Principle functionallity of CCD-cameras
(CCD = Charge-Coupled Device) consists of an array of silicon pixels. Light falling on such a
pixel is absorbed in the silicon (photoelectric effect). Special arrangements of electrodes create an
electrical potential which is transfered into a charge pulse that can be stored and measured by the
CCD-electronics. The potential increases with the amount of light that hits the pixel. After each exposure,
the pattern of charge - comming from these million of pixels a mondern ccd-chip has - is digitised and stored
as an image file on a computer. CCD's unfortunately also produce some signal in long exposures even if there is
no photon that reaches the sensor. This effect is called the dark-current and each image file therefore not only
contains the true information we seek for but also some dark current. Since the intensity of this unwanted signal
increases with temperature, all professional and semi-professional CCD-cameras have a cooling system that
dramtically reduces the amount of the dark current. Nevertheless, each image has to be corrected for this effect
by subtracting an equivalently exposed image without letting any photon reaching the detector (dark frame).
This procedure results in a first order calibrated image.
Simplified one can state that CCD-cameras are colorblind. So in order to produce colored pictures,
light particles have to be filtered. By using at least three different color filters (Red, Green and Blue)
so that only the corresponding light can pass and reach the detector, beautiful colored pictures can be
produced if a relatively time consuming image processing procedure is applied.