted to carry the beams from the entrance to the exit pupil with the appropriate rearrangement of the sub-apertures, which is convenient for next generation of interferometers, such as VLTI, CHARA, NPOI, MROI or OHANA.; Les interféromètres stellaires optiques sont en passe de devenir de véritables imageurs. Pour cela, il faut disposer d'un grand nombre de télescopes pour augmenter le nombre d'éléments de résolution (resel) dans l'image, et il faut cophaser activement les faisceaux pour observer des objets peu brillants en pose longue. Si ces deux conditions sont remplies, il devient plus intéressant de travailler en mode Imagerie Directe qu'en mode Synthèse de Fourier. Dès lors, une prospective est menée sur les futurs grands réseaux en mode hypertélescope, qui optimise les propriétés de l'image. En effet, un hypertélescope fournit une image directe instantanée, avec un fort gain en sensibilité sans perte de champ utile. Il a été démontré que le champ utile d'un interféromètre dilué est imposé par la géométrie du réseau, indépendamment du mode de recombinaison. Le fait de densifier la pupille optimise l'image en ajustant le champ d'imagerie direct avec le champ réellement exploitable par l'interféromètre.Un programme de simulation (HYPERTEL) étudie les propriétés d'une image directe à partir d'un ensemble de critères d'imagerie qualitatifs. Il est montré que le choix de la configuration du réseau est un compromis entre la résolution, la dynamique, le champ et l'objectif astrophysique. Un pavage régulier et non redondant des ouvertures améliore à la fois la dynamique, le contraste et la fidélité de l'image, mais minimise le champ d'imagerie. Les étoiles multiples requièrent un champ d'imagerie suffisant, tandis que les surfaces stellaires faiblement contrastées exigent de la dynamique.Un nouveau concept de densifieur de pupille à fibres optiques monomodes dans le visible (SIRIUS) a été développé au laboratoire optique de Grasse de l'Observatoire de la Côte d'Azur. Des études préliminaires sur l'influence des fibres dans le processus d'imagerie ont mis en évidence un optimum pour redéfinir la pupille de sortie du densifieur. Les premières images de SIRIUS ont montré que la densification monomodale améliore la qualité et la stabilité de l'image d'un hypertélescope, moyennant une perte de flux global. Le filtrage spatial des fibres monomodes convertit les perturbations atmosphériques en fluctuations photométriques plus faciles à étalonner. Ces fluctuations photométriques affectent peu la qualité de l'image densifiée, ce qui permet de simplifier la déconvolution de l'image et le cophasage des faisceaux. Enfin, la flexibilité des fibres permet une reconfiguration entrée/sortie plus aisée de la pupille, ce qui convient bien aux nouveaux interféromètres comme le VLTI, CHARA, NPOI, ou encore MROI et OHANA.