Study of adjustment influence on the autocollimating null-indicator accuracy

Null-indicator (NI) is an optical device setting up a reference direction in space for high-precision angle measurements by means of dynamic goniometry. During continuous rotation of the optical object under test (e.g. optical polygon or gauge) together with the angular scale the NI registers moments when reflecting faces of the object are normal to the optical axis of the NI. At these moments NI yields electronic logical pulse which triggers the readout from the angular scale. Conventionally interferometric null-indicators were considered as the most accurate, though they have few significant drawbacks. The interferometric NI is very sensitive to the quality of the reflecting surface and other factors that may bring aberrations to the NI emitted wavefront. This makes the interferometric NI suitable only for laboratory applications with test objects of high quality. Recent development has shown that autocollimating NI with digital signal processing can reach the same level of random error as the interferometric one. But during testing of the experimental model of the autocollimating NI authors have encountered systematic error that needed additional study. The encountered error was related to the adjustment of certain element of the NI optical scheme and the object under test relatively to the axis of rotation. The report presents theoretical and experimental study of the influence of aforementioned factors on the accuracy of setting the reference direction by the autocollimating NI. Also authors give recommendations on the adjustment procedure of the dynamic goniometer utilizing autocollimating NI in order to reach required measurement accuracy.