The focal plane examined has a small fill factor; in other words, there is a large dead space between detectors. From the size of the blur spot, one can calculate the fraction of the target energy, from a point source, which is detectable. This fraction is a function of the position of the center of the blur spot. As a result, the detected target intensity varies dramatically as the target moves across the focal plane, even when the target intensity is a constant. Intensity measurements can thus be extremely inaccurate. The effect is studied for several fill factors, from 100% down to 50%,the approximate fill factor for the proposed system. The effect can be compensated for if the precise position of the blur spot on the detector is known. One method, which estimates that position using intensities from adjacent detectors, is shown. This method's value, unfortunately, declines as the fill factor decreases. Furthermore, the method can only work when the detector response as a function of target position is known precisely. The effect of the focal plane design on separation of closely-spaced objects (CSOs) is then derived. Several cases are shown in which multiple targets, separated by substantial fractions of a detector width, are indistinguishable from a single target. The effect of changing the fill factor is also demonstrated. As the fill factor decreases, the effect worsens. Proposed changes to the sensor design include increasing the fill factor and/or defocussing the blur spot. Results are shown for various combinations of these parameters.