Interferometers are used to measure errors in the shape of mirror surface. They are classified into two groups, common-path interferometers and non-common-path interferometers. In common-path interferometers, the measurement and reference paths pass through essentially the same space and therefore the interference fringes are scarcely affected by air turbulence in the optical paths. The zone-plate interferometers proposed by Smartt~1 belong to the group of common-path interferometers. The zone palte used in the interferometer is manufactured to generate a diffracted wavefront which corresponds to the design shape of the mirror surface.
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