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Imaging of remote objects from space with the use of high-spatial resolution telescopes is a vital problem
for a number of applications. Enlarging the primary mirror diameter aimed at increasing the telescope resolution is
limited, on the one hand, by a dramatic growth of its cost with diameter, and on the other by the complexity and
cost of a system that is necessaiy to maintain the mirror shape with high accuracy in realistic conditions of thermal
and gravitational loads. Recently various adaptive techniques for correction of static and dynamic aberrations in
telescopes have been developed. They allow in principle to enlarge the primary mirror due to the use of
multisegment constructions when separate segments are phased to achieve the ultimate system resolution determined
by the priinaiy mirror diameter. However, all these techniques, based on the use of beam wavefront analyzers, active
minors with a lot of actuators and control electronics, are rather sophisticated and limited in response time, the
number of actuators and of segments to be phased. For this reason, in parallel with traditional adaptive optic
techniques some other approaches to the problem, based on the use of nonlinear optics techniques, have been
progressing last years.
One of the promising solutions for high-resolution imaging with the use of poor - quality optics is based
on illumination of remote objects by laser radiation and their observation in reflected coherent light 2, It gives the
opportunity to apply phase conjugation (PC) techniques, developed for coherent laser radiation, for correction of
aberrations of the segmented mirror. The correction concept uses such a design that the reflected from the object
laser beam coming to a certain point of the primary passes through this point twice, first propagating from the object
and second after phase conjugation, when it goes in backward direction. As a result the distortions introduced by the
primal)' mirror are canceled. To create the image of the object not at the object itself, as it should be due to phase
conjugation, but at some plane located within the telescope, the additional quadratic phase correction can be
introduced into the beam with the use of polarization technique when it goes back from the phase conjugate mirror (so called "by - pass concept") 2 The difference in angles of incidence of the beams at the direct and backward passes
is one of the most important factors limiting a degree of phase -conjugation correction of aberrations; especially at
large f - numbers. However, the degree of primary aberrations reduction due to phase conjugation can be very high in
such systems and amounts to tens or hundreds.
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