Effects of a misaligned photodetector in autocollimators on angle measurements

As one of the key errors in autocollimators, misalignment of the photodetector is analyzed and modeled carefully in this paper. Effect of misalignment of the photodetector on angle measurements is characterized, when the photodetector in any position and orientation in space with respect to the theoretical image plane of autocollimators. It is shown that the angular measurement errors of autocollimators induced by a misaligned photodetector increase with greater measuring range L, larger angle θ and smaller focal length f of the collimating object lens. When f=300 mm, L=100 mm, θ=20″, the angular measurement error caused by a misaligned photodetector is 0.004 5″. The effects of each photodetector misalignment error on angle measurements in autocollimators are characterized. The model proposed in this paper is validated. Among all kinds of photodetector misalignment errors, the defocusing error has the greatest influences on autocollimators. Hence, it is critical to choose an imaging objective with longer focal length, reduce the measurement distance, and improve the installation accuracy of the photodetector along the axis. The model proposed in this paper helps to systematically obtain the angular measurement errors caused by a misaligned photodetector, which will play a key role in building a better error analysis model for autocollimators.