Multi-degree-of-freedom measurement (MDFM) system is an effective way to measure multiple degrees of freedom errors simultaneously. In the MDFM system, the pitch angle and yaw angle are often measured based on the autocollimation measurement principle. Due to the lens fabrication and installation errors, the angle detector will deviate from the focal plane, causing defocus error. To address the problem, a defocus error model is established to analyze the influencing factors and solve the position of the focal plane. It is found that the straightness is the main factor of defocus error. When the defocus amount is present, the defocus error is larger with the increase of straightness. And the sensitivity of angle detector has a specific relationship with the intensity density of spot, which is related to the defocus amount. By using a precision displacement table, the position of the focal plane and the corresponding angle detector output can be accurately obtained. The experiment was carried out to verify the feasibility of this method. The angle detector and lens were installed precisely according to the angle detector output. When the vertical straightness is within ±400 μm, the influence of the defocus error on the pitch angle is less than 2 μrad. The results showed that the method can reduce the defocus error of auto-collimation measurement effectively, which can improve the accuracy of angle measurement in MDFM system.
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