Pentaprism measurements of large flat mirrors

In view of the difficulty in measuring large flat mirrors, this paper introduces a pentaprism system that measures the angle differences between points on the mirror surface with a scanning pentaprism. Based on the angle differences, many equations were established and the mirror surface was expressed through a least squares calculation. The error analysis reveals that the system accuracy was 11.6nm root-mean-square (rms) over a 1.5m flat mirror. Then, the pentaprism measurement was compared with Ritchey-Common test. The comparison shows that the difference between the two results is 10.2nm rms over a 1.5m flat mirror, which is within the error range of the pentaprism system. Thus, the proposed pentaprism system is applicable to the measurement of large flat mirrors. © 2017 AMSE Press. All rights reserved.