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Chapter 6 Advanced Topics 75
Shutter Open Close Open Close Open Close
Read Read Read
First exposure
Data
stored
Third
exposure
Data
stored
t
exp
t
c
t
R
Second
exposure
Data
stored
NOT SCAN
Figure 29. Free Run Timing Diagram
External Sync
In this mode all exposures are synchronized to an external source. As shown in the
flowchart,
Figure 30, this mode can be used in combination with Normal or PreOpen
Shutter operation. In Norm
al Shutter mode, the controller waits for an External Sync
pulse, then opens the shutter for the programmed exposure period. As soon as the
exposure is complete, the shutter closes and the CCD array is read out.
External synchronization depends on an edge trigger (negative- or positive-going) which
m
ust be supplied to the Ext Sync connector on the back of the camera. The type of edge
must be identified in the application software to ensure that the shutter opening is
initiated by the correct edge (in WinView/WinSpec, this is done on the
Experiment
Setup|Timing
tab page). Depending on the shutter, it may require up to 28 msec to fully
open. Therefore, the External Sync pulse provided by the experiment should precede the
actual signal by at least that much time. If not, the shutter may 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
Sy
nc pulse 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
sy
nchronized to the experiment. As soon as the controller is ready to collect data, the
shutter opens. Upon arrival of the first External Sync pulse 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.