What are diffraction spikes?

BBC Sky at Night Magazine's Scope Doctor Steve Richards solves your astronomy ailments.

Diffraction spikes seen from stars in globular cluster NGC 6397. Credit: NASA, ESA, and H. Richer (University of British Columbia)

What are diffraction spikes?

Steve says: “Light from bright objects like stars is diffracted when it passes any straight edge in its path and this diffraction manifests itself as bright spikes extending in two directions from the core of a star.

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For example, the spider vanes that support the secondary mirror in a Newtonian reflector each produce 2 spikes, although with the normal 4 vanes you only see 4 spikes because the second 4 are neatly overlaid on top of the first 4!

Wide-field images captured using a camera lens often show multiple diffraction spikes caused by the leaves of the camera’s aperture iris, so a 7-leaf iris will produce 14 spikes.”

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Email your astronomy queries to Steve at contactus@skyatnightmagazine.com and your question could be answered in a future issue of BBC Sky at Night Magazine.