Diffraction limits image detail
A telescope objective has a finite circular aperture. Radiation from a distant point source diffracts as it passes through this aperture, so its image is not an infinitely small point.
Instead, a circular diffraction pattern forms with a bright central region surrounded by alternating dark and bright rings.
The bright central maximum is called the Airy disc. The name refers only to this central region, not to the complete diffraction pattern.
- The central maximum is approximately twice as wide as the later bright maxima.
- Each distant point source produces its own diffraction pattern.
- Two nearby sources therefore produce patterns that can overlap.
- If the overlap is too great, two separate sources appear as one unresolved source.
Diffraction at the aperture therefore places a limit on the finest detail that the telescope can distinguish.
Increasing the aperture diameter or observing at a shorter wavelength reduces the effect of diffraction on angular resolution.
Exam Tip: Use Airy disc for the central maximum only. Refer to the complete arrangement of the central maximum and surrounding rings as the diffraction pattern.