Aperture masking interferometry and single-mode fibers

Since the pioneering work of Haniff et al. (1987)(1), aperture-masking interferometry has been demonstrated on large class telescopes. The usual implementation lays in the avoidance of redundancies in the pupil plane, which, in presence of aberrations and turbulence, depress the transfer function of the telescope. In a recent experiment on Keck 1, a non-redundant pupil geometry allowed diffraction-limited imaging, with dynamic range in excess of 200:1 (Tuthill et al., 2000)(2). Yet, the final image quality is still limited by the optical defects induced by turbulence in sub-pupils. We propose to overcome this issue by using the same technique of spatial filtering by single-mode fibers that we have used in long-baseline interferometry. Each sub-pupil element is focused in a single-mode fiber thus eliminating spatial phase fluctuations and trading these against instantaneous intensity fluctuations which can be directly measured. Therefore, each sub-pupil becomes spatially coherent. Simulations show that the dynamic range would be dramatically increased. Moreover, the idea of using fibers in the pupil plane could lead to outstanding prospects, like filtering the whole aperture, sub-divided into a filled array of sub-apertures.