Study On The Alignment Technique Of Large Ground-based Electro-optical Imaging Telescope

Following the development of the telescope, the telescope alignment techniques also have a long history. At first, the development history of the telescope and the representative of alignment and test tools were approximately introduced. As the most important part of the telescope, the alignment qualitative of main optical system was vital to the imaging quality. On the basis of referring to lots of literatures, the alignment and test techniques of the telescopes both internal and abroad were gathered, and the alignment process as to the assemble and adjustment of single mirror group and system were particular introduced. According to two parts of the system alignment process, the mechanical gross-adjustment and optical fine-adjustment, the corresponding alignment tools’property and applicable situation were elaborately analyzed. As to the optical test process, lab system testing and on sky system measurement techniques were separately introduced.Secondary, the alignment theory was analyzed. In the presence of the Seidal optical aberration theory, third order aberration caused by the main mirror and secondary mirror positions’errors were analyzed in detail. As the key point of the decreasing imaging aberration, coma was particularly studied. When the telescope was kept in the Coma-free Scheifspiegler, the astigmatism was very important to get the optical axis’angle of the main and secondary mirrors by calculating different fields’data. The longitude error between the main and secondary mirrors was recurred to three order spherical aberration. Because of the important role played by the alignment process, the property and error analysis of astigmatism were studied by oneself. The part of the discourse was the theoretical basement of the alignment techniques of the telescope.Thirdly,an example of the whole alignment process according to a actual 1.23 alt-azimuth telescope system was analyzed, and the limitation of the traditional alignment was studied. The traditional alignment method contained the null compensation test of the main mirror, the secondary mirror test in the presence of the main mirror and large aperture reflecting mirror, and two-pass autocollimation interference test of the system. The new alignment method of the large aperture electro-optical imaging telescope such as 4m aperture was derived according to the limitation of the traditional alignment method, including the feasibility scheme of the main mirror test and the secondary mirror test. The mechanical gross-adjustment process was investigated for the implementation of the coaxial of the system, and good imaging quality could be achieved by the alignment and test process.Optical fine-adjustment was very important in the alignment process. Fourthly, the natural star was used as the light source to get the defocused star for larger aperture telescope, whose geometry was used as the reference data to adjust the secondary mirror’s position error. Third order coma, third order astigmatism and third order spherical aberration may separately lead to special geometric property of the defocused star, which individually corresponding to the decenter and tilt of the secondary, the tilt angle of Coma-free Scheifspiegler and the longitude error between the main mirror and secondary mirror. Zemax was used to simulate to construct the optical mode and the software Matlab was also used to handle the data for proving the analytic process.At last, according to the third order aberration and fifth order aberration individually, another alignment technique was analyzed, computer aided alignment technique, which was mainly used as the test and alignment of the position error of the mirrors by appending the field data to Zernike components by use of wave aberration theory. Third order aberration was applied by simple two-mirror system, while fifth order aberration was applied by complicated multi-mirror system. Zemax was used to Model and get the corresponding Zernike coefficients, Matlab was also used to construct the sensitivity matrix and the least square was applied to achieve the test of the misalignment errors. The simulation process proved the feasibility of the method. The simulated results showed that the computer aided alignment was a rigid tool for alignment. For simulating the noise disturb process, the Zernike coefficients was purposefully added some random perturbing errors. The good convergence of the results proved that the method had well ability to conquer the interfere. Computer aided alignment could widely used to achieve the optical fine alignment process of actual telescope.