The extreme cleaning of stellar images, needed for distinguishing their planets
and especially those analogous to the Earth, requires coronagraphy and
apodization. These techniques suppress the diffraction rings, but not the residual
speckles resulting from the bumpiness of the optical surfaces.
Several methods, coherent and incoherent, can in principle remove these speckles,
but some of them are affected if the star is slightly resolved. With the future
large hypertelescopes, spanning 100 kilometers or more to resolve the details of
exo-Earths, this could require cleaning procedures applied separately for each
sub-aperture before recombining the beams.