Filter
Top Products
Chapter 5 Timing Modes 47
completed, to prevent the CCD from getting "out of step." As expected, the response
latency is on the order of one vertical shift time, from 1-30 µsec depending on the array.
This latency does not prevent the incoming signal from being detected, since photo
generated electrons are still collected over the entire active area. However, if the signal
arrival is coincident with the vertical shifting, image smearing of up to one pixel is
possible. The amount of smearing is a function of the signal duration compared to the
single vertical shift time.
Note: If EXT SYNC is still active at the end of the readout, the hardware will interpret
this as a second sync pulse, and so on.
Shutter (Normal)
Open Close Open Close Open Close
Read
Read Read
Shutter (Preopen)
Open Close Open Close Open Close
External Sync
NOTSCAN
Figure 15. Continuous Cleans Timing Diagram
Internal Sync
Internal Sync operation, in which the synchronization handshake is implemented via the
backplane of an ST-133, does not require a connection to the Ext Sync connector. With
respect to timing considerations, the mode is very similar to the Ext Sync mode. Note that
the Int. Sync. mode is only available if an ST-133 has a PTG installed.
Frame Transfer Mode
In frame transfer operation, half the CCD is used for sensing light and the other half for
storage and readout. Not all CCD arrays are capable of readout in this mode, as it requires
that charge be shifted independently in the two halves of the array. See Chapter 6 for a
detailed discussion of readout in the frame-transfer mode operation; the primary focus of
this section is frame-transfer timing.
There are two timing options available in frame transfer mode, Free Run and External
Sync. Both are similar to their counterparts in full frame (standard) operation, except that
in frame transfer operation a shutter is not generally used. Because there is no shutter (or
the shutter is only closed after the camera has collected a series of frames), shutter
Normal, PreOpen, or Disable have no physical meaning here. The exposure half of the
array sees light continuously. The actual exposure time is the time between data transfers
from the exposure half of the array to the storage half of the array, and may be longer
than the programmed exposure, t
exp
. Data transfer from the exposure half of the array to
the storage half occurs very quickly at the start of each read. During the read, the stored
data is shifted to the array’s output port, the same as in standard operation.