It is frequently assumed that the virtual image of a target viewed through a rotating mirror moves with respect to the observer at twice the angular rate of mirror rotation. This assumption is false, and leads to imprecise treatment of open-loop tracking systems. Of particular interest is a class of dynamic visual acuity experiments in which acuity targets are viewed through a rotating mirror, where control of image velocity, exposure time, and image dimensions are of critical importance. Expressions are derived which describe the direction of the target image with respect to the observer as a function of mirror position. This relationship is nonlinear, and depends upon the distances from the center of rotation of the mirror (A) to the observer (C), and to the target (B), and upon the included angle BAC. Expressions are further derived for image velocity, acceleration, mirror intercept, and image dimensions as functions of mirror position.
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