Taking the Image

A CCD (charge-coupled device) is an electronic instrument for detecting light.  In the case of an astronomical CCD camera, this light is very dim.  We will see that this has certain implications for how the CCD operates.

A CCD uses a thin silicon wafer chip.  The chip is divided into thousands or millions of tiny light sensitive squares (or sometimes rectangles) called photosites.  Each photosite corresponds to an individual pixel in the final image and photosites are often referred to simply as pixels.  For clarity in this discussion, "photosites" will refer to the CCD chip and "pixels" will refer to an image.  Each photosite is surrounded by a non-conductive boundary which contains the charge that is collected during an exposure within the photosite.

Above:  A CCD chip consists of an array of photosites (squares) and a serial register for reading out the image data.

So, where does this charge collected during an exposure come from?  The photoelectric effect.

In 1921, Albert Einstein  received the Nobel Prize in physics, not for his famous theory of relativity, but rather for explaining the photoelectric effect.  It is this effect operating in a CCD chip which produces the electric charge stored in the photosites.  When a photon of light strikes the surface of certain metal materials (like the silicon in a CCD chip) the energy imparted by the photon can release an electron from the metal.  In a CCD, this electron is stored within the walls of a photosite.  During a long exposure, photons rain down from the celestial object being imaged and strike the CCD detector.  The photosites act like wells and begin to fill up with electrons (generated by the photons impacting the chip).

Above:  The photosites (squares on top) collect light and convert it to electric charge.  The "wells" begin to fill with charge.  Brighter areas (light grey) fill faster than dark areas (dark grey and black).

If an area of the CCD is imaging a bright object such as a star (which gives off lots of photons), the photosites in that area fill up with more electrons than those in an area imaging something dim like faint nebulosity or the black night sky.  (We will see shortly that even the photosites imaging black sky will end up containing some electrons for several reasons.)

Once the exposure is finished (usually done by closing a shutter on the camera), the charge must be transferred out of the CCD and displayed on a computer monitor.

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