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As described in Sect. 3.5.3, the position of an event in the PSPC is determined from the amplitudesof signals on the two cathode grids. The specific algorithm uses the signals from each cathode grid. For events with large pulse height amplitudes, this works very well since the strength of the signals on the cathode segments is related to the strength of the signal on the anode. However, for events with small pulse height amplitudes, occasionally only one cathode segment of a grid will have a signal above the lower discriminator. This still gives a well specified, but unfortunately incorrect, position. If two segments of one grid have signals but only one of the segments from the other grid has a signal, then the determined position will lie along a line. If only one segment from each grid has a signal, then the determined position will lie at a point. This structure is very apparent when looking at an image of events in detector coordinates with pulse heights less than channel 15 ( 0.15 keV). There is a cross-work pattern of lines with bright spots at the intersections. There are secondary bright spots in the centers between the lines since the probability is greater there to get signals on four cathodes (see App. 7.1.1).