Princeton 4300 User Manual

Chapter 5 

Operation 

49 

 

 

 

49 

When signal levels in some part of the image are very high, charge generated in one pixel 

may exceed the "well capacity" of the pixel, spilling over into adjacent pixels in a process 

called "blooming." In this case a more frequent readout is advisable, with signal 

averaging to enhance S/N (Signal-to-Noise ratio) accomplished through the software.  
For signal levels low enough to be readout-noise limited, longer exposure times, and 

therefore longer signal accumulation in the CCD, will improve the S/N ratio 

approximately linearly with the length of exposure time. There is, however, a maximum 

time limit for on-chip averaging, determined by either the saturation of the CCD pixels 

by the signal or the loss of dynamic range due to the buildup of dark charge in the pixels  

After the exposure time has elapsed, the charge 

accumulated in the array pixels needs to be read 

out of the array, converted from electrons to 

digital format, and transmitted to the application 

software where it can be displayed and/or 

stored. Readout begins by moving charge from 

the CCD image area to the shift register. The 

charge in the shift register pixels, which 

typically have twice the capacity of the image 

pixels, is then shifted into the output node and 

then to the output amplifier where the electrons 

are grouped as electrons/count. This result 

leaves the CCD and goes to the preamplifier 

where gain is applied.  
WinView and WinSpec allow you to specify the 

 

type of readout (full frame or binned), the output amplifier (if dual amplifiers are 

available), and the gain (the number of electrons required to generate an ADU).  

In this section, a simple 6 



 4 pixel CCD is used to demonstrate how charge is shifted and 

digitized. Full frame readout, for full frame CCDs, reads out the entire CCD surface at 

the same time.