CCDs

Charge coupled devices, or CCDs are sensitive detectors of photons that can be used in telescopes instead of film or photographic plates to produce images. CCDs were invented in the late 1960s and are now used in digital cameras, photocopiers and many other devices.

A CCD is a tiny microchip onto which the light that the telescope collects is focused. The microchip consists of a large grid of individual light sensing elements called pixels. There are 2000 pixels along each side of the chip in the Merope Camera in Faulkes Telescope North. Each pixel is 15 micrometers(µm) across. For comparison, a strand of human hair is about 100 µm wide. When light falls onto one of the pixels, electrons are released from atoms in the pixel. To measure the amount of light that fell onto each pixel, the number of electrons that was released has to be counted. This is done by measuring the charge on the pixel at the end of the last row in the grid. Then that charge is discarded and all the other charges in the row are made to move along one pixel. The next charge in line is then measured, and so on – until all the charges in that row have been dealt with. Then all the charges in all the remaining rows are made to move over one row, and the whole process is repeated. Amazingly, the entire chip can be "read" in less than 10 seconds. The animation below gives a visualization of how this works.

CCDs are increadibly powerful tools for astronomers because when a telescope’s motion is synchronized with the Earth’s rotation, the camera can “stare” at one spot in space for hours at a time. The longer the CCD is exposed to the sky, the more photons will land on it, and fainter, more distant objects can be imaged than are otherwise visible. CCDs exposures are so long in astronomy (seconds, minutes or even longer) compared to digital cameras (normally a fraction of a second), that CCDs in telescopes must be kept very cold (−50° to -100°C) to keep the sensor from picking up charge from the warmth of the environment.