An optical interference effect due to the bending of light around
obstacles in its path (the edges of a telescope tube or its internal
light baffles, for example), similar to the way ocean or lake waves are
bent or deflected around dock pilings or the edge of a jetty. All
telescopes show faint light and dark diffraction rings around a star's
Airy disk at high power, as the diffracted light waves alternately
cancel out and reinforce each other. Diffraction rings are very faint
and an observer's inability to see them should not be a cause for
concern. For example, in a perfect refractor about 84% of the light
would be imaged in the Airy disk, with half of the remainder falling in
the first diffraction ring and the balance scattered among the second,
third, fourth rings, etc. Since the first diffraction ring is about six
times the area of the Airy disk itself, its fainter light is spread over
a much larger area, so that the brightness of the first diffraction
ring is actually less than 2% that of the Airy disk. The other rings are
dimmer still. It is easy to see how the beginning observer can have
difficulty separating the very faint diffraction rings from the much
brighter Airy disk. Catadioptric and reflector diffraction rings start
out about twice as bright as those of a refractor due to the additional
diffraction caused by their secondary mirror obstructions, but their
brightness is still low in relation to their Airy disk (only 4% as
bright in the case of the first ring). A catadioptric's higher
diffraction ring brightness shows itself as lower contrast and some loss
of sharpness on planets, binary stars, and star clusters when compared
with a refractor. The spider vanes holding a reflector's diagonal mirror
create additional contrast-lowering diffraction spikes radiating out
from each star's image, an effect particularly visible on long exposure
photos. The first illustration below simulates the Airy disk of a
slightly out-of-focus star in a properly collimated reflector. The
shadows of the diagonal mirror and spider vanes are shown, as are the
diffraction spikes of the spider vanes supporting the diagonal. A
catadioptric telescope also has a circular secondary mirror shadow, as
shown in the next illustration, but does not have diffraction spikes and
spider vane shadows.