Teledyne PIXIS-XF, System Manual

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Chapter 6 Advanced Topics 61
The PreOpen {Open Before Trigger} mode is useful in applications where an External
Sync pulse cannot be provided 8 ms (or the length of time a mechanical shutter takes to
open) before the actual signal occurs. Its main drawback is that the CCD is exposed to
any ambient light while the shutter is open between frames. If this ambient light is
constant, and the triggers occur at regular intervals, this background can also be
subtracted, providing that it does not saturate the CCD. As with the Normal Shutter
{Normal} mode, accurate background subtraction may not be possible for the first
frame.
Also note that, in addition to signal from ambient light, dark charge accumulates during
the wait time (t
w
). Any variation in the external sync frequency also affects the amount
of dark charge, even if light is not falling on the CCD during this time.
Figure 6-4 illustrates the timing diagram for External Sync mode.
Figure 6-4: Timing Diagram: External Sync Mode (+ edge trigger)
NOTE:
If EXT SYNC is still active at the end of the readout (i.e., it is
still HIGH in
Figure 6-4 ,) the hardware may interpret this as a
second sync pulse, and so on.
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4
11
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