Design of the Rotation Angle Measurement System for the Atmospheric Dispersion Corrector (ADC)

In order to measure the rotation angle error of the worm and gear mechanisms in the atmospheric dispersion corrector (ADC), a novel measurement system based on the autocollimator is designed in this paper. Based on the position relations between the shaft and spot converged by rays of autocollimator, the rotation angle can be calculated quickly and conveniently using a brief algorithm. An optical wedge is introduced to the measurement system for suppressing the awful measuring error caused by the axial wobbly error. The measurement system can measure the shaft rotation angle at any rotation position, which is the novel usage of the two-dimensional autocollimator. Its optical layout is very simple. Only an optical wedge and two plane mirrors are needed besides the autocollimator. The measurement accuracy of the proposed method is less than ±0.5 arcmin. In the measurement of two worms and gears mechanisms in ADC, the rotation angle error are ±0.05° and ±0.07° respectively, which all satisfies the design demand (±0.1°). The proposed measurement system can be also suits for some engineering fields.