A charge-coupled device (CCD) is a silicon chip divided into a grid of millions of tiny light-sensitive picture elements, called pixels. CCDs have replaced photographic film and the eye for recording astronomical images.How it works:
• incident photons cause electrons to be released in the silicon of each pixel
• the electrons are trapped in potential wells under each pixel during the exposure
• the charge collected in each pixel is proportional to the intensity of the light that fell on it
• after the exposure the charges are moved out pixel by pixel and measured, giving a digital image
The quantum efficiency is the percentage of incident photons that release an electron. For a CCD it is above 70% (about 80% is typical), compared with only about 1% for the eye, so a CCD can detect far fainter objects.Comparison with the eye:
• spectral range: CCDs respond from infrared to ultraviolet; the eye only detects about 400-700 nm
• resolution: CCD pixels are about 10 μm across and can give finer detail than the eye
• convenience: CCD images can be stored, processed and shared, and long exposures collect light over minutes or hours, which the eye cannot doWorked example: 5000 photons strike a pixel with quantum efficiency 80%. Electrons released = 0.80 × 5000 = 4000. The eye would detect only about 50 of them, so the CCD detects about 80 times as many.
• incident photons cause electrons to be released in the silicon of each pixel
• the electrons are trapped in potential wells under each pixel during the exposure
• the charge collected in each pixel is proportional to the intensity of the light that fell on it
• after the exposure the charges are moved out pixel by pixel and measured, giving a digital image
The quantum efficiency is the percentage of incident photons that release an electron. For a CCD it is above 70% (about 80% is typical), compared with only about 1% for the eye, so a CCD can detect far fainter objects.Comparison with the eye:• spectral range: CCDs respond from infrared to ultraviolet; the eye only detects about 400-700 nm
• resolution: CCD pixels are about 10 μm across and can give finer detail than the eye
• convenience: CCD images can be stored, processed and shared, and long exposures collect light over minutes or hours, which the eye cannot doWorked example: 5000 photons strike a pixel with quantum efficiency 80%. Electrons released = 0.80 × 5000 = 4000. The eye would detect only about 50 of them, so the CCD detects about 80 times as many.