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Figure 3.5: The quantum efficiency of the ROSAT PSPC as a function of energy.
Included are the window transmission, the average transmission of
the window support mesh, and the gas stopping efficiency
[ROSAT AO-21991, NRA 91-OSSA-31991]
The quantum efficiency of PSPC-B (the curve for PSPC-C would be nearly
identical) as a function of energy is shown in Fig. 3.5.
The curve includes the window transmission, average transmission of the
window support mesh (79%),
and the absorption of the counter gas.
The window becomes almost completely transparent by 2 keV,
where the counter gas becomes no longer optically thick.
The X-ray absorption of the counter gas is close to 100%
in the 0.1-2.0 keV band.
The fall-off at 
keV occurs where the effective area
of the X-ray mirrors also cuts off
(see Fig. 2.5).