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Section 2 - Introduction to CCD Cameras
Page 6
horizontal register of pixels. This register collects a line at a time and then
transports the charge packets in a serial manner to an on-chip amplifier. The
final operating step, charge detection, is when individual charge packets are
converted to an output voltage. The voltage for each pixel can be amplified off-
chip and digitally encoded and stored in a computer to be reconstructed and
displayed on a television monitor."
1
X=1 X=M
Y=1
Y=N
Output
Readout Register
Amplifier
Figure 2.1 - CCD Structure
2.2.1. Full Frame and Frame Transfer CCDs
In the ST-7E, ST-8E and ST-9E, the CCD is read out electronically by shifting each row of pixels
into a readout register at the Y=0 position of the CCD (shown in Figure 2.1), and then shifting
the row out through an amplifier at the X=0 position. The entire array shifts up one row when
a row is shifted into the readout register, and a blank row is inserted at the bottom. The
electromechanical shutter built into the camera covers the CCD during the readout to prevent
streaking of the image. Without a shutter the image would be streaked due to the fact that the
pixels continue to collect light as they are being shifted out towards the readout register. CCDs
with a single active area are called Full Frame CCDs.
For reference, the ST-5C, ST-237, STV and ST-6 CCD cameras use a different type of
CCD which is known as a Frame Transfer CCD. In these devices all active pixels are shifted
very quickly into a pixel array screened from the light by a metal layer, and then read out. This
makes it possible to take virtually streak-free images without a shutter. This feature is typically
called an electronic shutter.
2.3. Camera Hardware Architecture
This section describes the ST-7E, ST-8E and ST-9E CCD cameras from a systems standpoint. It
describes the elements that comprise a CCD camera and the functions they provide. Please
refer to Figure 2.2 below as you read through this section.
1
"History and Advancements of Large Area Array Scientific CCD Imagers", James Janesick, Tom
Elliott. Jet Propulsion Laboratory, California Institute of Technology, CCD Advanced Development
Group.