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54 ST-133 Controller Manual Version 3.B
levels, magnetic fields and RF radiation. They are easily cooled and can be precisely
thermostated to within a few tens of millidegrees.
Because CCD arrays, like film and other media, are always sensitive to light, light must not
be allowed to fall on the array during readout. Unintensified full-frame CCD cameras like the
ST-133 use a mechanical shutter to prevent light from reaching the CCD during readout.
ICCD (intensified) cameras use an image intensifier to gate the light on and off.
The software allows the user to set the length of time the camera is allowed to integrate
the incoming light. This is called the exposure time. During each scan, the shutter or
intensifier is enabled for the duration of the exposure period, allowing the pixels to
register light.
Exposure with a Mechanical Shutter
For some CCD arrays, the ST-133 uses a mechanical shutter to control exposure of the
CCD. The diagram in Figure 26 shows how the exposure period is measured. The
NOTSCAN signal, provided at the
BNC on the ST-133 Analog/Control panel,
can be used to monitor the exposure and readout cycle (t
R
). This signal is also shown in
Figure 26. The value of t
c
is shutter type dependent, and will be configured automatically
for cameras shipped with an internal shutter.
NOTSCAN
Mechanical Shutter
Acquire Readout
ClosedOpen
Exposure time
Shutter compensation
t
exp
t
c
Figure 26. Exposure of the CCD with Shutter Compensation
NOTSCAN is low during readout, high during exposure, and high during shutter
compensation time.
Since most shutters behave like an iris, the opening and closing of the shutter will cause
the center of the CCD to be exposed slightly longer than the edges. It is important to
realize this physical limitation, particularly when using short exposures.
Exposure with an Image Intensifier
Although the standard camera is not intensified, it is possible to connect it to a
lens-coupled intensifier, making the following general discussion of intensified operation
applicable.
ICCD (intensified) cameras use an image intensifier both to gate light on and off and to
greatly increase the brightness of the image. In these cameras the image intensifier
detects and amplifies the light, and the CCD is used for readout.
The exposure programmed by software in this case refers to duration of gating of the
intensifier. For shorter exposures, a Princeton Instruments pulser is required.
The MCP (microchannel plate) of the intensifier is composed of more than 10
6
individual
miniature electron multipliers with an excellent input to output spatial geometric