9.1.8 Charge-Coupled Devices (CCDs) in Astronomy

• A charge-coupled device (CCD) is a detector which is highly sensitive to photons, making it ideal for use in the detection system of modern telescopes
  • Incident photons cause electrons to be released
  • The number of electrons released is proportional to the intensity of the incident light
  • An image is formed on the CCD, which can be processed electronically to give a digital image

A charged-coupled device

A charged-couple device has had a huge impact on modern astronomy

Quantum Efficiency

• Quantum efficiency (QE) is defined as

The percentage of incident photons which cause an electron to be released

• It can also be written as

quantum efficiency (QE) =

• In a perfect device, the quantum efficiency will be 100% if every photon generates a photoelectron
  • However, in practice, the quantum efficiency will be less than 100% since there will usually always be unavoidable losses

• Some values of QE for different devices are shown in the table

Device	Quantum efficiency (%)
human eye	1−4%
photographic film	4−10%
CCD	70−90%

 

• Comparison of the eye and a CCD:
  • CCDs are renowned for achieving high values of quantum efficiency, generally upwards of 80%, whereas a human eye is only capable of achieving around 1%

Resolution of a CCD

• The resolution of a CCD is related to the total number of pixels per unit area, and their size
  
  • The smaller the size of the pixel, the better the resolution, hence the clearer the image will be
• The typical resolution of a CCD is about 10 μm
• In comparison, the typical resolution of the human eye is about 100 μm, but it can vary widely

• In most cases, the overall resolution of a telescope is limited by the diameter of the objective
  • Hence, the resolution of the CCD (or the eye) is not likely to make a difference to the final image observed

Convenience of a CCD

• CCDs have an edge over the eye in terms of convenience because:
  • The number of images captured in a time period and exposure time can be easily adjusted
  • The information stored on a CCD can be accessed remotely
  • The generated images can be stored and analysed digitally
  • They can detect a larger range of wavelengths, including beyond the visible spectrum

Comparison of a CCD with the human eye 

• The main comparisons between the eye and a CCD are summarised in the table

Device	Quantum efficiency (%)	Resolution	Convenience of use
human eye	Very low ~1%	Typical resolution ~100 μm	No additional equipment required
CCD	Very high 70%+ Able to detect much fainter objects	Typical resolution ~10 μm Resolution can be increased by using smaller pixels	Remote viewing Images can be stored and analysed digitally Long exposure times Can detect a wider range of wavelengths