In a data communication system such as a high capacity DBS system, dynamic mapping of broadcast resources is provided to exploit occasional redundancy in the program content of two or more input data streams, freeing at least one broadcast resource to carry alternate bitstreams, such as additional programs or existing programs at higher quality. A tank holds coupling liquid; the system includes a membrane for separating the liquid container from the coupling liquid. The prism includes a reflecting surface reflecting the light flux passing through the fourth lens to a direction along a second optical axis intersecting with the first optical axis.