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62 ST-133 Controller Manual Version 3.B
54
12
Charge accumulates in
unmasked cells during
exposure.
Accumulated charge in
exposed cells is quickly
transferred under mask.
Shifting continues until all masked
data has been shifted into serial
register and from there to the Output
Node.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
Charges in serial register shift into
Output Node, emptying the register
so the next line can be transferred in.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
A2
A1
B2
C2 D2
A3 B3 C3 D3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
B1 C1 D1
A3 B3 C3 D3
A4 B4 C4 D4
B3
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
C3 D3
3
Charge from cells A1-D1 shifted
to serial register. Exposed cells
accumulate new charge.
6
All data from first exposure has been
shifted out. Second exposure continues.
Initial conditions are restored.
A2
A1
B2
B1
C2
C1
D2
D1
A3 B3 C3 D3
A3 B3 C3 D3
A4 B4 C4 D4
A4 B4 C4 D4
A6 B6
C6
D6
A5 B5 C5 D5
Figure 30. Frame Transfer Readout
Only the exposed region collects charge. At the end of the exposure, the charge is quickly
shifted into the masked region. Since the shifting is accomplished in a short time, i.e., a
few milliseconds, the incident light causes only minimal "smearing" of the signal. While
the exposed region continues collecting data, the masked region is read out and digitized.
The percentage of smearing given by the equation below is simply the time needed to
shift all rows from the imaging area divided by the exposure time.
Nt
t
xi
exp
(4)
Digitization
During readout, an analog signal representing the charge of each pixel (or binned group
of pixels) is digitized. The number of bits per pixel is based on both the hardware and the
settings programmed into the camera through the software. One A/D converter (one
digitization speed) is standard with the ST-133. However, the ST-133 will support multiple
digitization speeds (software-selectable readout rates) if the Dual A/D Converters option is
ordered or if a 2 MHz version of the ST-133 is ordered for the system.
Multiple digitization provides optimum signal-to-noise ratios at all readout speeds.
Because the readout noise of CCD arrays increases with the readout rate, it is sometimes
necessary to trade off readout speed for high dynamic range. In the most common ST-133
configurations, there will be a 1 MHz conversion speed for the fastest possible data
collection and a 100 kHz or 50 kHz conversion speed for use where noise performance is
the paramount concern. Switching between the conversion speeds is completely under
software control for total experiment automation.