Continuous Cleans Cycles; Figure 16. Timing Diagram: External Sync Without Continuous Cleans - Princeton Instruments PI-MAX System Manual

56 PI-MAX/PI-MAX2 System Manual Version 5.F
Note: The start of the exposure is signaled by the NOT SCAN output of the
connector going high but will not occur until the current clean cycle and the additional
user-defined number of cleans (typically 0) have finished. "Number of Cleans" is defined
on the Setup|Hardware Setup|Cleans/Skips tab page. If you enter a value other than
"0", you will further delay the start of the exposure by that number of clean cycles.

Continuous Cleans Cycles

The Continuous Cleans function is provided when the start of an exposure is tied to an
external trigger (i.e., the experiment is being run in External Sync timing mode) to ensure
that the array will continue to be cleaned until the External Sync pulse is received. The
continuous clean cycles are defined by the same parameter values as the standard clean
cycles. The difference is that continuous clean cycles occur between NOT SCAN going
high and External Sync pulse arrival. Continuous cleans cycles ensure that ambient light
and dark current signal integrating on the array is continuously removed while the ST-133
is waiting for an External Sync pulse after the receipt of the Start Acquisition signal.
Note: When the External Sync arrives during a continuous clean cycle, that cycle must
be completed before the exposure will begin. In time critical experiments, the number of
rows per clean (set on the Hardware Setup| Cleans/Skips tab page) should be 1 or 2 to
minimize the delay.
Figure 16 shows the timing diagram for an experiment with External Sync trigger active
on the negative edge. Note that the timing diagram shows two possible setups for the
photocathode. In the first setup (Normal), the photocathode is biased on when External
Sync goes low. Because it takes time to bias the photocathode on, part of the incoming
signal may be missed while the photocathode is turning on. In the second setup
(PreOpen), the photocathode is biased on when the following conditions are met: the
command is received, clean cycles finish, and the NOT SCAN signal goes high. The
advantage is that the photocathode is fully on when the exposure (triggered by External
Sync) begins. The disadvantage is that ambient light is no longer being blocked from the
array. Continuous cleans provide a way to get rid of the signal that accumulates on the
array during that period.

Figure 16. Timing Diagram: External Sync without Continuous Cleans