2.3 Charge-Coupled-Devices

Charge-Coupled-Devices (CCDs) are an integral part of the modern telescope and a basic tool of modern astronomy. A review of the basic semiconductor physics, methods of construction and astronomical uses can be found in McLean (1997) and Mackay (1986).

Filippenko (1992) outlines many of the advantages of CCDs, which have been summarised by Bode (1995), the most advantageous of these are: (i) they have relatively high detective quantum efficiency, the number of electrons emitted per photon incident upon a photodetector, with respect to a photomultiplier tubes enabling fainter objects to be observed, (ii) the effective entrance aperture is defined by the image itself and can therefore be matched more flexibly to seeing conditions, thus decreasing the sky contribution optimally in a given exposure, and (iii) CCD data are extremely well adapted to computer manipulation due to their digitally quantised and discrete pixelated nature, which provides information in a natural form for computers.

The core structure of a CCD is a thin slice of semi-conductive material (i.e., silicon, germanium, etc.). Semi-conductors are used due to the size of their “energy-gap”, most semi-conductors have energy-gaps of around 1eV (c.f. insulators 2eV and conductors « 1eV). Red photons are able to pass further in the silicon before absorption than blue photons. Most CCDs used in astronomical work are cooled to reduce dark current, which in turn reduces the CCDs response to red light (see Section 2.4.2). This is because (i) the band gap increases and (ii) the absorption of lower energy photons requires the presence of higher energy phonons, which are rarer at lower temperatures. CCDs have a very large dynamic range, meaning they are able to measure light intensities of vastly different sizes without the need for neutral density filters. They are also highly linear detectors. Photometry therefore can be carried out to high precision without the difficulty of the removal of non-linear effects.

While CCD technology has greatly improved observational astrophysics it has also introduced new problems to overcome. All images taken using CCDs require software corrections for imperfections caused by electronic noise and optic distortions, this increased data reduction is one of the disadvantages of CCD photometry (Kaitchuck, 1992).