Features of application of elastic membrane primaries in observing telescopes with nonlinear optical correction of images

ABSTRACT Some ten last years the problem of compensation for dynamic distortions of primary mirrors in low-weight large-sizeimaging telescopes by methods of dynamic holography is actively being investigated'3. Today the performance of suchsystems has been experimentally proved4. Thus, the idea to use in such a telescopic system a low-weight-film primarymirror, which can, in principle, be of large diameter in an unfolded mode, is very attractive one. In this paper the problemsare considered related to integration of a thin-film primary mirror in a telescope with a non-linear corrector for an objectimage. Namely, there will be discussed the architecture of the telescope with the thin-film primary, optical quality of themirror, requirements on the non-linear corrector, the telescope's field ofview, and quality ofthe image of an object.Keywords: telescope, primary mirror, membrane, optical distortion correction, diffraction optical element. 1.INTRODUCTION Progress in the field of nonlinear optical correction of image distortions in telescopic systems aimed at both observationobjects with high resolution and forming the highly directed laser beams allows one to expect this research direction to beactively developed in the nearest future. In the last decades the great majority of works"2 has appeared directed at study ofthe imaging telescopes operating with nonlinear optical image correction in a broad spectral range. Thus, for instance, in thesurvey paper the problems were analyzed corresponding to nonlinear optical techniques for compensation for beamdistortions in the beam-forming telescopic systems operating with pulse-repetition-C02-laser radiation. An obviousadvantage of such telescopes intended first of all for future fundamental researches is that they enable one to reduceessentially the requirements imposed on optical quality of the telescope's primary on its fabrication as well as on itsexploitation conditions. Apparently, promising could be the telescopes with segmented large-aperture primary mirrors, ifimage distortions due to poor quality of the specular segments, their mutual misalignments and piston shifts, and vibrationsof the mirror as a whole would be automatically compensated by a nonlinear dynamic corrector.A thin-film elastic mirror deployed from its compact state into working state might be taken as a further modification of theprimary. Such a mirror could be of optical quality sufficient for its operation within the telescopic system equipped withsome dynamic nonlinear corrector. Apparently, along with compactness, such a mirror seems also to be attractive for its lowweight and ability to have the specular surface replaced in the case of its damage. A rather important advantage of theelastic mirror is that its focal length can be varied in some range without any deterioration of optical quality of its workingsurface.Creation of large-aperture elastic mirrors of a high aperture ratio and the required optical quality is a problem having beenyet unsolved. Poor optical quality of specular surfaces of such elements made it impossible to apply them to real systemsuntil the telescopes with compensation for image distortions of their primaries have been developed. The magnitudes of theallowable (being corrected) image distortions in these telescopes depend on the large majority of factors, for instance, on thetype of the observed object and the required field-of-view of the telescope, on the spectral bandwidth of the light used and