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SECTION 3
CCD Imaging Overview
This section is intended only as a brief overview of CCDs and CCD Imaging. If you are new to
CCD imaging there are a number of excellent books and lots of information on the internet that
you can use to gain a deeper understanding of the issues and techniques. The subject matter is
discussed in the context of astronomical imaging, but the basic principles apply to any
application.
How CCDs work
Charge Coupled Devices (CCD) work by converting photons into electrons which are then
stored in individual pixels. A CCD is organized in a two-dimensional array of pixels. The CCDs
used in the QSI 600 Series cameras at the time of printing range from roughly 400,000 pixels
(768W x 512H) to 16 million pixels (4499 W x 3599 H).
Each pixel can hold some maximum number of electrons. CCDs currently used in the QSI 600
Series can hold from 25,500 to as many as 100,000 electrons depending on the specific model
of CCD. While integrating (exposing) an image, photons strike individual pixels and are
converted to electrons and stored in each pixel well. The effectiveness of this process is referred
to as Quantum Efficiency (QE). The number of electrons stored in each pixel “well” is
proportional to the number of photons that struck that pixel. This linear response is one of the
key traits that make CCDs exceptionally well suited to astronomical imaging. A subject that is
twice as bright will build up twice as many electrons in the CCD. After an exposure is complete,
the electrons in each pixel are shifted out of the CCD and converted to a number, indicating how
dark or light each particular pixel was. Those brightness values for each pixel are then stored in
the image file, typically a FITS file for astronomical imaging.
Types of CCDs
CCDs are available in a variety of designs and technologies. QSI cameras currently employ two
different types of CCDs, Full Frame and Interline Transfer, with numerous optional features.
Full-frame CCDs
Full-Frame CCDs generally provide the highest sensitivity and the widest linear response range
of these two types of CCDs. These characteristics make full-frame CCDs ideally suited to
astronomical imaging. Full-frame CCDs must employ a mechanical shutter to prevent light from
falling on the CCD surface while the image is being shifted out of the CCD.
Interline transfer CCDs
Interline transfer CCDs work somewhat differently. In an interline transfer CCD, next to every
column of pixels is a specialized storage column that is covered by a mask to prevent light from
hitting the storage pixels underneath. When an exposure is complete, the entire image is
shifted in a single operation into this masked storage column. The pixels which are now under
the mask stop building additional charge and are shifted out of the CCD in the same fashion as
