The spatial infrared imaging telescope (SPIRIT III) radiometer is the primary instrument aboard the Midcourse Space Experiment (MSX), which was launched on April 24, 1997. The Space Dynamics Laboratory at Utah State University (SDL/USU) developed and implemented a ground-based procedure to optimize the focus of the SPIRIT III radiometer. The procedure used point source data acquired during ground measurements. These measurements were obtained with a calibration source consisting of an illuminated pinhole near the focus of a cryogenically cooled collimator. Simulated point source measurements were obtained at multiple focus positions by translating the pinhole along the optical axis inside and outside the optimum focus of the collimator. The radiometer was found to be slightly out of focus, and the detector focal plane arrays were moved to positions indicated by the test results. This method employed a single cryogenic cycle to measure both the distance and direction needed to adjust each array for optimal focus. The results of the SPIRIT III on-orbit stellar point source observation demonstrate the success of the technique. The method and hardware used to achieve focus optimization are described.