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Operational Modes

  

There are three software ``switches'' in the WFC which have a direct influence on the scientific data received from the focal plane detector.

Zoom:

The pixel output of the WFC can correspond to the full diameter field of view (ZOOM off), or can be `zoomed' so as to correspond to only the central region of the field (ZOOM on). The angular size of each pixel is therefore halved when the ZOOM is on. This facility is not normally needed, and has only been used for a small percentage of observations, mainly during the earlier phases of the mission.   

Data rate:

The on-board storage rate for detected events (i.e. photons or background particles) is normally restricted to 200 counts per second. This corresponds to filling the entire WFC allocation of the on-board tape recorder in 24 hours.  There is a high data rate of 400 counts per second which can be used for limited periods, for example to take advantage of the storage allocation not used during intervals when the detector is switched off (e.g. during radiation belt transits). The data rate is normally optimised automatically by on-board software. During the pointed phase of the mission the detector count rate has rarely exceeded the 200 count per second threshold during observations. The dead time in the WFC, due primarily to losses in the queueing of events by the on-board software, depends on the count rate and which of the two data rates is selected (Fig. 6.2).   

  
Figure 6.2: Curves showing the telemetered count rate against the true count rate for the WFC in low speed (solid line) and high speed telemetry modes (dashed line). The background count rate in the WFC rarely exceeds 300 counts/s for extended periods of time.

Electronic windowing:

This allows the events stored by the WFC to be restricted to those which are detected within, or outside of, 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). The main use for electronic windowing would be in situations where the total count rate expected from the whole field of view exceeds the normal maximum data rate of 200 counts per second (and would thus lead to large dead times). 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. To date, neither of the windowing facilities has been used.