Specifications

54 ST-133/ST-133A Controller Manual Version 2.H
signal or the loss of dynamic range due to the buildup of dark charge in the pixels (see
below).
Dark Charge
Dark charge (or dark current) is the thermally induced buildup of charge in the CCD over
time. The statistical noise associated with this charge is known as dark noise. Dark
charge values vary widely from one CCD array to another and are exponentially
temperature dependent. At the typical operating temperature of an RTE/CCD camera, for
example, dark charge is reduced by a factor of ~2 for every 6º reduction in temperature.
In the case of cameras such as the RTE/CCD-768-K and RTE/CCD-1317-K, which have
MPP type arrays, the average dark charge is extremely small. However, the dark-charge
distribution is such that a significant number of pixels may exhibit a much higher dark
charge, limiting the maximum practical exposure. Dark charge effect is more pronounced
in the case of cameras having a non-MPP array.
With the light into the camera completely blocked, the CCD will collect a dark charge
pattern, dependent on the exposure time and camera temperature. The longer the
exposure time and the warmer the camera, the larger and less uniform this background
will appear. Thus, to minimize dark-charge effects, you should operate with the lowest
CCD temperature possible.
Note: Do not be concerned about either the DC level of this background or its shape
unless it is very high, i.e., > 1000 counts with 16 bit A/D or 300 counts with a 12 bit
A/D. What you see is not noise. It is a fully subtractable readout pattern. Each CCD has
its own dark charge pattern, unique to that particular device. Simply acquire and save a
dark charge “background image” under conditions identical to those used to acquire the
“actual” image. Subtracting the background image from the actual image will
significantly reduce dark-charge effects.
Note: Offset and excess noise problems are more likely to occur if the controller and
camera weren’t calibrated and tested as a system at the factory.
If you observe a sudden change in the baseline signal you may have excessive humidity
in the vacuum enclosure of the camera. See your camera manual for information on
having the camera vacuum repumped.
Readout of the Array
In this section, a simple 6 × 4 pixel CCD is used to demonstrate how charge is shifted
and digitized. As described below, two different types of readout are available. Full
frame readout, for full frame CCDs, reads out the entire CCD surface at the same time.
Frame transfer operation assumes half of the CCD is for data collection and half of the
array is a temporary storage area.
Full Frame Readout
The upper left drawing in Figure 28 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.
CAUTION