Specifications

Chapter 5 Timing Modes 45
shifted midway when the External Sync pulse arrives, the pulse is saved until the row
shifting is 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
SCAN
Cont.
Cleans
Cont.
Cleans
Cont.
Cleans
Cont.
Cleans
Cont.
Cleans
Figure 16. Continuous Cleans timing diagram
Internal Sync
Internal Sync operation, in which the synchronization handshake is implemented via the
backplane of an ST-133A, 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-133A 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