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72 PIXIS System Manual Version 2.C
Figure 32. Continuous Cleans {Clean Until Trigger} Flowchart
Once the External Sync pulse is received, cleaning of the array stops as soon as the
current row is shifted, and frame collection begins. With Normal Shutter {Normal}
operation the shutter is opened for the set exposure time. With PreOpen Shutter {Open
Before Trigger} operation the shutter is open during the Continuous Cleaning {Clean
Until Trigger}, and once the External Sync pulse is received the shutter remains open for
the set exposure time, then closes. If the vertical rows are shifted midway when the
External Sync pulse arrives, the pulse is saved until the row shifting is completed, to
prevent the CCD from getting "out of step." As expected, the response latency is on the
order of one vertical shift time, from 1-30 s depending on the array. This latency does
not prevent the incoming signal from being detected, since photo generated electrons are
still collected over the entire active area. However, if the signal arrival is coincident with
the vertical shifting, image smearing of up to one pixel is possible. The amount of
smearing is a function of the signal duration compared to the single vertical shift time.
Note: If EXT SYNC is still active (in Figure 33, this means that if it is still HIGH) at the
end of the readout, the hardware may interpret this as a second sync pulse, and so on.