Charge-Coupled Devices.

Traditionally astronomical observations were made by eye and subsequently photographic plates were used to obtain deeper imaging and spectroscopy. The present detector of choice is the charge-coupled device (CCD) providing a quantum efficiency ($QE$) of nearly 100%, where

$$QE = \frac{{\rm Number\ of\ photons\ detected}}{{\rm Number\ of\ incident\ photons}}$$ (10)

This is an enormous advantage over photographic plates which typically had $QE \approx 1\%$. A plot of $QE$ vs. wavelength is given in figure 2.4 for the SuprimeCam detector (see section 2.2.3).

Figure 2.4: Quantum efficiency vs. wavelength for the Subaru Prime Focus Camera.

CCDs are semi-conductor devices consisting of a grid of pixels. For example the UKIRT UFTI camera has $1024 \times 1024 \approx 1.05 \times 10^{6}$ pixels. Photons incident on the detector are photo-electrically absorbed by an individual pixel, causing a build up of charge. The charge can be integrated over a period of time after which the CCD is read out and the data processed. The response of a CCD is highly linear over a large range so the intensity of light is accurately measured at the position of each pixel. The linearity of UKIRT's UFTI camera is shown in figure 2.5. Furthermore the digital nature of the data is extremely handy when it comes to processing the data facilitating the use of computers in this task.

Figure 2.5: Linearity of the UKIRT Fast Track Imager.