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Chapter 6 Advanced Topics 79
another as rapidly as possible as shown in Figure 34. In these figures, Shutter indicates
the programmed exposure time. If a shutter were present and active, it would also be the
actual exposure time.
Prior to the first readout, clean cycles are performed on the array. When the software issues a
Start Acquisition com
mand, the first readout is generated in hardware and the rapid data
transfer from the exposure half of the array to the storage half of the array occurs (marking
the beginning of the first exposure). The initial data read are discarded because they are not
meaningful. The first exposure continues until the next data transfer, which occurs at the
beginning of the next readout, 50 ns after the first readout ends. The data acquired during the
first exposure is then read out. This pattern continues for the duration of the experiment so
that, during each frame, the data acquired during the previous frame is read out.
NOT SCAN
Shutter
External Sync
(negative polarity shown)
t
exp
t
w1
50ns min.pulse between frames
t
R
t
R
t
R
t
R
actual exposure time
cleans
acquisition
Figure 34. Frame Transfer where t
w1
+ t
exp
+ t
c
< t
R
Figure 35 shows the case where the programmed storage time is greater than the time
required to read out the storage half of the array, that is, where t
w1
+ t
exp
+ t
c
> t
R
. In this
case, the programmed exposure time will dominate in determining the actual exposure
time. In the situation depicted in
Figure 35, the External Sync pulse arrives during the
readout. As alway
s, the External Sync pulse must be detected before the next readout can
occur. However, there is no requirement as to when it must be applied or even that it be
periodic. The timing of the External Sync pulse is entirely at the user’s discretion. In
Figure 36, the External Sync pulse is shown arriving after the read. Detection of the
External Sy
nc pulse enables a new readout to occur on completion t
exp
+ t
c
.
NOT SCAN
Shutter
External Sync
(negative polarity shown)
t
exp
t
w1
t
R
t
c
cleans
acquisition
t
R
t
R
t
R
t
R
actual exposure time
Figure 35. Frame Transfer where t
w1
+ t
exp
+ t
c
> t
R