Study On Low Order Aberrations’ Testing Technology For Large Optical Flat Mirror

With the development of science and technology, optical system with large aperture is rapidly expanding in space optics, astronomy optics and other fields. The flat mirror’s size is larger and larger, and could be used as reflection mirrors in big telescope systems, transmission lens of large interferometer, and standard mirror in some optical system. Taking the Thirty Meter Telescope-TMT project as an example, TMT’s tertiary mirrors(M3M) is an elliptical flat mirror, whose size is up to 2.5 m × 3.5 m.The large aperture optical system has high requirements on surface accuracy, and high-accuracy test method is needed. For the extremely large M3 M, the traditional methods such as CMM, Ritchey-Common, sub-aperture stitching etc. are difficult to simultaneously meet the demand of testing accuracy, cost, and efficiency. How to test the large flat mirror efficiently and accurately, especially the test of low order surface errors, is difficult. Our primary work is as follows:1) Research on online profile measuring technology for grinding large flat mirror. Traditional profile measurement methods such as CMM have disadvantages like low measurement accuracy, and long measurement cycle. In this thesis, a novel dual-gauge profilometer is presented. The entire low order surface profile is achieved from surface slope which is tested by two high-precision non-contact displacement sensors, by least square method. The error accumulation of uncertainty caused by displacement sensor could be avoided by using dual-gauge slope measurement, so the testing system has low dependence on highly accurate rail. A highly accurate testing result could be achieved by ordinary machine.2) Development of dual-gauge profilometer. The design of structure and control system, system alignment, precision calibration, measurement path and data processing are completed combined with previous theory, as well as the machine’s structure and motion feature. M3MP(a prototype of TMT’s tertiary mirrors, 600 mm × 900mm) was tested by the proposed dual-gauge profilometer. The difference of the testing result between the proposed method and CMM is 0.05μm rms, which satisfies the accuracy demand for large flat mirror’s online profile measurement.3) Theoretical analysis of scanning pentaprism technology. Scanning pentaprism technology could be as a testing method of low order profile during the polishing process of large flat mirror. In this dissertation the scanning pentaprism system’s mathematical model is built, based on ray tracing vector method of matrix manipulation. The form and order of testing accuracy influenced by various error sources are evaluated. For non-circular large flat mirror, singular value will be caused by the traditional least squares fitting algorithm, which results in the failure of the data fitting. In order to solve this problem, a method combining the inner products of polynomials and cholesky decomposition is adopted, which can be applied to arbitrary non-circular aperture. A simulated testing is performed on M3 MP, and the relevant results have passed the preliminary review or TMT project.4) Development of the scanning pentaprism system. According to the demands of TMT project, structural design, motion control, and data acquisition of the scanning pentaprism system have been completed. System’s main components, including the auto-collimation, pentaprism, 3D adjustor, high precision optical rails, feedback control are carefully calibrated in our laboratory to verify the system’s reliability. A Φ300mm standard mirror was tested by the well aligned scanning pentaprism system. The experimental results show that the testing precision of 300 mm mirror’s low order aberrations could be 60 nm rms, system’s measurement uncertainty for single testing point can be better than 0.1 arcsec, which satisfies the testing requirements of low order aberrations during polishing flat mirror.5) Application of relevant testing technologies in large flat mirrors’ manufacturing process. Combining with relative testing methods, this thesis confirms the testing strategy for large flat during different phases, then an manfactouring strategy based on CCOS technology is presented for manufacturing large flat mirrors. The proposed strategy is used to guide the actual manufacture process of M3 MP. Up to now, the whole polishing of M3MP’s back surface and preliminary polishing of its front surface has been completed. The surface convergence rate can be up to 56% in a single processing, proving the efficiency of proposed strategies in manufacturing large flat mirrors.