English: Left panel: Mirror R is stationary. Lens L (not shown) forms an image of slit S on spherical mirror M. The reflected image of the slit reforms at the original position of slit S regardless of how R may be tilted. Right panel: Mirror R is rotating rapidly. The reflected light from mirror M bounces from mirror R that has advanced an angle θ during the transit of the light. The telescope detects the reflected image of the slit at angle 2θ relative to the position of slit S.