Princeton 4411-0106 User Manual

Chapter 6 

Advanced Topics 

71 

 

71 

The PreOpen {Open Before Trigger} mode is useful in cases where an External Sync 
pulse cannot be provided 8 ms or ~ 20 ms (the length of time the 25 mm or 45 mm 
mechanical shutter takes to open) 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 {Normal} 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.  

 

Another timing mode available with the PIXIS is called Continuous Cleans {Clean Until 
Trigger}. In addition to the standard "cleaning" of the array, which occurs after the 
camera is enabled, this mode will remove any charge from the array until the moment the 
External Sync pulse is received.