Measuring Parallelism for Two Thin Parallel Beams Based on Autocollimation Principle

Abstract

In order to measure the parallelism of two thin parallel beams in multi-axis interferometers and differential interferometers, the autocollimation principle was used by locating a CCD at the focal plane of collimation lens, the two beams converge on CCD image plane at different points when they are not parallel, and the distance between the two converging points is used to calculate the parallelism of the two beams. To achieve high accuracy and low system error, the CCD defocus and tilt should be strictly controlled according to the accuracy requirement. A centroid algorithm with grey threshold was used to reduce the influence of image noise. A beam-selected structure was designed to let the two beams converge to the CCD image plane in sequence to avoid superposition of beam spots. An experimental setup is built to verify the validity of the method. Experimental results show that the system has a centroid position resolution of 0.05 arcsec and a centroid position stability of 0.4 arcsec. It is therefore concluded that the method can be used to measure parallelism that is more than few arcsecs for two thin beams. Moreover, the measuring accuracy of this method can be improved when higher centroid position stability is achieved.