Precise measurement of curvature radius for spherical mirror with large aperture

Abstract

Conventional testing methods for the curvature radius of spherical mirror with large aperture are introduced briefly,and a novel method using combined rods and laser interferometer is proposed. Firstly,the surface figure of spherical mirror is tested with laser interferometer. The position of interferometer and tested mirror is adjusted to make the focus of the exit wave front and the center of curvature of tested mirror to be confocal. Then the suitable combined rods are setup,and the position of probe spheric head which is close to interferometer is adjusted to make the focus of the exit wave front and the center of the probe spheric head confocal. Next,another probe spheric head is adjusted to contact the surface of the tested mirror. The curvature radius of the spheric mirror can be calculated by the data. The basic principle of the testing method is analyzed. A telescope spheric primary mirror with a aperture of 600 mm is tested many times with this method. The average radius of curvature is calculated to be 2 836. 774 mm,and the root mean square is 0. 071 mm. Finally,the measurement uncertainty of this method is analyzed,and the main factors affecting the measurement accuracy are found out,and the composed standard uncertainty is 0. 061 mm.