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This allows the events stored by the WFC to be restricted to those which are detected within a restricted spatial window within the field of view of the WFC. The window can be any rectangular region specified with a resolution of 32 pixels (i.e. the smallest window possible corresponds to a region in extent with ZOOM off). Events from this window can be either included or excluded from storage, and hence eventual transmission to the ground. The main use for electronic windowing is in situations where the total count rate expected from the whole field of view exceeds the normal maximum data rate of 200 counts s (and would thus lead to large dead times). This situation will occur, for example, when making observations with the long wavelength filter (P2), since the geocoronal background for this filter is relatively high (see § 7.8.4 ). The appropriate window will then be selected by operational staff.
Electronic windowing can also be used to ``mask out'' detector hot-spots or other defects which would otherwise saturate the data rate; windowing of such features, which are expected to be essentially fixed, is achieved within the WFC CDHS using a similar but independent mechanism.