An oscillating mirror is frequently used as a scan mechanism in thermal imagers. The correct location of the rotation axis to minimise unwanted displacements of a beam whose direction is being changed has been determined using a ray-tracing technique well suited to many problems in optomechanical scanner design. Numerical analysis reveals that a ray may be rotated by at least 20° about a point 10 units in front of a plane mirror if the mean deflection angle at the mirror is 120° and the rotation axis of the mirror is located 16.7 units in front and 5.8 units on the incidence side of the mirror.
"Optimum Rotation Axis For A Flapping Mirror In Thermal Imaging Scanners", Proc. SPIE 0197, Modern Utilization of Infrared Technology V, (17 December 1979); doi: 10.1117/12.957987; https://doi.org/10.1117/12.957987