What is a charge-coupled device (CCD)?
• It can be defined as a light-sensitive integrated circuit imprinted
on a silicon surface to form light-sensitive elements called pixels,
and each pixel is converted into an electrical charge
• It can be treated as CCD sensor, which is used in the digital and
video cameras for taking images and recording videos through
photoelectric effect. It is used for converting the captured light
into digital data, which is recorded by the camera.
• They transfer incoming photons into an electrical charge in the
• The photons are received in a photoactive region of silicon
• A lens is used to focus the light onto the silicon
• A charge is accumulated in the capacitors proportional to the light
• Each photo site corresponds to an individual pixel in the finial
image and photo sites are often referred to simply as pixels
• Each “photo sites” will refer to the CCD chip and “pixels” will refer to an
• When a photon of light strikes the surface of certain metal
materials the energy imparted by the photon can release an
electron from the metal
• in a CCD , this electron is stored within the walls of a photo site
• Basically, the camera turns light into electricity. A CCD camera
forms light sensitive elements called pixels which sit next to each
other and form a particular image. CCD cameras have been in
production for a long period of time and tend to have high quality
pixels that produce a higher quality, low-noise image than any
• CCDs are used in digital cameras , optical scanner s and video
cameras as light- sensing devices .
• CCD cameras used in astrophotography .
• CCDs are typically sensitive to infrared light, which allows
infrared photography, night-vision devices, and zero lux (or near
zero lux) video-recording/photography.
• CCD are use to take exposures of galaxies and nebulae .
The CCD camera consist of three main parts :
1. Camera Body
• the camera body is the mechanical part of the camera and it houses the
CCD chip and protects it against unwanted light from outside.
2. CCD Chip
• A CCD chip is a light sensitive device made of silicon. its an array of light
sensitive pixels , it will be converted to a charge , this charge is captured
in the pixel . it can't go to other pixels
• the amount of charge is measure of the amount of light that felt on the
• the electronics of the camera are the intermediate between the CCD
chip and the computer
• the computer must be able to control the camera through its electronics
• to shift the lines on the CCD chip ,it needs clock signals or pulses that
must be generated by the electronics or by the computer
• another important task is the conversion of the analog output signal of
the charge detection node to a digital format the computer is also to
How a CCD chip registers image?
• The telescope projects an image on the CCD chip. High intensity
areas will produce more charge in the pixels than low intensity
• The amount of charge in every pixel makes the image,The
computer can read this charge and convert it to an image.
• The value in every pixel gives the charge, which is related to the
How is an image made with a CCD chip?
There are three main phases:
• Clearing phase
• Exposure or integration phase
• Readout phase
• Mostly you don’t have a shutter in your camera, this means that
there is always light falling on the CCD chip.
• When you want to make an image, all existing charge has to be
removed from the pixels.The CCD chip can shift lines downward with
• All charge in a line will move to the line below of it.This happens with
all lines of the chip on the same moment.
• The lowest line will move the charge to a clearing line, where it will
• When all lines are shifted away, there is no charge left anymore.This
phase takes a few ms.
Exposure or integration phase
• The telescope projects the image on the CCD chip, so charge will
accumulate in the pixels.
• The longer you wait, the more light will be registered by the chip.
• This phase controls the exposure time, also called: integration
• because the chip integrates the amount of light in a certain
period.With lunar and planetary photography it usually takes a
few tenths of a second, but it can take minutes or even hours
• To get the image in your computer, it will have to read the amount of
charge of every pixel.
• There is a device, the charge detection node , in the CCD chip that is
able to measure the charge of a pixel.
• Every pixel must be shifted in this device.The lowest line is able to
shift pixels to the left in the direction of the device. So these pixels can
be measured one by one.
• When all pixels of the line are measured, all lines are shifted one line
• Then again the lowest line can be measured.This will be repeated until
all lines are measured.
• he computer can read the measurements through the camera
electronics.This phase can take a few seconds.
The most used types of CCD chips
This is the most simple device. It is used in the CB211 camera.This
type can’t be used with lunar and planetary photography without a
shutter. Making images of this kind of objects typically requires an
integration time of a few tenths of a second.The problem is the
readout time, which can take a few seconds. During readout, the
chip is still receiving light, which will destroy the original image,
before it’s completely read by the computer.
•Frame transfer device
With this kind of chip, the lowest half of the chip is protected
against light.The image is only made on the upper half of the chip.
When you are finished making your image, the image will be
transferred from the upper half to the lower half, where it will be
protected from light.This will take only a few ms.There is enough
time, in this area, to read the image. A frame transfer device can be
used with lunar and planetary photography.
The most used types of CCD chips
•Interline transfer device
One line of every two lines is protected from light. During
integration, only one line receives light. At the end, this line is
shifted to the protected line, where it will be read by the computer.
An interline transfer device can be used with planetary
photography, Lunar photography will be more difficult, because of
it’s brightness.The bright light can influence the protected line.The
protected area of a frame transfer device is safer.
The most used types of CCD chips
• Relative simple
• More sensitive than photographic film
• Cheaper to replace if failure
• Modularity- easy upgrades
• Detector costs simple
• No chemical processing is needed
• CCD sensors consume much more power
• HD video- less pixel rates.
• CCDS are expensive as they require special manufacturing.
• CCD sensors may be wired/wireless- mastering the use requires
some effort and learning period