Sparse optical interferometric arrays of many apertures can produce direct images in the
densified-pupil mode, also called “hypertelescope” mode. Pending the introduction of
adaptive optics for cophasing, indirect images can also be reconstructed with speckle
imaging techniques. But adaptive phasing is preferable, when a sufficiently bright guide
star is available. Several wave sensing techniques, by-products of those used on
monolithic telescopes for some of them, are potentially usable. For cophased direct images
of very faint sources in the absence of a natural guide star, a modified form of the Laser
Guide Star techniques demonstrated on conventional and segmented telescopes is described.
Preliminary testing in laboratory suggests further investigation. Recorded images, assumed
co-phased, are also improvable post-detection with optical aperture-synthesis techniques
such as Earth rotation synthesis, where data from successive exposures are combined
incoherently. Nevertheless, the gain becomes modest if hundreds of sub-apertures are used.
Image deconvolution techniques are also applicable, if suitably modified as demonstrated
by Aime et al. (2012), and Mary
(2012). Their modified deconvolution algorithms
can extend the Direct Imaging Field (also called Clean Field) of hypertelescopes. More
sub-apertures at given collecting area, implying that their size is reduced, improve the
direct-imaging performance. The predictable trend thus favors systems combining hundreds
of sub-apertures of modest size, if workable designs can be evolved. One such design, the
“Ubaye Hypertelescope” entering the initial testing phase in the southern Alps, has a
fixed spherical meta-mirror with a 57 m effective aperture, expandable to 200 m.
Preliminary results suggest that larger versions, whether spherical or active
paraboloidal, can reach a kilometric aperture size at terrestrial sites having a suitable
concave topography. In space, hypertelescope meta-apertures spanning up to 100 000 km are
in principle feasible in the form of a flotilla of mirrors, driven by micro-thrusters or
by the radiation pressure of the Sun or lasers.