Study On The Technique Of Off-Axis Mirrors Test

Off-axis three-mirror system is with no obstruction. It is for this reason that onecan expect diffraction limited quality over a wide field of view. It should be clear thatOff-axis telescope is the next generation of earth observation telescopes. However, itis well known that fabrication of off-axis apsheres and alignment of off-axis systemare quite difficult. The fabrication of off-axis optical element requires thecorresponding testing method. The optical measurement of off-axis mirrors for thenext generation telescopes present a serious challenge.This thesis is dedicated to solve the key theoretical and technical problems forthe test of off-axis aspheric optics. Here, the following researches in the test arecarried out:.1. The transformation matrix of Zernike aberration coeffients is developedbetween scaled and decentered pupils. So is rotaion matrix for rotation of wavfront.When the test piece changed from the rotationally symmetric parent asphere to theoff-axis asphere, it means that the pupil of the test system is scaled and decentered.Therefore, the transformation matrix is the theorical base for analysis of therelationship between the off-axis aspheric mirror and its parent mirror in the test.2. Based on the third order theory, design theory is established for Offner, Dalland Zero power null lens. The impact of structure parameter of the null lens on theresidual aberration is discussed, and the principle and value scope of parameterselecting is determined.3. Tolerance analysis is performed for the null corrector of the tertiary off-axisaspheric mirror. Optical fabrication errors, alignment errors and environmental effectsare all considered.4. The measurement of aspheric surface with CGH (Computer generatedhologram) and the alignment technology of the CGH are illustrated. The design ofoptical null testing off-axis aspheric mirror is developed. To test the steep off-axisaspheric, the null optics are often expensive to produce accurately. We can combine aspherical mirror and a CGH. The spherical mirror is tilted to compensate much of theastigmatism and some coma, and the CGH compensates rest of aberrations. At last wegive the design example for an off-axis parabolic mirror.5. Based on the mathematic description for surface departure of the off-axisasphere from the paraxial sphere, misfigure induced by the error on the off-axis distance is analyzed. The coma observed in the test for rotationally symmetric parentaspheric is the source of the distance error. In our discussing off-axis interferometrictest for off-axis aspheric mirror, we showed that the error on the distance of theoff-axis aspheric is linear with the field of view. Meanwhile, a way to fine thetolerance on the allowable rotation error is developed by use of rotation matrix.6. In interferogram the null lens test and stigmatic null test for off-axis asphericmirror, we analyze the effects of alignment error with optical path method by ray trace.An expression for wavefront aberrations induced by misalignment of the test piece isderived, and an optimum method for separating misalignment from misfigure ininterferograms on off-axis aspheres is developed. The method are applied to thealignment of two off-axis mirrors in the null lens test and one off-axis mirrors instigmatic null test. The test result of all the three off-axis mirrors can reach up to1/40λ (λ=632.8nm).The research in this thesis is as a theoretical and technical guide for off-axisasphere test. Finally, the primary and tertiary off-axis aspheric mirrors in XX cameraare successfully fabricated. This supplies a basic technical guarantee for finalsuccessful development and manufacture of the off-axis telescope.