Charge-Coupled Devices(CCDs) in Astronomy

• What is a charge-coupled device (CCD)?

  A detector highly sensitive to photons
• Why are CCDs ideal for modern telescopes?

  They are highly sensitive to photons
• What happens when incident photons hit a CCD?

  Electrons are released from the device
• How is the number of electrons released by a CCD related to light?

  It is proportional to the intensity of light
• What is formed on a CCD when it detects light?

  An image that can be processed digitally
• What is quantum efficiency (QE)?

  The percentage of photons causing electron release
• How is quantum efficiency (QE) calculated?

  QE = (electrons produced / photons absorbed) × 100%
• What is the ideal quantum efficiency of a perfect device?

  100%
• Why is the quantum efficiency usually less than 100%?

  Due to unavoidable losses in the device
• What is the quantum efficiency of the human eye?

  1-4%
• What is the quantum efficiency of photographic film?

  4-10%
• What is the quantum efficiency of a CCD?

  70-90%
• How does the resolution of a CCD relate to pixel size?

  Smaller pixels yield better resolution
• What is the typical resolution of a CCD?

  About 10 μm
• What is the typical resolution of the human eye?

  About 100 μm
• What limits the overall resolution of a telescope?

  The diameter of the objective lens
• What advantages do CCDs have over the human eye?

  They can capture more images and store data
• How can CCDs detect a wider range of wavelengths?

  They can detect wavelengths beyond the visible spectrum
• What are the main comparisons between the human eye and a CCD?

  • Quantum efficiency:
  • Human eye: ~1%
  • CCD: 70%+
  • Resolution:
  • Human eye: ~100 μm
  • CCD: ~10 μm
  • Convenience:
  • Human eye: No additional equipment
  • CCD: Remote viewing, digital storage, long exposure times
• What is the focus of current exam questions regarding CCDs?

  Comparison between CCDs and the human eye