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In order for the PSPC to record an incoming X-ray photon as an event, the photon must first be transmitted through the PSPC entrance window, and, second be absorbed in the counter volume. The PSPC entrance window is a 1 m foil of polypropylene coated with 50 g cm carbon and 40 g cm lexan. As a consequence, the window is basically opaque at energies just above the carbon edge at 0.28 keV, but transmitting at about 50 percent just below the carbon edge. The X-ray absorption of the counter gas is close to 100 percent almost up to 2 keV where the X-ray mirror cuts off. The resulting quantum efficiency as a function of energy is shown in Figure 4.5 .