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48 ST-133 Controller Manual Version 3.B
In Free Run frame-transfer mode operation, half the array is exposed for the set exposure
time (t
exp
). Then the data transfer to the storage half of the array takes place, marking the
start of the read and the beginning of a new exposure.
In External Sync frame-transfer mode operation, the camera reads out one frame for
every External Sync pulse received, providing the frequency of the External Sync pulse
doesn’t exceed the maximum rate possible with the system. Other than for the first
readout, initiated by starting acquisition, a Sync Pulse must be detected before the
subsequent readout can occur. If operating without a shutter, the actual exposure time is
set by the period of the sync signal. There is one exception, if the programmed exposure
time is less than the readout time, then the actual exposure time is simply equal to t
R,
the
readout time (marked by NOTSCAN low). More specifically, if the readout time, t
R
, is
greater than the sum of t
w1
, the time the controller waits for the first External Sync pulse,
plus t
exp
, the programmed exposure time, plus t
c
, the shutter compensation time, then the
actual exposure time will equal t
R.
If an External Sync pulse is detected during each read,
frames will follow one another as rapidly as possible as shown in Figure 16. 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 command, 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.
NOTSCAN
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 16. Frame Transfer where t
w1
+ t
exp
+ t
c
< t
R
Figure 17 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 17, the External Sync pulse arrives during the
readout. As always, 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 18, the External Sync pulse is shown arriving after the read. Detection of the
External Sync pulse enables a new readout to occur on completion t
exp
+ t
c.