This paper examines the focal plane for the proposed Brilliant Pebbles sensor. Each Pebble is designed to be autonomous -- to locate and identify thrusting targets, determine which it can reach, and to attempt to intercept the closest target. Tracking a booster is not possible with a single sensor, so each Pebble identifies targets by comparing the measured intensity histories with those for known targets. The proposed focal plane has a large dead space between detectors. The detected target intensity thus 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. Knowing the precise position of the blur spot on the detector helps to compensate for this effect. One method, which estimates position using intensities from adjacent detectors, is shown. But this method's value 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 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.