A method for designing and predicting the performance of large aperture sensors is described. The sensor is modeled as an elementary optical system combined with a focal plane consisting of many discrete detectors. The output signals of the detectors are filtered and combined to compute signal-to-noise ratio and to estimate centroiding (target location) error. The dependence of these two performance criteria upon major design parameters is described. The quantum efficiency of the detectors is a critical factor. Optical aperture is also a major factor in design, but lack of a cost function for optical subsystems is a drawback. It is possible to design an optimal focal plane layout for given detector and optics models. The electrical filter is optimized separately. Centroiding error in the crossscan direction depends in an interesting way upon the focal plane layout. Two mathe-matical appendices describe the sensor as a linear, time-invariant system and some of the signal processing.