The sky testing with the new densification scheme confirmed the high-resolution imaging capabilities of hypertelescopes and their applicability to observing methods highly sensitive to the exit pupil shape, such as phase-mask coronagraphy. The image quality has been noticeably improved. The filling and uniformity of the exit pupil is indeed easily controllable, and the densifier system can be a compact package installed at the Fizeau focus of a large multi-mirror array. Its use is straightforward if adaptive optics provides the required piston phasing and also phasing within each sub-aperture if their size exceeds Fried's parameter. Un-phased images can be exploited by speckle interferometry. Direct imaging occurs within the unaliased narrow field of view (ZOF) and the images are usable for spectroscopic, spectro-imaging and also coronagraphic uses with a focal mask such as Roddier's phase dot or Rouan's Four Quadrant Phase Mask (Riaud et al. 2001). A hypertelescope coupled to a coronagraph has been proposed for NASA's Terrestrial Planet Finder project (Riaud et al. 2002). The concept is being further explored in our group. At given collecting area, more apertures of smaller size indeed enlarge the direct field (ZOF) and more sources are tolerable within the HOF (the size of which increases). Thus, current designs of Extremely Large Telescopes can be "exploded'' for higher resolution with the same limiting magnitude, while retaining their deep-field imaging capability: this announces ground and space interferometric instruments with powerful observing characteristics.
Copyright ESO 2003