Disclosed is an apparatus, method, and computer program product for ridge map formation. Two flexible bands moving around a generally rectangular path about the aperture independently control the positions of each set of shutters. Therefore, a very fast response compared to the conventional sluggish voltage loop can be achieved. With this configuration, for example, a frequency of generation of the refresh execution signal in response to one supply of the refresh command can be changed dynamically, flexible control can be performed by a controller.