"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".