Principles of CCD
Here we look at the basic principles - How does a CCD work?
What is a CCD?
In a digital camera the traditional photographic film is replaced by a
Charge Coupled Device (CCD).
A CCD is a mosaic of tiny light sensitive detectors called pixels
The pixels are arranged as a flat rectangular surface onto which an image is
projected using a camera or telescope lens. Each pixel accumulates an
electrical charge depending on the amount of light falling upon it.
When an image is 'captured' the electrical charge from each pixel is
measured and converted to a number (digitised) by the electronic circuits
within the camera. These numbers are transmitted to a computer (immediately
or at some later time) where they are used to control the brightness of
points on the computer screen (screen pixels), thus reproducing the original
image projected onto the CCD.
Sets of numbers representing a complete image are stored in the computer
as 'image files'. There are many powerful software programs
available to process these files and enhance the image by adjusting the
contrast, colour balance etc.
Signal and Noise
The efficiency with which CCD pixels can capture faint images is much
better than traditional photography, but there are problems that need
to be understood. Most of these are relate to 'noise'.
The variation in the brightness of pixels across the image
shows the 'picture' to our eyes. The greater the variation, the greater the
contrast. However, variation can come from:
- Signal: Pixel variation due to the features of the object being
- Noise: Pixel variation due to other unwanted causes.
For a good image we need to have a good signal-to-noise ratio. If there
is too much noise we will have an image with poor contrast, speckles or other unpleasant features.
(See this example of a noisy image of C/2004 Q4)
A CCD will introduce noise through bias, dark current, quantum noise and
inhomogeneities, each of which is described below. Modern, cooled
cameras developed for astrophotography will suffer relatively little from
these problems, but for long exposures they can still degrade image quality. Webcams
and ordinary digital cameras are generally very 'noisy'.
Each CCD pixel will have a certain minimum electrical charge even if the
exposure is of zero duration. This means that pixel values will always be
greater than zero even for the shortest exposures. This is called
'bias' or 'offset'.
Each CCD pixel will accumulate additional charge during the exposure even
if there is no light falling on it. This means that pixel values will be
higher than they should be by an amount that depends on the length of the
exposure. This is 'dark current'.
The effect of Bias and Dark Current is that all parts of the image are a
bit brighter than they should be. The process of Calibration aims to remove this excess brightness by 'Dark Frame
Flat Field Inhomogeneities
The sensitivity of pixels may vary across the surface of the CCD. Some
pixels may be 'dead' (giving no response) or 'hot' (always giving the
maximum value). Bias or dark current may vary from pixel to pixel. Light or
heat leakage inside the camera can cause bright areas or spots. All these
effects contribute to CCD flat field inhomogeneities.
In addition, (and as for traditional photography), there may defects in
the optical system such as vignetting, internal reflections and 'do-nut'
shaped shadows from dust particles on the CCD window.
These defects can cause unwanted light/dark areas or points in the image.
The process of Calibration aims to minimise
these defects by 'Flat Fielding'.
There are random variations in the charge of each pixel due to the
physical processes going on within them. This means that each pixel may not
return the same value each time it is exposed to the same amount of light.
These random variations apply to
bias, dark current and flat field inhomogeneities as well as the desired
The effect is that an image has a grainy look and fine detail may be
lost. There are a number of approaches to quantum noise reduction
Cosmic Ray Hits
Cosmic Rays are high energy particles from space that occasionally strike the
surface of the CCD leaving a small bright spot that may be mistaken for a star
or other object. Unfortunately there is not much you can do about this. If a
spot appears on one image of an area of sky but not on other images then it is
probably a cosmic ray hit (not a new discovery!). The best thing to do is
discard the spotty image.