Precision Measuring Machine Project

A CCD detector, also called a ``chip'', consists of a regular rectangular array of picture elements, called pixels, each of which converts any light falling on it into an electrical charge. The amount of this charge is proportional to the intensity of the light which falls on that pixel. The charge accumulated in a pixel is recorded as a ``data number'', or DN. After the detector has been exposed to light (just like photographic film being exposed in a conventional camera), the DNs are read out, one pixel at a time, row by row. Circuitry on the chip moves the collected charge to the readout device (an amplifier) in a regular pattern. This pattern is usually described as a sort of bucket brigade, where the charge is read from the pixel nearest the amplifier and then the charges in all the other pixels in that same row are shifted over one pixel and the next pixel is read out. When all the pixels in a row have been read out, the charges in the remaining rows are all shifted down one row, and then the next row is read out. By reading out the charges in this orderly way, it is possible to keep track of the place on the chip where each charge was recorded and to reconstruct the image.

CCDs are commonly used in camcorders and television cameras, and are now the detectors of choice for most astronomical telescopes and other imaging systems.