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88 ST-133 Controller Manual Version 3.B
External Sync
(negative polarity shown)
NOTSCAN
Shutter
t
w1
t
c
t
exp
cleans
acquisition
t
R
t
R
t
R
t
R
actual exposure time
Figure 46. Overlapped Mode where Pulse arrives after Readout
Exposure
CCD arrays perform three essential functions: photons are transduced to electrons,
integrated and stored, and finally read out. The software allows you to set the length of
time that incoming light will be allowed to integrated on the CCD. This time is called the
exposure time. Interline transfer CCDs contain alternate columns of imaging and storage
cells that work in pairs. Light impinging on the imaging cells cause a charge buildup. As
previously explained, the operating mode is always overlapped unless the exposure time
is shorter than the readout time, in which case non-overlapped operation is automatically
selected.
Note: The storage cells of an interline chip are quite light insensitive (the ratio of the
light sensitivity of the storage cells, which are masked, to the light sensitivity of the
imaging cells is ~4000:1). However, even with a rejection ratio of ~4000:1, there may be
situations where this may not be sufficient to prevent light leakage from significantly
affecting the data. That this is so becomes apparent when the on/off time factors are
considered. In an experiment with a very short exposure compared to the readout rate, the
ratio of the readout time to the exposure time may easily be of the same order as the
rejection ratio of the interline chip storage cells. Where this is the case, the signal buildup
in the storage cells during the readout time may equal the signal transferred from the
imaging cells to the storage cells at the end of the exposure time. The effect of this signal
will be to cause data smearing. The only solutions to this problem at this time are to
increase the exposure time to where the effect is insignificant, use a shutter, or to use a
gated light source.
Exposure with a Mechanical Shutter
As previously discussed, even though an interline CCD ordinarily doesn’t require a
mechanical shutter, a mechanical shutter can be incorporated into the system
advantageously in certain situations. The diagram in Figure 47 shows how the exposure
period is measured in shuttered operation. The
output can be used to monitor the
exposure and readout cycle (t
R
). This signal is also shown in Figure 47. The value of t
c
is
shutter type dependent, and will be configured automatically for systems shipped with an
internal shutter.