A CCD (for charge-coupled device ) is a type of electronic
device generally used to detect light, in such cases,
incorporating an array of tiny electronic light sensors.
It provides a mechanism for unloading the sensor-data, so as to
capture an image focused on the sensors as digital data.
Generally, such arrays are implemented as integrated circuits.
The development and adoption of such electronic light-sensing arrays
constituted a revolution in astronomy: they are used in telescopes
(as well as many digital cameras) where they provide the data in
electronic digital form, convenient for image storage and processing,
and offering the possibility of automating the whole
observation process. They have improved the efficiency of
astronomical observation, vastly increasing the number of images captured
and their use is advantageous for remote operation of telescopes,
including space probes, and for fully-automated surveys.
The term CCD actually refers to the mechanism for unloading the data:
it consists of a string of (basically) identical
circuit elements that each sense and store a pixel, and are
coupled together such that they that can
pass their data to a neighbor in lock-step (shifting the data), so
after a number of such steps, all the data has been output.
The typical CCD circuit is a row of these strings in parallel, on a
single chip, laid out in a grid. This technology and name ("CCD")
were initially designed for data storage, but the technology to
charge an electronic circuit element from incident EMR was
already known and easily incorporated. Unloading a CCD's data
takes some time, and faster means have been developed at the expense
of more complex circuitry: such faster sensors are not termed CCDs
(though undoubtedly people sometimes refer to one as a "CCD"), e.g.,
some fall under the term CMOS sensor. Such ability to unload data
faster allows more time to collect data and more frequent unloading
and resetting the device, which offers a number of benefits such
as allowing a faster cadence to avoid saturation.