A system and method for focusing infrared detectors operable at cryogenic temperatures. The invention includes a sensor (10) for detecting electromagnetic energy comprising a first detector (14) operable over a first temperature range and a predetermined number of auxiliary detectors (12) operable over a second temperature range, wherein the auxiliary detectors (12) are adjacent to and in the same optical plane as the first detector (14). In the illustrative embodiment, the energy is infrared or visible light, the first temperature range is a range of cryogenic temperatures, and the second temperature range is a range of ambient temperatures. The first detector (14) is a focal plane array and the auxiliary detectors (12) are uncooled detector arrays. In the preferred embodiment, the focal plane array (14) and the uncooled detectors (12) are disposed on a common substrate. In accordance with the teachings of the present invention, the novel sensor (10) can be used to focus an optical system at cryogenic temperatures. The inventive method includes illuminating energy onto the sensor (10) through the optical system at ambient temperatures and then adjusting the position of the sensor (10) until maximum illumination is received by the auxiliary detectors (12). This determines the location of the focal plane of the system at ambient temperatures. The location of the focal plane at cryogenic temperatures can then be calculated using the location of the focal plane at ambient and a model of the thermal characteristics of the system.