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Chapter 6 Advanced Topics 57
57
programmed exposure period. As soon as the exposure is complete, the shutter closes and
the CCD array is read out.
Because the external shutter requires a finite amount of time to open completely (shutter
open time may be 5-28 ms depending on the shutter), the External Sync pulse trigger
edge provided by the experiment should precede the actual signal by at least that much
time. If not, the shutter will not be open for the duration of the entire signal, or the signal
may be missed completely.
Also, since the amount of time from initialization of the experiment to the first External
Sync pulse trigger edge is not fixed, an accurate background subtraction may not be
possible for the first readout. In multiple-shot experiments this is easily overcome by
simply discarding the first frame.
In the PreOpen Shutter mode, on the other hand, shutter operation is only partially
synchronized to the experiment. As soon as the controller is ready to collect data, the
shutter opens. Upon arrival of the first External Sync pulse trigger edge at the ST-133, the
shutter remains open for the specified exposure period, closes, and the CCD is read out.
As soon as readout is complete, the shutter reopens and waits for the next frame.
Figure 24. Chart Showing Two External Sync Timing Options
The PreOpen mode is useful in cases where an External Sync pulse trigger edge cannot
be provided 5-28 ms (shutter open time) 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 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.