Study on the calculation and application for asphericity of rectangle off-axis aspherics

ABSTRACT Asphericity which affects the manufacturing process and the grinding quantity is an important parameter of optical aspherics. In this paper, the approximation, the least square fitting, wavefront aberration, sag methods are given for the calculation of optical asphericity. The effects of different definitions of optical asphericity are analysed to optimize the process of manufacture. A sag method is presented to solve the best-fit sphere and asphericity for an off-aixs aspheric mirror with a rectangle aperture of 180mm×145mm, and polishing quantity and distribution plots are given. The optical manufacturing and testing technology for different choices of best-fit spheres and asphericity are described and analyzed. Therefore, proper definition of optical asphericity which is useful to manufacturing is very important for optical fabrication and test. Keywords: off-axis aspherics, Asphericity, Manufacturing, Test 1. INSTRUCTION With the development of optical manufacturing and testing technology, aspheric surfaces are widely applied in the optical system. Because off-axis aspherics can avoid the beam of light blocked, increase the system resolution and field angle to a large extent, correct the optical aberration, improve the image quality, simplify the instrument structure and reduce the instrument size and weight, it has been adopted broadly in optical systems, especially in the fields of space camera and astronomical telescope with segmented mirrors. Generally, the fabrication of aspherics starts from the best-fit sphere to the designed aspherics. Therefore, it is significant to calculate the best-fit sphere and ashpericity for the fabrication of aspherics. The grinding quantity and initial error distribution decide the manufacture and test method. The calculation method and asphericity distribution affect the difficulty and efficiency of manufacture. In this paper, the best-fit sphere and error distribution are calculated in different methods for a 180mm×145mm concave off-axis aspheric. Considering manufacture technique and test method, the calculation method of asphericity is analyzed and appraised for the off-axis aspherics.