Charge-Coupled Devices (CCDs)

How a CCD works

• 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 from the surface of the CCD via the photoelectric effect.
• 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.

Quantum efficiency

• Quantum efficiency (QE) is defined as:

• In a perfect device, the quantum efficiency would 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.
• The key part is the comparison between different devices:
  • The human eye has a quantum efficiency of approximately 1-4%.
  • Photographic film has a quantum efficiency of approximately 4-10%.
  • A CCD has a quantum efficiency of approximately 70-90%.
• 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 $\mu$m.
• In comparison, the typical resolution of the human eye is about 100 $\mu$m, but it can vary widely.
• Crucially, in most cases, the overall resolution of a telescope is limited by the diameter of the objective (via the Rayleigh criterion), not the resolution of the detector. 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.
• They allow for long exposure times, enabling detection of much fainter objects.

Comparison of a CCD with the human eye

• The main comparisons between the eye and a CCD are:
  • Quantum efficiency: The human eye has a very low QE ($\sim$1%), whereas a CCD has a very high QE (70%+). Consequently, a CCD is able to detect much fainter objects.
  • Resolution: The human eye has a typical resolution of $\sim$100 $\mu$m; a CCD has a typical resolution of $\sim$10 $\mu$m, and resolution can be increased further by using smaller pixels. However, both are limited in practice by the telescope's objective diameter.
  • Convenience: The eye requires no additional equipment but cannot store images. A CCD allows remote viewing, digital storage and analysis, adjustable exposure times, and detection of a wider range of wavelengths.