How to achieve diffraction limited resolution with large space telescopes

Large space telescopes cannot be made diffraction-limited at least for short wavelengths. Methods for reconstructing diffraction-limited images are reviewed including active optics, Michelson stellar interferometry, non-redundant aperture arrays, deconvolution and shearing interferometry. Expressions are given for the signal-to-noise ratio in each case and the result of laboratory simulations are presented. It is concluded that diffraction-limited images could be obtained in the visible up to magnitude 21 with large deployable reflectors planned to be used in the far I.R. or millimetric range.