Overview of CCD Detectors

Before the introduction of photography to astronomy the only way of recording images of extended objects seen through a telescope was to sketch them. This approach worked moderately well for the planets, which are illuminated by reflected light, but was much less successful for nebulæ  and other objects beyond the solar system, both because they are much fainter and because of the inherent difficulty in reproducing the gradations in brightness of an extended luminous object using drawing techniques. Photographic plates were first used to record images of regions of the sky around the middle of the nineteenth century. The techniques proved successful and photographic plates were ubiquitous in astronomy for more than a century. The advantages that they offered were basically threefold:

• unlike the eye they were an integrating detector: fainter objects could be detected by making longer exposures to accumulate more light,

• the images were objective and reproducible (unlike a sketch),

• the photographic image constituted a quantitative measure of the light distribution across the luminous object (at least in principle).

Nonetheless there were problems with photographic plates: they had only a limited dynamic range and their response to the brightness of the illuminating light was non-linear, leading to persistent calibration problems. In the middle years of the twentieth century photoelectric photometers were developed: electronic devices which were more sensitive, accurate, linear and had a wider dynamic range than the photographic plate. However, they were not imaging devices: they merely produced a single output corresponding to the brightness of one point on the sky.

In many ways CCDs (Charge-Couple Devices) combine the advantages of both photographic plates and photoelectric photometers, though their principles of operation are very different from either. They have a high sensitivity, linear response, large dynamic range and are imaging devices which record a picture of the region of sky being viewed. (Imaging devices are sometimes called, perhaps somewhat grandiloquently, panoramic detectors.)

The CCD was invented in 1969 by W.S. Boyle and G.E. Smith of the Bell Laboratory. They were not interested in astronomical detectors (and were, in fact, investigating techniques for possible use in a `picture-phone'). Indeed, most of the applications of CCDs are not astronomical. CCDs were first used in astronomy in 1976 when J. Janesick and B. Smith obtained images of Jupiter, Saturn and Uranus using a CCD detector attached to the 61-inch telescope on Mt Bigelow in Arizona. CCDs were rapidly adopted in astronomy and are now ubiquitous: they are easily the most popular and widespread imaging devices used at optical and near infrared wavelengths.

Figure: Examples of various CCD chips

A CCD is best described as a semiconductor chip, one face of which is sensitive to light (see Figure ). The light sensitive face is rectangular in shape and subdivided into a grid of discrete rectangular areas (picture elements or pixels) each about 10-30 micron across. The CCD is placed in the focal plane of a telescope so the the light-sensitive surface is illuminated and an image of the field of sky being viewed forms on it. The arrival of a photon on a pixel generates a small electrical charge which is stored for later read-out. The size of the charge increases cumulatively as more photons strike the surface: the brighter the illumination the greater the charge. This description is the merest outline of a complicated and involved subject. For further details see some of the references in Section  or the web pages on CCD construction maintained by the University of Oregon. The CCD pixel grids are usually square and the number of pixels on each side often reflects the computer industry's predilection for powers of two. Early CCDs used in the 1970s often had 64x64 elements. 256x256 or 512x512-element chips were typical in the 1980s and 1024x1024 or 2048x2048-element chips are common now.

A CCD in isolation is just a semiconductor chip. In order to turn it into a usable astronomical instrument it needs to be connected to some electronics to power it, control it and read it out. By using a few clocking circuits, an amplifier and a fast analogue-to-digital converter (ADC), usually of 16-bit accuracy, it is possible to estimate the amount of light that has fallen onto each pixel by examining the amount of charge it has stored up. Thus, the charge which has accumulated in each pixel is converted into a number. This number is in arbitrary `units' of so-called `analogue data units' (ADUs); that is, it is not yet calibrated into physical units. The ADC factor is the constant of proportionality to convert ADUs into the amount of charge (expressed as a number of electrons) stored in each pixel. This factor is needed during the data reduction and is usually included in the documentation for the instrument. The chip will usually be placed in an insulating flask and cooled (often with liquid nitrogen) to reduce the noise level and there will be the usual appurtenances of astronomical instruments: shutters, filter wheels etc. The whole instrument is often referred to as a CCD camera. Other synonyms sometimes encountered are area photometer, panoramic photometer or array photometer.

The electronics controlling the CCD chip are interfaced to a computer which in turn controls them. Thus, the images observed by the CCD are transferred directly to computer memory, with no intermediate analogue stage, whence they can be plotted on an image display device or written to magnetic disk or tape. Normally you will return from an observing run with a magnetic tape cartridge of some sort containing copies of the images that you observed.