60 PI-MAX/PI-MAX2 System Manual Version 5.F
Automatic: This approach requires that you activate "Background" and specify the
background filename on the Acquisition|Experiment Setup…|Data Corrections
tab before acquiring an image. When you acquire an image, the specified background
file data will automatically be subtracted from the raw image data before the
corrected data is displayed and is available for storage to disk.
Post-Processing: If you prefer to acquire and preserve the raw image data, make sure
that "Background" is not active on the Acquisition|Experiment Setup…|Data
Corrections tab. Then, acquire the image, save the raw image data to disk, and, via
the Image (or Spectra) Math function (accessed from the Process menu), subtract
the background file data from the raw image data. The subsequent data can then be
saved to a separate file.
Readout of the Array
At the end of the exposure time, the charge from the pixels is shifted to the shift register,
amplified, and digitized. In this section, a simple 6 4 pixel CCD is used to demonstrate how
charge is shifted and digitized. As described below, three different types of readout are
available: full frame, interline, and binned. Full frame readout, for full frame CCDs, reads out
the entire CCD surface at the same time. Interline readout, for interline CCDs, can readout the
array using either overlapped or non-overlapped mode. Binned readout (also known as
Hardware Binning) adds the charge from adjacent pixels together to form a single pixel
(sometimes called a super-pixel) before the preamplifier reads out the signal.
Full Frame Readout
The upper left drawing in Figure 18 represents a CCD after exposure but before the
beginning of readout. The capital letters represent different amounts of charge, including
both signal and dark charge. This section explains readout at full resolution, where every
pixel is digitized separately.
Readout of the CCD begins with the simultaneous shifting of all pixels one row toward
the "shift register," in this case the row at the top. The shift register is a single line of
pixels along one side of the CCD, not sensitive to light and used for readout only.
Typically the shift register pixels hold twice as much charge as the pixels in the imaging
area of the CCD.
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