In the field of optic instruments comprising at least one optical architecture, a photoreception assembly and means for acquisition and analysis of the images arising from the said photoreception assembly, the acquisition and analysis means comprising an algorithm of phase diversity type, an optical architecture comprises an optical plate of low or zero optical power arranged in the vicinity of the photoreception assembly and disposed so as to form on all or part of the said assembly a first focused image and a second image defocused by a first predetermined value and shifted by a second predetermined value with respect to the first image. The optic instrument can advantageously be a space telescope.