Research On Testing Large Diameter Off-axis Aspherical Surface Using Shack-Hartmann Wavefront Sensor

The optical system using aspherical elements not only can improve the aberration correction ability, but also can reduce the number of optical components, which can simplify the system structure and reduce the optical system weight. So more and more aspherical mirrors are applied in military, aerospace, astronomy and other scientific fields. With the development of space science and technology, the requirements on the resolution of the optical system and the size of the field of the optical system are increasing, which need large diameter, high-precision off-axis aspherical elements to complete. Manufacturing large diameter, high-precision off-axis aspherical surface needs corresponding high-precision processing equipment, testing equipment and testing technologies, which bring new challenges to the field of processing and testing.At present, large diameter aspherical mirror processing technology uses both grinding method and polishing method. The mainly testing technologies at grinding stage and polishing stage are profile measurement and null compensation interferometry respectively. However, at the stage of fine grinding and coarse polishing, the accuracy of the common profilometer is in the same order of magnitude with the aspherical surface error, which means the accuracy of the profile measurement is not enough. At the same time, the aspherical surface error is beyond interferometer’s dynamic range, and the surface finish of the tested mirror is very low, so null compensation interferometry can’t test the full aperture surface error. Profilometer measurement range and interferometry measurement range can’t overlap each other at the stage of fine grinding and coarse polishing, so they can’t guide optical processing effectively. Shack-Hartmann wavefront sensor has large dynamic range, high measurement accuracy, low environmental requirements and short exposure time, etc. Based on these advantages, this paper presents a method using Shack-Hartmann wavefront sensor to test large diameter off-axis aspherical mirror at the fine grinding stage and coarse polishing stage. This dissertation includes following three parts:1. Non-null testing of large diameter off-axis aspherical surface by using a Shack-Hartmann wavefront sensor. After an in-depth study on the principle of Shack-Hartmann wavefront sensor and a theoretical analysis of the error sources of testing system, we separate the systematic errors correctly and utilize ray trace software to calculate the system’s aberrations, and also make the reference file for non-null testing aspherical surface. With the help of the reference, a360mm*240mm rectangle off-axis aspherical mirror is tested by using a SHWS, we compare the surface error to the result that gets from null testing the aspherical mirror by using interferometer. This experiment verifies the feasibility and operation ability of the testing system.2. Null testing of large diameter off-axis aspherical surfaces by using a SHWS and a compensator. A CGH that used for null testing aspherical surface is designed, and I take a imitation and simulation on null testing of a large diameter off-axis aspherical surface by using a SHWS and the CGH. Then I take an analysis on using a SHWS and an null lens to test aspherical surface.3. Null testing of large diameter off-axis asphcrical surfaces by using a SHWS and an interferometer. With the help of the splitter and expander, we can null test a large diameter off-axis aspherical surfaces by using a SHWS and an interferometer at the same time. By design a CGH to simulate the tested mirror, we can measure the system error, and then take a relative testing on the mirror, it can increase the precision of the surface error testing result.