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.