Teledyne Princeton Instruments PI-MTE, System Manual

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Chapter 6 Advanced Topics 63
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 primary
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.
NOTE:
In Figure 6-5 , when EXT SYNC is still active (i.e., it remains
LOW,) at the end of the readout, the hardware may interpret
this as a second sync pulse, and so on.
Figure 6-5: Timing Diagram: External Sync Mode, Negative Edge Trigger
6.1.3 External Sync with Continuous Cleans
Another timing mode available with the ST-133 controller is Continuous Cleans. In
addition to the standard cleaning of the array, which occurs after the controller is
enabled, Continuous Cleans removes any charge that integrates on the array between
the time that the Start Acquisition command is received by the controller and the
moment that the External Sync pulse trigger edge is received.
Figure 6-6 shows a flow
chart for this mode.
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4
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