One of the main disadvantages of the IPCS is its behaviour at high count rates. At count rates much above 0.1 counts per pixel per second the IPCS starts becoming nonlinear. The way in which it does this is rather complex, depending on both the format of the IPCS and the extent of the emission, and is discussed in some detail by Jenkins (Monthly Notices of the Royal Astronomical Society, vol 226, p341, 1987). The maximum count-rate allowable for a particular observation depends on what degree of nonlinearity is acceptable. A common rule of thumb is that the count-rate should not exceed 0.5 counts per pixel per second. At this count-rate we might expect the deviation from linearity to be between 10 and 20 per cent. For observations which would give a higher count rate than this, it is necessary to reduce the count rate using neutral density filters.