A catadioptric Cassegrain optical system is described that permits simultaneous operation as an infrared (IR) imager and a millimeter wave (MMW) radar through a common aperture. Separation of IR and MMW focal points is achieved by use of suitable dichroic coatings and substrate materials. We have evaluated our design by building a prototype and measuring its performance in the laboratory with particular attention given to isolate the degradation suffered by each mode due to the presence of the other. We present our test results and conclude from an analysis of the data that although our design is feasible, improvements are needed both in the design of the secondary element and in the technology for conductive dichroic coatings. We suggest a straightforward solution to the problem with the secondary element and present evidence to show that requirements for conductive coatings will soon be achieved.