An astronomical detector is a device, typically located in the focal plane of a telescope or instrument, that has the ability to record the photons incident upon it. We have seen that for imaging, photometry and spectroscopy, detectors composed of a two-dimensional array of picture elements, or pixels, are essential for efficient operation. Furthermore, detectors with the ability to record as many of the incident photons as possible are highly desirable - wasting photons is a cardinal sin in astronomy, given how faint astronomical sources are and how much money and effort goes into building bigger telescopes.

Towards the end of 1969, Willard Boyle and George Smith, whilst trying to develop a video phone at Bell Labs in the USA, invented the charge-coupled device or CCD, a discovery for which they were awarded a share of the 2009 Nobel Prize in Physics. CCDs are almost perfect astronomical detectors. They are multi-element, small, linear, stable, low-power, low-cost devices with excellent sensitivity over a wide wavelength range. Astronomers pioneered the use of CCDs in the 1970s and nowadays you would find it extremely difficult to find any other type of detector in use at a major telescope. For this reason, we shall concentrate exclusively on CCDs in this part of the course.