Development of null test measurement system of aspherical mirror shape by scatter plate interferometer

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Main-mirror of big astronomical telescope has concave and rotationally symmetric aspherical shape. The performance of the telescope is greatly influenced by the precision of the shape of the mirror.As a method for measuring the surface shape and/of the characteristics of optical systems, conventional interferometers have been widely used. But, if the conventional interferometer such as Twyman-Green one is used, many interference fringes generate and the interval of fringes becomes very narrow when deep aspheric surface is measured.For overcoming these defects, computer generated hologram ( CGH ) or special optical elements are used. But these interferometers are fundamentally different-path interferometers, and can not be used for aspheric surfaces with large aperture-size and long radius of curvature.For the interferometric measurement of these surfaces, the common-path interferometric method is inevitable. So, the purpose of this research is to develop null-test scatter-plate interfe … More rometer for measuring concave deep aspheric surfaces with rotationally symmetry.The scatter-plate interferometer is one of common-path interferometers, and the principle of null-test by this interferpometer is as follows. A scatter-plate with larger aperture size of which characteristics are point-symmetry are used and a pinhole is set apart from the position behind the scatter-plate. In this optical set-up, the reference wave-front becomes aspheric and can be made almost equal to the wave-front which is reflected from the aspheric concave mirror under test. As a result, null-test can be realized.A zone-plate interferometer is also common-path interferometer and a specially optically made zone-plate can be regarded as equivalent with a scatter-plate when the zone-plate is used in the above mentioned interfero-meter instead of the scatter-plate.In this researh, at first, the priciple of null-test on scatter-plate interferometer is made clear, and next, the relation between the scatter-plate and the zone-plate interferometer is compared. Then, two kinds of fringe-scanninig techniqes which can be applied to common-path interferometer are developed and practised. And finally, computer-controlled fringe pattern aquisition and analysis system is constructed. Less