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With the increase of telescope size, the manufacture of monolithic primaries becomes increasingly difficult. Instead, the
use of segmented mirrors, where many individual mirrors (the segments) work together to provide good image quality
and an aperture equivalent to that of a large monolithic mirror, is considered a more appropriate strategy. But, with the
introduction of large telescope mirror comprised of many individual segments, the problem of insuring a smooth
continuous mirror surface (co-phased mirrors) becomes critical. One of the main problems arising in the co-phasing of
the segmented mirrors telescope is the problem of measurements of the vertical displacements between the individual
segments (piston errors). Because of such mirrors to exhibit diffraction-limited performance, a phasing process is
required in order to guarantee that the segments have to be positioned with an accuracy of a fraction of a wavelength of
the incoming light.The measurements become especially complicated when the piston error is in order of wavelength
fractions. To meet the performance capabilities, a novel method for phasing the segmented mirrors optical system is
described. The phasing method is based on a high-aperture Michelson interferometer. The use of an interferometric
technique allows the measurement of segment misalignment during daytime with high accuracy, which is a major design
guideline. The innovation introduced in the optical design of the interferometer is the simultaneous use of both
monochromatic and white-light sources that allows the system to measure the piston error with an uncertainty of 6nm in
50µm range. The description about the expected monochromatic and white-light illumination interferograms and the
feasibility of the phasing method are presented here.
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