Diffraction

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.


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