Nauka i Obrazovanie (Jan 2015)
Testing the Secondary Mirror Surface Form of a Radiotelescope «Millimetron»
Abstract
A new method to test a surface form of the convex high-aperture hyperbolic mirrors has been developed with the secondary mirror of a telescope «Millimetron» as an example. It was proposed to use the selfsame auxiliary spherical mirror relatively small in diameter for testing the inner central part and outer part of a hyperbolic mirror.To test the central part of a hyperbolic mirror, an aberration-free points method is offered. It uses the non-traditional ray path for the secondary mirror. As a result, the overall length of interferometer measurement branch optical system is significantly reduced. Working wave front is reflected trice from the auxiliary spherical mirror surface and twice from the hyperbolic mirror under test. Double interaction of the working wave front with tested surface increases sensitivity of optical testing.The feature of the working bundle ray path is that the rays fall towards the auxiliary mirror once perpendicular to the surface and twice at the arbitrary angles to the surface. As a result, the homocentricity of the ray bundle is violated, i.e. the spherical aberrations arise. To compensate aberrations, a Mangin mirror compensator is used. It is placed in the central non-working zone of a hyperbolic mirror.The outer part of hyperbolic mirror is tested using the selfsame auxiliary spherical mirror according to Hindle scheme in several stages with serial imposition of the auxiliary mirror until information about the whole surface is collected. The interferometer provides measurements recording the interferogram from each part of the surface of tested mirror. Then to obtain complete information about the mirror surface form the interferograms are united.The article presents calculation results of the optical systems for testing the hyperbolic secondary mirror of the radiotelescope «Millimetron», which has extremely high aperture. It has been shown that the auxiliary spherical mirror of 665 mm in diameter and with 500 mm radius of curvature provides testing of hyperbolic mirror of 492 mm in diameter with the aperture angle of an edge ray exceeding 90 degrees. The wave front aberration residuals of the interferometer measurement branch optical system with auto-collimation ray path make 0.2 of He-Ne laser wavelength.
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