The charge coupled device (CCD) camera

In recent years there have been enormous advantages in the application of CCD technology. The charge coupled device, or CCD, is a means of capturing an image electronically. As developments have progressed and the number of CCD cameras ha increased the price has fallen and it is now possible to buy a CCD camera quite cheaply.

The optical parts of a CCD camera are similar to those of one using 'conventional' film. It is the method of detecting and storing the image that is different.

With a conventional film the image of the object is focused on a light sensitive surface and as light falls on the film a chemical change takes place. The resolution of fine detail depends on the size of the 'grains' of chemical on the film, typical grain having a diameter of around 10^-6 m.

The imaging area, or chip, in the CCD is composed of 'cells' made of thin layers silicon. As light falls on its surface electron- hole pairs are formed and a charge is built up. The charge depends on the total energy of the light that falls on the cell during the exposure. Therefore the longer the exposure and the brighter the light the more charge is built up on the individual cell. The cells are scanned electronically and a digital version of the image is built up. This can be downloaded to a computer for processing by the appropriate software.

The imaging area of a CCD camera consists of a grid of a large number of these cells or pixels. Cameras with 2 million pixels on the chip are extensively used although 4 million is now becoming the accepted standard. The chip is typically about 10 mm x 5 mm and so each pixel has an area of roughly 2.5x10^-5 mm. In my CCD camera that I use with my telescope the chip has 752x852 pixels in a 6.5x4.8 mm active area.

Three examples of the kind of images obtained with this camera can be seen in Figure 1.

The MX7C colour CCD camera uses a Sony chip which is 6.47 mm x 4.83 mm and contains 752x580 pixels. The pixel size is 8.6x8.3μm, some 100 times larger than the grain on a photographic film.

The CCD camera has advantages and disadvantages when compared with the 'ordinary' film camera.

Advantages:

The chip can be 'used' many times
The are becoming relatively cheap
Unwanted exposures can be discarded
The image taken can be viewed more of less immediately after taking the photograph

Disadvantages:

At present the pixel size is larger than the grain size on a good quality film.

The pixel nature of the image can easily be seen in Figure 2. This is an enlarged image of one of the stars from the picture of the Orion nebula in Figure 1.

A further example is shown below. The circled area of the Rocky Mountains has been enlarged to show the pixel nature of the image.