Objective  As the core optical element of the off-axis optical system, the off-axis aspheric mirror has the function of reducing the volume of the optical system, increasing the field of view and improving the imaging quality. Aluminum alloy material is one of the commonly used materials for off-axis aspheric mirrors because of its high processing efficiency, low cost and the ability to realize the athermalized design of the optical systems. Due to the special application field of the off-axis optical system, the accuracy requirements of the off-axis aspheric surface are very strict. In the process of high-precision turning of large-diameter off-axis aspheric aluminum alloy mirrors, the micron-level surface shape error caused by centrifugal force becomes non-negligible. In order to reduce the surface shape error caused by centrifugal force deformation of off-axis aspheric aluminum alloy mirror during turning, it is necessary to study the single-point diamond turning process of off-axis aspheric aluminum alloy mirror.

  Methods  Based on the basic principle of centrifugal force generation, a machining method to suppress centrifugal force error is proposed (Fig.4). Through the finite element simulation method, the translation displacement and rotation angle of the centrifugal force coordinate transformation machining model are optimized. Taking an off-axis aspheric aluminum alloy mirror with an aperture of 320 mm as an example, the optimal coordinate transformation parameters are reflected in the optimization results (Fig.6), and the off-axis aspheric mirror is used for ultra-precision turning.

  Results and Discussions  Using the processing method of suppressing centrifugal force error, the off-axis aspheric surface of aluminum alloy with a diameter of 320 mm is processed by Nanoform 700 ultra diamond lathe, and the processing results are detected by Zygo interferometer. A high-precision off-axis aspheric aluminum alloy mirror with PV of 3.121λ and RMS of 0.198λ (Fig.8) is obtained. The machining method of surface suppression centrifugal force error can provide theoretical guidance for the ultra-precision machining of large-aperture off-axis aspheric aluminum alloy mirror, and provide theoretical support for the large-aperture turning of other materials.

  Conclusions  Aiming at the problem that the surface shape accuracy of large-aperture off-axis aspheric mirror is deteriorated due to the centrifugal force error in single-point diamond turning process, a machining method to suppress centrifugal force deformation is proposed. According to the mechanism of centrifugal force generation, a machining model for suppressing centrifugal force coordinate transformation is established. The influence of different translation displacements and rotation angles on the surface shape accuracy in the process of coordinate transformation is analyzed and optimized by finite element simulation. Finally, the ultra-precision turning experiment of RSA6061 aluminum alloy off-axis aspheric mirror with an aperture of 320 mm is carried out by using the coordinate transformation parameters obtained by the simulation analysis, and a high-precision off-axis aspheric aluminum alloy mirror with RMS of 0.198λ is obtained. The proposed processing method of suppressing centrifugal force deformation realizes the ultra-precision machining of large-diameter off-axis aspheric aluminum alloy mirrors. While improving the surface shape accuracy of the off-axis aspheric aluminum alloy mirror, the limitation of the aperture, off-axis amount and vector height difference on the processing stroke in the manufacturing process of the off-axis aspheric aluminum alloy mirror is reduced, and the manufacturing accuracy of the aluminum alloy optical element is effectively improved.