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Chapter 6 Advanced Topics 59
59
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 µs 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 27, this means that if it is still LOW) at the
end of the readout, the hardware may interpret this as a second sync pulse, and so on.
Figure 27. Continuous Cleans Timing Diagram (-edge trigger)
Fast Mode and Safe Mode
The Experiment Setup|Timing tab allows you to choose Fast Mode or Safe Mode.
Figure 28 is a flowchart comparing the two modes. In Fast Mode operation, the ST-133 runs
according to the timing of the experiment, with no interruptions from the computer. In Safe
Mode operation, the computer processes each frame as it is received. The ST-133 cannot
collect the next frame until the previous frame has been completely processed.
Fast Mode operation is primarily for collecting "real-time" sequences of experimental data,
where timing is critical and events cannot be missed. Once the ST-133 is sent the Start
Acquisition command by the computer, all frames are collected without further intervention
from the computer. The advantage of this timing mode is that timing is controlled
completely through hardware. A drawback to this mode is that the computer will only
display frames when it is not performing other tasks. Image display has a lower priority, so
the image on the screen may lag several images behind. A video monitor connected to the
VIDEO output will always display the current image. A second drawback is that a data
overrun may occur if the number of images collected exceeds the amount of allocated
RAM or if the computer cannot keep up with the data rate.
Safe Mode operation is primarily useful for experiment setup, including alignment and
focusing, when it is necessary to have the most current image displayed on the screen. It is
also useful when data collection must be coordinated through software with external devices
such as external shutters and filter wheels. As seen in Figure 28, in Safe Mode operation, the
computer controls when each frame is taken. After each frame is received, the camera sends
the Stop Acquisition command to the camera, instructing it to stop acquisition. Once that
frame is completely processed and displayed, another Start Acquisition command is sent from
the computer to the camera, allowing it to take the next frame. Display is therefore, at most,
only one frame behind the actual data collection.